*Editorial* **Research in the Atlantic Sector of the Southern Ocean and Propagation of Antarctic Bottom Water in the Atlantic**

**Eugene Morozov**

Shirshov Institute of Oceanology, Russian Academy of Sciences, 117997 Moscow, Russia; egmorozov@mail.ru

This article belongs to the Special Issue "Physical and Biological Properties of Waters in the Region of the Antarctic Peninsula and Adjacent Basins of the South Atlantic,": a Special Issue that is dedicated to recent research in the Atlantic sector of the Southern Ocean and Antarctica Peninsula region. The papers in this issue are focused on the field and theoretical research of the physical properties and ecology of this key region in the Southern Ocean. The main objects of the study were the properties of seawater, currents, the ecosystem, and biological communities in the South Atlantic, the Powell Basin of the northwestern Weddell Sea, the Bransfield Strait, and the Antarctic Sound. The Antarctic marine ecosystem is unique was terms of its biodiversity and high productivity of basic trophic levels of the ecosystem. This ecosystem was efficiently used by organisms such as seabirds, seals, and whales that fed on Antarctic krill.

The Antarctic Circumpolar Current System (ACC) and oceanic circulation at higher latitudes near the Antarctic continent are the oceanographic basis for the functionality of the Antarctic marine ecosystem. The processes in this region of the World Ocean have been intensely studied in recent decades but still remain not completely understood because of an insufficient number of expeditions to this region. The processes in Antarctica require more and more investigation by scientists. In recent decades, the Southern Ocean has experienced significant changes associated with global climate trends.

These issues of the *Water* MDPI journal have been composed on the basis of analyzing the results of multi-disciplinary studies based on the physics and biology of the Atlantic sector of the Antarctic during two cruises in January-February 2020 and in January-February 2022 on the R/V *Akademik Mstislav Keldysh* (cruises 79 and 87) [1,2].

This issue presents the results of multidisciplinary studies in Antarctica. The issue includes articles on marine physics, marine chemistry, and marine biology, which jointly compose a comprehensive multidisciplinary approach to a modern view of the processes that govern the mean state of the ocean and its variability. Research has been performed in the region of the Antarctic Circumpolar Current, Weddell Gyre, Weddell Sea, and Bransfield Strait. The most important areas of study were the Bransfield Strait between the Antarctic Peninsula [3], the South Shetland Islands, and the Powell Basin east of the northern tip of the Antarctic Peninsula [2].

Research has also been conducted in the region of the South Orkney Islands and Orkney Passage east of the Powell Basin. The Orkney Passage is the main pathway for Antarctic Bottom Water flow from the Weddell Sea [4]. On the route of the ship to the Antarctic continent, the propagation of Antarctic Bottom Water has been studied in the Vema Channel [5]. The upwelling on the Patagonian shelf and Malvinas Current became a topic of multidisciplinary research during the expedition. We also studied the processes across an eddy dipole in the interaction zone between Subtropical and Subantarctic waters in the Southwest Atlantic [6].

Biological papers on this issue present an analysis of the recent changes in the composition and distribution of zooplankton [7,8] and Antarctic worms [9]. The biological analysis is based on the assessment of water quality and biota in this region.

**Citation:** Morozov, E. Research in the Atlantic Sector of the Southern Ocean and Propagation of Antarctic Bottom Water in the Atlantic. *Water* **2023**, *15*, 2348. https://doi.org/10.3390/ w15132348

Received: 1 June 2023 Revised: 13 June 2023 Accepted: 20 June 2023 Published: 25 June 2023

**Copyright:** © 2023 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

**Funding:** This research was supported by the Russian Science Foundation grant 21-77-20004.

**Conflicts of Interest:** The author declares no conflict of interest.

### **References**


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## *Article* **Water Exchange between Deep Basins of the Bransfield Strait**

**Dmitry I. Frey 1,2,\* , Viktor A. Krechik 1,3, Eugene G. Morozov 1,2 , Ilya D. Drozd 1,4,5, Alexandra S. Gordey <sup>1</sup> , Alexander A. Latushkin <sup>2</sup> , Olga S. Mekhova 1,6, Rinat Z. Mukhametianov 1,7, Svetlana A. Murzina 1,8 , Sofia A. Ostroumova 1,6, Vladimir I. Ponomarev <sup>9</sup> , Pavel A. Salyuk <sup>9</sup> , Daria A. Smirnova 1,4 , Sergey A. Shutov <sup>2</sup> and Oleg A. Zuev <sup>1</sup>**


**Abstract:** The Bransfield Strait is a relatively deep and narrow channel between the South Shetland Islands and the Antarctic Peninsula contributing to the water transport between the Pacific and Atlantic sectors of the Southern Ocean. The strait can be divided into three deep separate basins, namely, the western, central, and eastern basins. The sources of deep waters in the three basins are different, leading to differences in thermohaline properties and water density between the basins. The difference in water density should in turn cause intense deep currents from one basin to another through narrow passages over the sills separating the basins. However, there are still no works dedicated to such possible overflows in the Bransfield Strait. In this study, we report our new CTD and LADCP measurements performed in 2022 over the watersheds between the basins. Quasisimultaneous observations of the main circulation patterns carried out at several sections allowed us to analyze the evolution of thermohaline and kinematic structures along the Bransfield Strait. Volume transports of waters in the strait were estimated on the basis of direct velocity observations. These new data also indicate the existence of intense and variable deep current between the central and eastern basins of the strait. The analysis of historical data shows that the mean flow is directed from the central to the eastern basin. In addition, LADCP data suggest the intensification of the flow in the narrow part of the sill between the basins, and the possible mixing of deep waters at this location.

**Keywords:** Bransfield Strait; deep overflow; CTD; LADCP; bottom circulation

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

### **1. Introduction**

The Bransfield Strait (BS) is an important passage for Antarctic waters in the region of the Antarctic Peninsula, which contributes to the zonal water transport between the Pacific and Atlantic sectors of the Southern Ocean. The strait extends over 460 km from the west–southwest to the east–northeast (true direction is 60◦ relative to the north), and is bounded by the South Shetland Islands from the northwest and the Antarctic Peninsula from the southeast. The BS region is characterized by strong climatic changes [1] that affect

**Citation:** Frey, D.I.; Krechik, V.A.; Morozov, E.G.; Drozd, I.D.; Gordey, A.S.; Latushkin, A.A.; Mekhova, O.S.; Mukhametianov, R.Z.; Murzina, S.A.; Ostroumova, S.A.; et al. Water Exchange between Deep Basins of the Bransfield Strait. *Water* **2022**, *14*, 3193. https://doi.org/10.3390/w14203193

Academic Editor: Changming Dong

Received: 7 August 2022 Accepted: 5 October 2022 Published: 11 October 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

3

the oceanographic, meteorological, glaciological, and biological conditions [2]. The BS region is also important because of its high biological productivity [3]. In particular, the region is a key source of Antarctic krill (*Euphausia superba*) to the Southern Ocean [4–6], and the region is a significant spawning, breeding, and overwinter habitat of *E. superba* [7–9] and another abundant euphausiid in Antarctic waters, *Thyssanoessa macrura* [10]; the BS is one of the significant regions for commercial krill fishing [11–13]. gion is also important because of its high biological productivity [3]. In particular, the region is a key source of Antarctic krill (*Euphausia superba*) to the Southern Ocean [4–6], and the region is a significant spawning, breeding, and overwinter habitat of *E. superba* [7–9] and another abundant euphausiid in Antarctic waters, *Thyssanoessa macrura* [10]; the BS is one of the significant regions for commercial krill fishing [11–13]. The hydrography of the Bransfield Strait is highly dependent on the complicated ba-

Pacific and Atlantic sectors of the Southern Ocean. The strait extends over 460 km from the west–southwest to the east–northeast (true direction is 60° relative to the north), and is bounded by the South Shetland Islands from the northwest and the Antarctic Peninsula from the southeast. The BS region is characterized by strong climatic changes [1] that affect the oceanographic, meteorological, glaciological, and biological conditions [2]. The BS re-

*Water* **2022**, *14*, x FOR PEER REVIEW 2 of 17

The hydrography of the Bransfield Strait is highly dependent on the complicated bathymetry [1,14]. From a geomorphological point of view, the BS can be considered as a sequence of three basins (Figure 1), namely, the western (WB), central (CB), and eastern (EB) basins [15,16]. Modern bottom topography data GEBCO2021 suggest that the maximal depths of these basins are 1370, 1960, and 2750 m, respectively. The basins are separated by relatively shallow sills. The maximal depth of the sill between the WB and CB is 630 m; the depths of the sill between the CB and EB slightly exceed 1000 m. The WB is connected to the Bellingshausen Sea through the Gerlache Strait and other gaps between Smith, Low, and Hoseason islands, and to the Drake Passage through the Boyd Strait (Figure 1). The EB is open to the Weddell and Scotia seas through relatively shallow sills; their depths do not reach 800 m. thymetry [1,14]. From a geomorphological point of view, the BS can be considered as a sequence of three basins (Figure 1), namely, the western (WB), central (CB), and eastern (EB) basins [15,16]. Modern bottom topography data GEBCO2021 suggest that the maximal depths of these basins are 1370, 1960, and 2750 m, respectively. The basins are separated by relatively shallow sills. The maximal depth of the sill between the WB and CB is 630 m; the depths of the sill between the CB and EB slightly exceed 1000 m. The WB is connected to the Bellingshausen Sea through the Gerlache Strait and other gaps between Smith, Low, and Hoseason islands, and to the Drake Passage through the Boyd Strait (Figure 1). The EB is open to the Weddell and Scotia seas through relatively shallow sills; their depths do not reach 800 m.

**Figure 1.** (**a**) Bathymetry of the Bransfield Strait and (**b**) the region of the overflow between the central and eastern basins. (**a**) The upper layer circulation schematic is shown by black arrows. **Figure 1.** (**a**) Bathymetry of the Bransfield Strait and (**b**) the region of the overflow between the central and eastern basins. (**a**) The upper layer circulation schematic is shown by black arrows. Western, central, and eastern basins of the strait are shown with green, yellow, and orange, respectively. CTD/LADCP stations performed in 2022 are shown by white dots; historical CTD stations from World Ocean Database (WOD2018) used in this study are shown by grey dots. The bottom relief is shown according to the GEBCO2021 database; the shoreline is based on the GSHHS data [17]. Station 7390 (not shown) repeats station 7352.

The thermohaline structure of the upper BS layer is formed by two water masses, namely, transitional zonal water with Bellingshausen Sea influence (TBW) and transitional zonal water with Weddell Sea influence (TWW) [18,19]. According to [1], typical characteristics of these waters are θ > 1 ◦C, S < 34.1 psu (TBW flow) and θ < 1 ◦C, S > 34.1 psu (TWW flow). TBW waters propagate to the northeast along the South Shetland Islands in the form of a narrow high-velocity jet called the Bransfield Current [19–22]. Additionally, modified Circumpolar Deep Water (mCDW) with temperatures θ > 1 ◦C and salinities >34.5 psu is stably observed within the Bransfield Current in a depth range of 200–450 m [23,24]. The maximal velocities of the Bransfield Current are observed at the sea surface and linearly decay towards the bottom [22,25,26]; its transport is approximately 1 Sv on the basis of direct velocity measurements [27]. The authors in [28] showed that the diurnal tide essentially affects the Bransfield Current; the same effect was observed in [29]. The TWW is located in the southern part of the strait and spreads southwestward along the Antarctic Peninsula [23]; the velocities of this flow are much lower than those in the Bransfield Current and usually do not exceed 20–30 cm/s [30]. Further inflows of TWW waters from the BS to the West Antarctic Peninsula slope are caused by wind forcing [31]. TWW and TBW waters are separated by two fronts: the Peninsula Front divides these waters at the sea surface and the Bransfield Front divides them in the deeper layers. The Bransfield Front is located much closer to the South Shetland islands than the Peninsula Front is.

The thermohaline structure of waters within the Bransfield Strait was repeatedly studied on the basis of in situ CTD data [15,24,32–36]. Deep layers of the BS basins are filled with relatively cold, saline, and dense waters from the continental shelf of the western Weddell Sea [32,33]. During the last few decades, the freshening and lightening of these waters have been observed [24,34,37]; their variability is caused by changes in source waters, and negatively correlated with Southern Annular Mode [38]. As the water density in the deep layers differs from one BS basin to another, one might expect that deep overflows can exist over the sills between the basins. However, there are no studies dedicated to direct measurements over the sill points between the WB and CB, and between the CB and EB. Regarding available data, the WOD2018 database contains 27 CTD profiles in the CB and 18 profiles in the EB, but there are no stations near the sill between these basins. The same lack of data is observed in the region of the sill between the WB and CB. LADCP data are even less available than CTD data. Regarding Shipboard ADCP measurements (for example, [27,30]), the maximal depth of such velocity profiles does not exceed 300–400 m, which is not sufficient for the studies of the bottom circulation between the deep basins of the BS. A recent work by [39] also showed that geostrophic velocity calculations based on hydrographic data do not reproduce actual ocean circulation patterns in the strait. This fact emphasizes the importance of direct velocity measurements in the BS. The objective of this paper is to study deep-water exchange between the basins of the BS based on the new data collected in January and February 2022. We used both CTD and LADCP profilers for the synchronous measurements of thermohaline and kinematic structures of currents.

The paper is structured as follows: we describe our in situ CTD and LADCP measurements in Section 2. In Section 3, we analyze the spatial thermohaline and kinematic structure of flows over the sill between the WB and CB (Section 3.1), and between the CB and EB (Section 3.2). The results are discussed in Section 4, followed by conclusions in Section 5.

#### **2. Materials and Methods**

This study is focused on the water exchange between the deep basins of the BS. For this purpose, we combined in situ measurements of temperature, salinity, and velocities performed during austral summer in January and February 2022. In this section, we describe our approach for station selection (Section 2.1), and the applied equipment and data processing techniques (Section 2.2).

#### *2.1. Sections across the Strait*

processing techniques (Section 2.2).

**2. Materials and Methods** 

*Water* **2022**, *14*, x FOR PEER REVIEW 4 of 17

A total of 34 stations were performed within the BS from 21 January to 14 February 2022 (Table 1) in the 87th cruise of the research vessel *Akademik Mstislav Keldysh*. The measurements were performed almost at the bottom from the ship that maintained its position at the station with accuracy not worse than 200 m. The stations were organized in three sections, up to nine stations each across the strait (Figure 1); two relatively short but high-resolution sections were located at the sill point between the central and east basins. The GEBCO2021 bathymetry grid of 15" resolution was used for the selection of the stations. The major part of the BS region has been covered by multibeam echosounder surveys. Their locations are shown with green in the right upper panel of Figure 2 based on the data from GEBCO2021 Type Identifier (TID) grid. Three BS basins are clearly seen along the thalweg of the strait (Figure 2). The first section was located over the ridge between the WB and CB; the second and third sections were located in the CB, allowing for us to trace how water properties change along the strait. Two additional sections were located across and along the sill between the CB and EB. These sections were performed with very high resolution (the distance between stations here was approximately 2 km), which allowed us to study deep overflow between the CB and EB. Because the GEBCO2021 grid contains multibeam echosounder data for the entire strait, the depths of our stations and own single-beam measurements using Kongsberg EA600 echosounder coincided very well with the GEBCO bathymetry. *2.1. Sections across the Strait*  A total of 34 stations were performed within the BS from 21 January to 14 February 2022 (Table 1) in the 87th cruise of the research vessel *Akademik Mstislav Keldysh*. The measurements were performed almost at the bottom from the ship that maintained its position at the station with accuracy not worse than 200 m. The stations were organized in three sections, up to nine stations each across the strait (Figure 1); two relatively short but highresolution sections were located at the sill point between the central and east basins. The GEBCO2021 bathymetry grid of 15″ resolution was used for the selection of the stations. The major part of the BS region has been covered by multibeam echosounder surveys. Their locations are shown with green in the right upper panel of Figure 2 based on the data from GEBCO2021 Type Identifier (TID) grid. Three BS basins are clearly seen along the thalweg of the strait (Figure 2). The first section was located over the ridge between the WB and CB; the second and third sections were located in the CB, allowing for us to trace how water properties change along the strait. Two additional sections were located across and along the sill between the CB and EB. These sections were performed with very high resolution (the distance between stations here was approximately 2 km), which allowed us to study deep overflow between the CB and EB. Because the GEBCO2021 grid contains multibeam echosounder data for the entire strait, the depths of our stations and own single-beam measurements using Kongsberg EA600 echosounder coincided very well with the GEBCO bathymetry.

This study is focused on the water exchange between the deep basins of the BS. For this purpose, we combined in situ measurements of temperature, salinity, and velocities performed during austral summer in January and February 2022. In this section, we describe our approach for station selection (Section 2.1), and the applied equipment and data

**Figure 2.** Depths along the thalweg of the Bransfield Strait and location of CTD/LADCP measurements. (**a**) Bottom topography according to the GEBCO2021 database, shoreline is based on GSHHS data [17], thalweg is indicated with dark red solid line, CTD/LADCP sections shown with black solid lines, colored dots show the locations of CTD stations along the thalweg presented in (**c**). (**b**) areas with ocean bottom measured with multibeam soundings indicated with green; the data were taken from TID file of GEBCO2021 database. (**c**) Ocean depth along the thalweg of the BS (shown with gray) and location of CTD stations along the strait; the deepest CTD/LADCP station was selected at each transect. Station numbers correspond to Table 1; colors are indicated in the bottom panel.


**Table 1.** Coordinates of stations carried out in the Bransfield Strait in January and February 2022.

#### *2.2. In Situ Measurements and Data Processing*

The stations were performed using the lowered acoustic Doppler current (LADCP) and conductivity, temperature, depth (CTD) profilers mounted on a General Oceanics GO1018 rosette water sampler. The CTD measurements were performed along all three sections; the LADCP data are available only for Sections 1 and 2 due to technical reasons. Exact information about the type of measurements at each station is presented in Table 1. The water sampler was equipped with a Valeport VA500 altimeter allowing for measurements close to the ocean bottom (3–7 m above the seafloor; see Table 1 for more details). An Idronaut Ocean Seven 320plus CTD probe was used for the measurements together with an MKplus Deck Unit. CTD data were collected using standard package REDAS5 version 5.78. The declared accuracy of CTD measurements is 0.001 ◦C for temperature, and 0.001 mS/cm for conductivity sensors. The CTD data from the World Ocean Database (WOD2018) were used for addressing potential temperature and salinity variations in the bottom layer of the CB and EB. This database contains 27 CTD profiles in the CB, and 18 profiles in the EB carried out in different years and seasons (only profiles deeper than 1500 m were taken into account). Most stations were performed during the austral summer season. Thus, 18 stations of 27 in the CB, and 14 stations of 18 in the EB were occupied from November to February. The observation period covered the 1980s and 1990s; three stations in the EB were performed in 1975–1976. The LADCP data measured with a TRDI WorkHorse Monitor 300 kHz profiler were processed using programming package LDEO Software version IX.10 [40]. Data from the shipborne acoustic doppler current profiler (SADCP) TRDI Ocean Surveyor 75 kHz were used for more reliable data processing in the upper ocean layer. The accuracy of velocity measurements estimated by the processing program is usually 3–4 cm/s. In the bottom layers, due to the bottom track signals, the errors decreased to 1–2 cm/s. The results of LADCP processing were corrected by subtracting the tidal velocities. The barotropic tide was calculated on the basis of the TPXO 9.1 global inverse tide model [41].

#### **3. Results**

We analyzed the spatial kinematic and thermohaline structures of the currents within the BS at several transects across the strait; their locations are presented in Figure 1 and Table 1. The observations included CTD and LADCP stations from the surface to the bottom. The results of these measurements are discussed separately for the western (Section 1) and eastern (Section 2) margins of the CB.

#### *3.1. Water Exchange between Western and Central Basins*

The results of our measurements along three sections in the western part of the CB are shown separately on the basis of LADCP (Figure 3) and CTD (Figure 4) data. Previously, such measurements were performed in the CB, and allowed for investigations of the spatial structure of currents [26,27,30,35] and thermohaline properties of waters within the strait [16,23,37,42,43]. Some of these studies were focused on a single transect in different parts of the strait [25,26,35,39]. However, there are no studies focused on the sills between deep basins or where the structure of the TBW and TWW flows were analyzed on the basis of several quasisimultaneous crossings of the strait with synchronous CTD and LADCP measurements. Velocity data were projected to the direction along the strait (true direction is 60◦ ). CTD data are available for all three sections; LADCP measurements were performed only at two western sections (Sections 1 and 2; see Figure 2 for their location). Two major circulation patterns were clearly observed at both sections: the fast and narrow Bransfield Current is located in the northwestern part of the strait, while the flow of waters from the Weddell Sea is observed along the Antarctic Peninsula. The main result is that TWW flow was observed at Section 1, the westernmost. It was previously suggested that TWW flow recirculates within the BS and does not propagate as far to the west through the entire strait [27,42]. The velocities of this flow do not decrease along the TWW path: the maximal velocities are 23 cm/s at Section 2 and 36 cm/s at Section 1. On the other hand,

the Bransfield Current that transports TBW waters significantly changes between these sections. The maximal velocities changed from 24 cm/s at Section 1 to 52 cm/s at Section 2. The maximum was located at a 260 m depth at Section 1; at Section 2, the maximal velocities were observed at the sea surface. The total volume transports of TBW waters based on LADCP measurements were 1.19 Sv (Section 1) and 2.54 Sv (Section 2). The corresponding values of TWW transports were 0.77 Sv and 1.82 Sv. No significant currents were observed in the bottom layer of the sill between the WB and CB; the conditions in the bottom layer over the sill between CB and EB were significantly different and are discussed below in more detail. these sections. The maximal velocities changed from 24 cm/s at Section 1 to 52 cm/s at Section 2. The maximum was located at a 260 m depth at Section 1; at Section 2, the maximal velocities were observed at the sea surface. The total volume transports of TBW waters based on LADCP measurements were 1.19 Sv (Section 1) and 2.54 Sv (Section 2). The corresponding values of TWW transports were 0.77 Sv and 1.82 Sv. No significant currents were observed in the bottom layer of the sill between the WB and CB; the conditions in the bottom layer over the sill between CB and EB were significantly different and are discussed below in more detail.

path: the maximal velocities are 23 cm/s at Section 2 and 36 cm/s at Section 1. On the other hand, the Bransfield Current that transports TBW waters significantly changes between

*Water* **2022**, *14*, x FOR PEER REVIEW 7 of 17

**Figure 3.** Along-strait LADCP velocity distributions at two sections across the BS: (**a**) Section 1 over the watershed between the WB and CB and (**b**) Section 2 in the CB. Locations of TBW and TWW flows are indicated at the top of each panel; stations are indicated by solid black lines. The bottom relief is shown according to the GEBCO2021 database. **Figure 3.** Along-strait LADCP velocity distributions at two sections across the BS: (**a**) Section 1 over the watershed between the WB and CB and (**b**) Section 2 in the CB. Locations of TBW and TWW flows are indicated at the top of each panel; stations are indicated by solid black lines. The bottom relief is shown according to the GEBCO2021 database.

The CTD data are available for all three sections across the CB of the strait (Figure 4). Potential temperature and salinity distributions showed quite similar structures; at all sections, TBW and TWW flows were clearly distinguished on the basis of different thermohaline properties. The warmest waters were observed within the Bransfield Current; the maximal water temperature changed along the strait from 1.27 ◦C (Section 1) to 1.56 ◦C (Section 2) and 1.52 ◦C (Section 3); the corresponding variations of the minimal salinity were 34.22, 34.14, and 34.24 PSU. At depths of 200–450 m, a core of mCDW waters was observed at all sections; their properties changed along the strait from 1.08 to 1.13 and 1.08 ◦C, and 34.73 to 34.73 and 34.71 psu. The minimal potential temperature in the upper 200 m was observed near the Antarctic Peninsula within the TWW flow. Along this flow, thermohaline properties changed from −0.73 ◦C and 34.60 psu (Section 3) to −0.62 ◦C and 34.57 psu (Section 2), and −0.66 ◦C and 34.58 psu (Section 1). The minimal potential temperature in the CB, −1.65 ◦C, was observed at the deepest point of the strait at station 7299; the minimal potential temperature over the watershed between the WB and CB was −0.95 ◦C (station 7321), confirming the well-known fact that the bottom waters of the CB originated directly from the waters of the Weddell Sea.

**Figure 4.** (**a**,**c**,**e**) Potential temperature and (**b**–**f**) salinity distributions at three sections across the Bransfield Strait. Data for (**a**,**b**) Section 1, (**c**,**d**) Section 2, and (**e**,**f**) Section 3. Isolines of potential density anomalies shown with dashed contours. Locations of CTD stations are indicated with solid lines. The bottom relief is shown according the GEBCO2021 database. **Figure 4.** (**a,c,e**) Potential temperature and (**b**–**f**) salinity distributions at three sections across the Bransfield Strait. Data for (**a,b**) Section 1, (**c,d**) Section 2, and (**e,f**) Section 3. Isolines of potential density anomalies shown with dashed contours. Locations of CTD stations are indicated with solid lines. The bottom relief is shown according the GEBCO2021 database.

#### The CTD data are available for all three sections across the CB of the strait (Figure 4). *3.2. Deep-Water Overflow between Central and Eastern Basins*

Potential temperature and salinity distributions showed quite similar structures; at all sections, TBW and TWW flows were clearly distinguished on the basis of different thermohaline properties. The warmest waters were observed within the Bransfield Current; the maximal water temperature changed along the strait from 1.27 °C (Section 1) to 1.56 °C (Section 2) and 1.52 °C (Section 3); the corresponding variations of the minimal salinity were 34.22, 34.14, and 34.24 PSU. At depths of 200–450 m, a core of mCDW waters was observed at all sections; their properties changed along the strait from 1.08 to 1.13 and 1.08 °C, and 34.73 to 34.73 and 34.71 psu. The minimal potential temperature in the upper 200 m was observed near the Antarctic Peninsula within the TWW flow. Along this flow, thermohaline properties changed from −0.73 °C and 34.60 psu (Section 3) to −0.62 °C and 34.57 psu (Section 2), and −0.66 °C and 34.58 psu (Section 1). The minimal potential temperature in the CB, −1.65 °C, was observed at the deepest point of the strait at station 7299; the minimal potential temperature over the watershed between the WB and CB was −0.95 °C The overflow between the CB and EB was studied on the basis of a transect performed over the sill between the basins (Figure 5). The deepest point of the watershed between the basins is 1050 m based on the GEBCO2021 database. The transect included five stations and was oriented across the deep overflow. Along-flow velocity, potential temperature, salinity, and potential density relative to the sea surface are shown in Figure 5. Velocities over entire water column along the section were quite low (less than 10–15 cm/s). TBW and TWW flows are located at some distance away from the transect between two basins; the Bransfield Current was observed north of our stations, closer to the South Shetland Islands, while the inflow of waters from the Weddell Sea is located closer to the Antarctic Peninsula. On the basis of the data of our section, the intensification of currents was observed in the bottom layers. Two separate high-velocity jets were observed at station 7357 and 7352. The first jet with maximal velocity equal to 22 cm/s was located at the deepest point of the section. The coldest (minimal potential temperature was −1.31 ◦C) and

directly from the waters of the Weddell Sea.

(station 7321), confirming the well-known fact that the bottom waters of the CB originated

the stations.

densest (maximal potential density anomaly relative to the sea surface was 27.87 kg/m<sup>3</sup> ) waters were observed at this point. The second jet with the maximal velocity, equal to 21 cm/s, was observed at station 7357. Its location near the steep slope could be caused by the Coriolis force, which displaced the jet to the left in the Southern Hemisphere. A very thin bottom layer of cold waters was also observed at station 7354; this layer was formed by the overflow of cold waters from the central basin. and densest (maximal potential density anomaly relative to the sea surface was 27.87 kg/m3) waters were observed at this point. The second jet with the maximal velocity, equal to 21 cm/s, was observed at station 7357. Its location near the steep slope could be caused by the Coriolis force, which displaced the jet to the left in the Southern Hemisphere. A very thin bottom layer of cold waters was also observed at station 7354; this layer was formed by the overflow of cold waters from the central basin.

*Water* **2022**, *14*, x FOR PEER REVIEW 9 of 17

*3.2. Deep-Water Overflow between Central and Eastern Basins* 

The overflow between the CB and EB was studied on the basis of a transect performed over the sill between the basins (Figure 5). The deepest point of the watershed between the basins is 1050 m based on the GEBCO2021 database. The transect included five stations and was oriented across the deep overflow. Along-flow velocity, potential temperature, salinity, and potential density relative to the sea surface are shown in Figure 5. Velocities over entire water column along the section were quite low (less than 10–15 cm/s). TBW and TWW flows are located at some distance away from the transect between two basins; the Bransfield Current was observed north of our stations, closer to the South Shetland Islands, while the inflow of waters from the Weddell Sea is located closer to the Antarctic Peninsula. On the basis of the data of our section, the intensification of currents was observed in the bottom layers. Two separate high-velocity jets were observed at sta-

deepest point of the section. The coldest (minimal potential temperature was −1.31 °C)

**Figure 5.** Structure of the deep overflow between the CB and EB of the strait: distribution of the (**a**) northeastern velocity (true direction is 45°) based on LADCP data, (**b**) potential temperature, and (**c**) salinity based on CTD data. Contours of potential density anomalies are shown with dashed black lines. Positions of stations are indicated with vertical solid lines together with the numbers of **Figure 5.** Structure of the deep overflow between the CB and EB of the strait: distribution of the (**a**) northeastern velocity (true direction is 45◦ ) based on LADCP data, (**b**) potential temperature, and (**c**) salinity based on CTD data. Contours of potential density anomalies are shown with dashed black lines. Positions of stations are indicated with vertical solid lines together with the numbers of the stations.

The measurements in the region of the deep overflow between the CB and EB were performed twice with an interval of two weeks on 1 and 14 February 2022. The first survey included measurements at five stations across the bottom flow; the second survey The measurements in the region of the deep overflow between the CB and EB were performed twice with an interval of two weeks on 1 and 14 February 2022. The first survey included measurements at five stations across the bottom flow; the second survey consisted of four stations along the overflow. The station at the sill point between the basins was performed twice during both surveys. The direction of the deep overflow changed during these two weeks. The LADCP velocities measured in the bottom layer are shown in Figure 6. We present the maximal velocities in the 100 m bottom layer and averaged velocities in the bottom layers with thicknesses of 50 and 100 m. The magnitudes and directions of these velocity vectors were very close. For example, the maximal velocity of the overflow was 22 cm/s, while average velocities were 19 and 18 cm/s for the 50 and 100 m layers, respectively. As for the measurements at the same point performed two weeks later, the flow was directed in the opposite direction, from the EB to CB. The velocities of this opposite flow reached 18 cm/s (maximal value), 16 cm/s (averaged in the 50 m bottom layer), and 14 cm/s (averaged in the 100 m bottom layer). These changes in currents are shown in Figure 7 in more detail. Corresponding changes in the potential temperature and salinity

served.

between the measurements over the sill point between the basins were 0.47 ◦C (from −1.31 to −0.84 ◦C) and 0.02 psu (from 34.62 to 34.64 psu), respectively. A sharp thermocline was observed at depths of 880–920 m at station 7352; the vertical potential temperature gradient reached 0.01 ◦C/m (1 ◦C per 100 m depth). When the current changed direction in two weeks (station 7390), such a strong thermocline was not observed. thermocline was observed at depths of 880–920 m at station 7352; the vertical potential temperature gradient reached 0.01 °C/m (1 °C per 100 m depth). When the current changed direction in two weeks (station 7390), such a strong thermocline was not ob-

*Water* **2022**, *14*, x FOR PEER REVIEW 10 of 17

consisted of four stations along the overflow. The station at the sill point between the basins was performed twice during both surveys. The direction of the deep overflow changed during these two weeks. The LADCP velocities measured in the bottom layer are shown in Figure 6. We present the maximal velocities in the 100 m bottom layer and averaged velocities in the bottom layers with thicknesses of 50 and 100 m. The magnitudes and directions of these velocity vectors were very close. For example, the maximal velocity of the overflow was 22 cm/s, while average velocities were 19 and 18 cm/s for the 50 and 100 m layers, respectively. As for the measurements at the same point performed two weeks later, the flow was directed in the opposite direction, from the EB to CB. The velocities of this opposite flow reached 18 cm/s (maximal value), 16 cm/s (averaged in the 50 m bottom layer), and 14 cm/s (averaged in the 100 m bottom layer). These changes in currents

and salinity between the measurements over the sill point between the basins were 0.47 °C (from −1.31 to −0.84 °C) and 0.02 psu (from 34.62 to 34.64 psu), respectively. A sharp

**Figure 6.** Measured LADCP velocities in the bottom layer over the sill point between the central and eastern basins of the BS. (**a**,**b**) Maximal velocities averaged (**c**,**d**) in the 50 m bottom layer and ((**e**,**f**)) the 100 m bottom layer. (**a**,**c**,**e**) Data measured on 1 February; (**b**–**f**) measurements performed two weeks later (on 14 February). The magnitude of currents is shown in both the colors and lengths of the arrows; all color and vector scales are the same in all panels. The relief is based on the GEBCO2021 database.

GEBCO2021 database.

**Figure 6.** Measured LADCP velocities in the bottom layer over the sill point between the central and eastern basins of the BS. (**a**,**b**) Maximal velocities averaged (**c**,**d**) in the 50 m bottom layer and ((**e**,**f**)) the 100 m bottom layer. (**a**,**c**,**e**) Data measured on 1 February; (**b**–**f**) measurements performed two

**Figure 7.** (**a**) Potential temperature, (**b**) salinity, and (**c**) along-flow velocity profiles measured at the same location over a sill point between the central and eastern basins on 1 February 2022 (station 7352, orange line) and 14 February 2022 (station 7390, blue line). Horizontal dashed line at a depth of 870 m indicates the upper boundary of the bottom layer with high velocities. **Figure 7.** (**a**) Potential temperature, (**b**) salinity, and (**c**) along-flow velocity profiles measured at the same location over a sill point between the central and eastern basins on 1 February 2022 (station 7352, orange line) and 14 February 2022 (station 7390, blue line). Horizontal dashed line at a depth of 870 m indicates the upper boundary of the bottom layer with high velocities.

#### **4. Discussion**

**4. Discussion** 

Different properties of waters from the separate BS basins are caused by different pathways of propagation of Weddell Sea waters, which is the source of these waters. Differences in temperature and salinity define the density of deep waters in these basins, which in turn define the dynamics of deep layers within the strait. Temperature–salinity diagrams (Figure 8) were used for the analysis of thermohaline water structure at all studied sections. The densest waters were observed in the deepest part of the CB (Section 3, Figure 8c); the maximal recorded potential density anomaly at station 7299 was 27.89 kg/m3. Such dense waters were not observed at Section 1 (Figure 8a) between the WB and CB, confirming the fact that the deepest layers of the CB are filled by TWW waters from the east. The minimal potential temperature in the bottom layer of the CB was −1.65 °C, which is quite close to the freezing-point temperature (−1.90 °C for waters with 34.65 PSU salinity). This fact indicates the possible importance of the local ice formation effects during austral winter. It is quite possible that cooling and ice formation over the shelves of the BS (which are sufficiently wide in the southern part of the strait near the Antarctic Peninsula) can cause the formation of such dense waters within the CB. More winter measurements are needed for studies of such processes in the strait. Different properties of waters from the separate BS basins are caused by different pathways of propagation of Weddell Sea waters, which is the source of these waters. Differences in temperature and salinity define the density of deep waters in these basins, which in turn define the dynamics of deep layers within the strait. Temperature–salinity diagrams (Figure 8) were used for the analysis of thermohaline water structure at all studied sections. The densest waters were observed in the deepest part of the CB (Section 3, Figure 8c); the maximal recorded potential density anomaly at station 7299 was 27.89 kg/m<sup>3</sup> . Such dense waters were not observed at Section 1 (Figure 8a) between the WB and CB, confirming the fact that the deepest layers of the CB are filled by TWW waters from the east. The minimal potential temperature in the bottom layer of the CB was −1.65 ◦C, which is quite close to the freezing-point temperature (−1.90 ◦C for waters with 34.65 PSU salinity). This fact indicates the possible importance of the local ice formation effects during austral winter. It is quite possible that cooling and ice formation over the shelves of the BS (which are sufficiently wide in the southern part of the strait near the Antarctic Peninsula) can cause the formation of such dense waters within the CB. More winter measurements are needed for studies of such processes in the strait.

> For addressing the temporal variability of thermohaline properties of the deepest waters within the Bransfield Strait, we analyzed the modern World Ocean Database (WOD2018). Temperature–salinity diagrams allowed us to compare the maximal densities of deep waters in the CB (Figure 9a) and EB (Figure 9b).These data show that the deep waters in the CB are colder, saltier, and denser than waters in the EB. The minimal potential temperature in the CB usually reaches −1.75 ◦C, while waters in the EB are always warmer than −1.42 ◦C. Salinity in the deep layers of the CB significantly varies, but it is usually greater than that in the EB, which contributes to the observed density differences. The maximal density in the CB reaches 27.89 kg/m<sup>3</sup> , but it does not exceed 27.82 kg/m<sup>3</sup> in the

EB. Thus, the mean bottom flow through the studied sill should be directed from the CB to the EB. Continuous observations are required to evaluate the magnitude of the deep overflow, and the question regarding the source of deep water in the central basin requires further studies. *Water* **2022**, *14*, x FOR PEER REVIEW 12 of 17

**Figure 8.** Temperature–salinity diagrams including the freezing-point temperature (magenta line) and sigma contours (gray lines) at the hydrographic stations at different sections in the BS. From west to east: (**a**) Section 1, (**b**) Section 2, (**c**) Section 3, (**d**) section across the deep overflow. The locations of the sections are shown in Figures 1 and 2. The colors of the dots indicate depths of measurements; see color scale in (**c**). The freezing-point temperature was calculated on the basis of EOS80 equations [44]. **Figure 8.** Temperature–salinity diagrams including the freezing-point temperature (magenta line) and sigma contours (gray lines) at the hydrographic stations at different sections in the BS. From west to east: (**a**) Section 1, (**b**) Section 2, (**c**) Section 3, (**d**) section across the deep overflow. The locations of the sections are shown in Figures 1 and 2. The colors of the dots indicate depths of measurements; see color scale in (**c**). The freezing-point temperature was calculated on the basis of EOS80 equations [44].

For addressing the temporal variability of thermohaline properties of the deepest waters within the Bransfield Strait, we analyzed the modern World Ocean Database (WOD2018). Temperature–salinity diagrams allowed us to compare the maximal densities of deep waters in the CB (Figure 9a) and EB (Figure 9b).These data show that the deep waters in the CB are colder, saltier, and denser than waters in the EB. The minimal potential temperature in the CB usually reaches −1.75 °C, while waters in the EB are always warmer than −1.42 °C. Salinity in the deep layers of the CB significantly varies, but it is usually greater than that in the EB, which contributes to the observed density differences. The maximal density in the CB reaches 27.89 kg/m3, but it does not exceed 27.82 kg/m3 in the EB. Thus, the mean bottom flow through the studied sill should be directed from the CB to the EB. Continuous observations are required to evaluate the magnitude of the deep overflow, and the question regarding the source of deep water in the central basin requires further studies. CTD data from several profiles are presented in Figure 10 for a more detailed analysis of water transformation along the strait. Stations 7321, 7317, 7299, 7352, and 7387 were established at the deepest points along the thalweg of the BS. The coldest (−1.65 ◦C) and densest (27.89 kg/m<sup>3</sup> ) waters were located in the CB (station 7299). Despite the fact that the EB is deeper (2750 against 1960 m in the CB), the waters in the EB are not as cold and dense as those in the CB. This indicates that the bottom water in the EB has not been directly transported from the TWW flow from the Weddell Sea, but appeared there due to the overflow from the CB. During propagation, these waters mix with overlaying warmer and more saline waters leading to the observed decrease in density. Regarding the overflow between the basins (station 7352), the bottom waters there are colder than any waters at these depths in the CB or EB. In fact, a potential temperature isotherm of −1.30 ◦C was observed at a depth of 1105 m in the CB (station 7299) and at a depth of 1000 at the sill point between the basins (station 7352), indicating that deeper layers of the CB are involved in the studied overflow.

**Figure 9.** Temperature–salinity diagrams including the freezing-point temperature (magenta line) and sigma contours (gray lines) at the available hydrographic stations in the WOD2018 database for the (**a**) central and (**b**) eastern basins. Only full-depth profiles deeper than 1500 m were selected from the database. The colors of the dots indicate depths of measurements; see color scale in (**b**). The freezing-point temperature was calculated on the basis of EOS80 equations [44]. **Figure 9.** Temperature–salinity diagrams including the freezing-point temperature (magenta line) and sigma contours (gray lines) at the available hydrographic stations in the WOD2018 database for the (**a**) central and (**b**) eastern basins. Only full-depth profiles deeper than 1500 m were selected from the database. The colors of the dots indicate depths of measurements; see color scale in (**b**). The freezing-point temperature was calculated on the basis of EOS80 equations [44]. *Water* **2022**, *14*, x FOR PEER REVIEW 14 of 17

**Figure 10.** Potential temperature, salinity, and potential density along the thalweg of the BS: (**a**) location of stations relative to the basins of the strait, (**b**) temperature-salinity diagram, (**c**) profiles of potential temperature, (**d**) salinity, and (**e**) potential density. The colors of stations are the same in all panels. All profiles at depths of the overflow (850–1050 m) are shown in the insets in more detail. between the basins. **Figure 10.** Potential temperature, salinity, and potential density along the thalweg of the BS: (**a**) location of stations relative to the basins of the strait, (**b**) temperature-salinity diagram, (**c**) profiles of potential temperature, (**d**) salinity, and (**e**) potential density. The colors of stations are the same in all panels. All profiles at depths of the overflow (850–1050 m) are shown in the insets in more detail.

tidal motions are intensified over underwater ridges such as the watershed between the CB and EB. However, the calculations of tidal velocities at the exact time and positions of stations 7352 and 7390 based on the TPXO9.1 model show that variability cannot be completely explained by tidal motions. Thus, tidal velocities were 8.1 cm/s to the northeast (direction 48° relative to the north) at the time of station 7352, and 7.8 cm/s to the east (direction 88°) at the time of station 7390. The measured LADCP velocities are sufficiently higher. One of the other possible causes of such a variability are internal seiches. Some similar mechanisms were suggested for the variability in the overflow in the Denmark Strait [45]. In any case, further measurements of currents over this sill point are needed for better understanding the properties of the observed overflow, their temporal and spatial variability, and dynamic aspects that drive this intense current in this narrow gap

Temporal variability in the flow between the CB and EB needs to be studied in more

Temporal variability in the flow between the CB and EB needs to be studied in more detail. Our two stations with a time interval of two weeks showed significant variability of currents between the basins. One of the possible reasons of such variability is tides; tidal motions are intensified over underwater ridges such as the watershed between the CB and EB. However, the calculations of tidal velocities at the exact time and positions of stations 7352 and 7390 based on the TPXO9.1 model show that variability cannot be completely explained by tidal motions. Thus, tidal velocities were 8.1 cm/s to the northeast (direction 48◦ relative to the north) at the time of station 7352, and 7.8 cm/s to the east (direction 88◦ ) at the time of station 7390. The measured LADCP velocities are sufficiently higher. One of the other possible causes of such a variability are internal seiches. Some similar mechanisms were suggested for the variability in the overflow in the Denmark Strait [45]. In any case, further measurements of currents over this sill point are needed for better understanding the properties of the observed overflow, their temporal and spatial variability, and dynamic aspects that drive this intense current in this narrow gap between the basins.

### **5. Conclusions**

Thermohaline structure and water dynamics in the BS during austral summer of 2022 were analyzed on a series of CTD/LADCP sections across the strait. The analysis was focused on the water exchange between the CB and adjacent basins; this exchange significantly differed in the western and eastern margins of the CB. The main result for the western margin is that there is a significant flow of TWW waters along the Antarctic Peninsula that continues up to the western basin. It was previously suggested that all these waters recirculate within the CB as a part of cyclonic circulation in the strait. Our direct velocity measurements show that the TWW flow transports 0.77 Sv to the WB. At the same time, there was no significant overflow of deep and dense waters between the WB and CB due to the relatively shallow sill separating the basins. The situation over the watershed between the CB and EB was completely different. The underwater ridge there was much deeper and reaches a depth of more than 1000 m at the sill point. At that point, a strong intensification of deep currents was observed; the maximal bottom velocities reached 22 cm/s. This bottom flow is very variable and sometimes changes its direction. More direct velocity measurements are needed at this point for the detailed analysis of the bottom overflow and its temporal variability.

**Author Contributions:** Conceptualization, D.I.F. and E.G.M.; methodology, V.A.K. and D.A.S.; software, A.S.G. and I.D.D.; validation, O.A.Z.; data curation, V.A.K. and S.A.S.; writing—original draft preparation, D.I.F.; writing—review and editing, E.G.M., A.A.L., S.A.M., V.I.P. and P.A.S.; visualization, O.S.M., R.Z.M. and S.A.O.; supervision, E.G.M.; funding acquisition, E.G.M. and D.I.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the State Task of Russia FMWE-2022-0001 (ship operations and expenses), 0211-2019-0007 (field studies), and FNNN-2022-0001 (bio-optical measurements). The LADCP data processing and analysis of circulation in the strait were supported with Russian Science Foundation grant 22-77-10004. CTD data processing by DIF was supported by a grant of the President of the Russian Federation for the state support of young Russian scientists—candidates of science, research project MK-1492.2021.1.5.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** All experimental data, including CTD and LADCP profiles, used in the publication are available in open access through the Mendeley Data service (http://dx.doi.org/ 10.17632/69v8599btr.1). GEBCO2021 data are available at the official service https://www.gebco.net/ data\_and\_products/gridded\_bathymetry\_data/gebco\_2021/ (accessed on 4 October 2022).

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


## *Article* **Antarctic Bottom Water Jets Flowing from the Vema Channel**

**Eugene G. Morozov \* , Oleg A. Zuev , Dmitry I. Frey and Viktor A. Krechik**

Shirshov Institute of Oceanology, Nakhimovsky Prospect 36, 117997 Moscow, Russia

**\*** Correspondence: egmorozov@mail.ru

**Abstract:** Properties of the abyssal current of Antarctic Bottom Water (AABW) from the Vema Channel are studied based on temperature, salinity, and velocity profiler (CTD/LADCP) data. Previous studies over a period of almost 30 years revealed that very intense current of AABW exists in the Vema Channel. Later, it was found that this current consists of two branches. One branch spreads over the bottom of the channel; the other branch is elevated over the western wall of the channel. The deepest branch decays after it passes approximately 100 km while the upper one continues further to the North Atlantic and is the source of abyssal waters in the Canary and Cabo Verde basins of the North Atlantic. Data analysis suggested that the upper jet splits into two. One of these descends down a canyon at 24◦300 S, while the other (the third one) remains on the continental slope, and indications of its existence are also found at 24◦000 S. This research analyzes the existence and pathway of this third branch that can be traced up to latitude 24◦ S. Velocity measurements in 2022 allowed us to confirm the existence of this third branch.

**Keywords:** Antarctic Bottom Water; Vema Channel; CTD/LADCP measurements; three jets of bottom current

### **1. Introduction**

Antarctic Bottom Waters originate from the Weddell Sea and spread to the north in the abyssal depths of the Atlantic Ocean. The Vema Channel is a pathway for the bottom waters through the Rio Grande Rise and Santos Plateau before they spread to the Brazil Basin and further to the north. Weddell Sea Deep Water (the densest abyssal water) flows only through the deep Vema Channel, which is approximately 4600–4800-m deep. The channel is a narrow passage situated between two terraces that are located on both sides. The narrowest channel width is about 15 km, and the channel length is about 700 km (Figure 1) [1].

Oceanographic studies of the Vema Channel have been carried out since the 1970s [2]. The largest number of CTD measurements was made over the section along 31◦120 S and at a point on this section at 39◦18.30 W. Since 1972, 29 visits have been made to the region; our group joined these studies in 2002 [3]. After 2005, CTD casts were supplemented with LADCP velocity profilers. The current of bottom water from the Vema Channel and circulation in the southern part of the Brazil Basin were studied for the first time in [4]. These studies revealed a strong current of AABW and displacement of the coldest core to the eastern wall of the channel [5]. A warming trend of Antarctic Bottom Water has been found in the Vema Channel [6–8]. The long-term temperature trend revealed on the basis of these data is shown in Figure 2. It can be observed that the temperature increase continues: our measurements taken in 2020 and 2022 reveal a continuing gradual increase in the bottom potential temperature at this point. The temperature increase is caused by the warming of Weddell Sea Deep Water in the Weddell Sea. The signal from the Weddell Sea to the Vema Channel has been propagating for more than 35 years [9]. Hence, a signal from the Weddell Sea, caused by warming approximately 40 years ago, has now been recorded. There are insufficient data to explain the abrupt change in temperature in the early 1990s as this process is related to the warming in the Weddell Sea in the 1950s–1980s.

**Citation:** Morozov, E.G.; Zuev, O.A.; Frey, D.I.; Krechik, V.A. Antarctic Bottom Water Jets Flowing from the Vema Channel. *Water* **2022**, *14*, 3438. https://doi.org/10.3390/w14213438

Academic Editor: Changming Dong

Received: 23 August 2022 Accepted: 25 October 2022 Published: 28 October 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Figure 1. Bottom topography in the region of the Vema Channel (based on the GEBCO2019 data). Our stations in 2020 are indicated by red dots and those in 2022 are indicated by black dots. The inset shows stations in 2022 on a larger scale and stations in 2003 (dark yellow red dots on the inset, not shown on the main map) approximately along the same line crossing the outflowing currents from the Vema Channel. **Figure 1.** Bottom topography in the region of the Vema Channel (based on the GEBCO2019 data). Our stations in 2020 are indicated by red dots and those in 2022 are indicated by black dots. The inset shows stations in 2022 on a larger scale and stations in 2003 (dark yellow dots on the inset, not shown on the main map) approximately along the same line crossing the outflowing currents from the Vema Channel. Water 2022, 14, x FOR PEER REVIEW 3 of 12

Figure 2. Long-term trend of potential temperature in the bottom layer of the Vema Channel. Blue dots indicate measurements during the visits for distinguishing the warming of AABW in the coldest part of the jet at 31°12° S, 39°18.3 W. The fit curve indicates that the warming rate is 0.0019°C per year. Red color between two dashed lines shows the 95% confidence interval. **Figure 2.** Long-term trend of potential temperature in the bottom layer of the Vema Channel. Blue dots indicate measurements during the visits for distinguishing the warming of AABW in the coldest part of the jet at 31◦12◦ S, 39◦18.3 W. The fit curve indicates that the warming rate is 0.0019 ◦C per year. Red color between two dashed lines shows the 95% confidence interval.

The existence of two branches of Antarctic Bottom Water current in the Vema Channel was later reported in [10, 11, 12]. The AABW outflow from the Vema Channel was also studied based on a regional version of the ocean circulation model [13, 14]. However, studies of the fine structure of the outflow require direct observations, which are extremely rare in this region. The pathways of bottom water propagation into the The existence of two branches of Antarctic Bottom Water current in the Vema Channel was later reported in [10–12]. The AABW outflow from the Vema Channel was also studied based on a regional version of the ocean circulation model [13,14]. However, studies of the fine structure of the outflow require direct observations, which are extremely rare in this region. The pathways of bottom water propagation into the Brazil Basin north of the

Brazil Basin north of the Vema Channel have been studied based on the measurements of temperature, salinity, and velocity profilers (CTD/LADCP). It was suggested in [12]

ence of this shallower jet (4200–4600 m) based on velocity measurements and map its

A long-term series of observations have been performed since the 1970s [3]. These measurements allow us to study long-term trends in the properties of the abyssal flow. Two regions of the channel were studied during the WOCE experiment in the 1990s. It is the standard section along 31°12′ S from 39°18′ W to 39°27′ W and the region in the northern part of the channel [15]. In addition, WOCE section A17 occupied by French scientists in 1994 and our stations in 2003 can be related to historical stations in the region. Complete information about the oceanographic stations with tables of coordinates and time is reported in [3]. Since this research concerns the northern part of the region, we emphasize that we performed investigations in the northern region of the Vema Channel in 2003, 2009, 2010, 2012, 2013, and, recently, in 2018, 2019, 2020, and 2022.

In cruise 87 of the R/V Akademik Mstislav Keldysh (AMK87) in 2022, we occupied one station over the section along 31°12′ S, which has been repeatedly occupied in the coldest jet at (31°12′ S; 39°18.3′ W). We also occupied a section of 13 stations north of the Vema Channel, which was planned to intersect all outflow jets and determine the position of individual stream jets of Antarctic Bottom Water outflow from the Vema Channel at lat-

pathway to the north up to 24° S.

2. Data

Vema Channel have been studied based on the measurements of temperature, salinity, and velocity profilers (CTD/LADCP). It was suggested in [12] based on historical potential temperature measurements at the bottom that there is a third jet of the bottom water outflow. The objective of this work is to confirm the existence of this shallower jet (4200–4600 m) based on velocity measurements and map its pathway to the north up to 24◦ S.

#### **2. Data**

A long-term series of observations have been performed since the 1970s [3]. These measurements allow us to study long-term trends in the properties of the abyssal flow. Two regions of the channel were studied during the WOCE experiment in the 1990s. It is the standard section along 31◦120 S from 39◦180 W to 39◦270 W and the region in the northern part of the channel [15]. In addition, WOCE section A17 occupied by French scientists in 1994 and our stations in 2003 can be related to historical stations in the region. Complete information about the oceanographic stations with tables of coordinates and time is reported in [3]. Since this research concerns the northern part of the region, we emphasize that we performed investigations in the northern region of the Vema Channel in 2003, 2009, 2010, 2012, 2013, and, recently, in 2018, 2019, 2020, and 2022.

In cruise 87 of the R/V *Akademik Mstislav Keldysh* (AMK87) in 2022, we occupied one station over the section along 31◦120 S, which has been repeatedly occupied in the coldest jet at (31◦120 S; 39◦18.30 W). We also occupied a section of 13 stations north of the Vema Channel, which was planned to intersect all outflow jets and determine the position of individual stream jets of Antarctic Bottom Water outflow from the Vema Channel at latitudes 25.8–26.8◦ S. This section also made it possible to determine the flow velocities in the jets (Figures 1 and 3). The coordinates of stations in the region are given in Table 1. Water 2022, 14, x FOR PEER REVIEW 4 of 12 itudes 25.8–26.8°S. This section also made it possible to determine the flow velocities in the jets (Figures 1 and 3). The coordinates of stations in the region are given in Table 1.

Figure 3. Distributions of velocity normal to the sections at 26°25′ S, 34°50′ W (western) and 26°40′ S, 34°15′ W (eastern) (see red dots in Figure 4) in April 2020. Numbers of stations are shown along the top axis [12]. **Figure 3.** Distributions of velocity normal to the sections at 26◦250 S, 34◦500 W (western) and 26◦400 S, 34◦150 W (eastern) (see red dots in Figure 4) in April 2020. Numbers of stations are shown along the top axis [12].

Figure 4. Bottom topography and stations in the study site. Bathymetry is based on GEBCO 2019, supplemented with our echo sounder measurements from 2010 to 2020. Locations of CTD/LADCP stations are indicated with different symbols. The bottom potential temperatures are indicated near the stations. Blue triangles show stations of the WOCE A17 transect in 1994. Black crosses (+) show the top axis [12].

itudes 25.8–26.8°S. This section also made it possible to determine the flow velocities in the jets (Figures 1 and 3). The coordinates of stations in the region are given in Table 1.

Figure 3. Distributions of velocity normal to the sections at 26°25′ S, 34°50′ W (western) and 26°40′ S, 34°15′ W (eastern) (see red dots in Figure 4) in April 2020. Numbers of stations are shown along

Figure 4. Bottom topography and stations in the study site. Bathymetry is based on GEBCO 2019, supplemented with our echo sounder measurements from 2010 to 2020. Locations of CTD/LADCP stations are indicated with different symbols. The bottom potential temperatures are indicated near the stations. Blue triangles show stations of the WOCE A17 transect in 1994. Black crosses (+) show **Figure 4.** Bottom topography and stations in the study site. Bathymetry is based on GEBCO 2019, supplemented with our echo sounder measurements from 2010 to 2020. Locations of CTD/LADCP stations are indicated with different symbols. The bottom potential temperatures are indicated near the stations. Blue triangles show stations of the WOCE A17 transect in 1994. Black crosses (+) show stations in 2003. Sign (x) shows the stations in 2009. Green color shows stations and data of section A09 in 2009 and 2018 (24◦ S). Small red squares show stations in 2012 and 2013. Purple stars show stations in 2018. Circles with beams ( stations in 2003. Sign (x) shows the stations in 2009. Green color shows stations and data of section A09 in 2009 and 2018 (24° S). Small red squares show stations in 2012 and 2013. Purple stars show stations in 2018. Circles with beams ( ) show stations in 2019. Red circles show stations in 2020. Yellow arrows show directions of currents and their speed (vectors). Deep water areas are shown in blue, especially the canyon at 24°40′ S. Two thin magenta lines show the previously analyzed AABW flows from the deep bottom of the channel. The thick magenta line shows the flow of the branch that initially was found above the western wall. The brown line shows the possible pathway of the shallowest jet (~4600–4700 m), which is plotted using only the CTD data from 1994 and 2003 [updated from 12]. The existence of this flow was confirmed in 2022. ) show stations in 2019. Red circles show stations in 2020. Yellow arrows show directions of currents and their speed (vectors). Deep water areas are shown in blue, especially the canyon at 24◦400 S. Two thin magenta lines show the previously analyzed AABW flows from the deep bottom of the channel. The thick magenta line shows the flow of the branch that initially was found above the western wall. The brown line shows the possible pathway of the shallowest jet (~4600–4700 m), which is plotted using only the CTD data from 1994 and 2003 [updated from [12]]. The existence of this flow was confirmed in 2022.

Table 1. CTD/LADCP stations at the exit from the Vema Channel in 1994–2022. Stations Date Coordinates R/V Maurice Ewing (only CTD) 81 02.02.1994 27°21.1′ S, 36°36.7′ W 82 02.02.1994 26°58.6′ S, 36°16.1' W 83 02.02.1994 26°34.6' S, 35°55.7' W The Sea-Bird 19plus (Sea-Bird electronics, Bellevue WA, USA), Idronaut 320 plus (Idronaut S.R.L., Brugherio MS, Italy), and RDI Monitor 300 kHz (LADCP) (Teledyne RDI, San Diego, CA, USA) instruments were used for the measurements. The cold current jet was tracked based on the coldest bottom potential temperatures and velocity measurements using the LADCP profiler (Teledyne RDI, San Diego, CA, USA)at the stations where these data were available. It was also assumed that the current of bottom water should flow along the isobaths or go deeper.

84 02.02.1994 26°11.2' S, 35°34.2' W 85 03.02.1994 25°47.8' S, 35°14.0' W 86 03.02.1994 25°24.4' S, 34°53.4' W 87 03.02.1994 25°01.1' S, 34°32.7' W The bottom topography used in this paper is based on satellite altimetry (GEBCO 2019) to plot our topographic maps. Some data of this digital bathymetry were corrected based on the measurements along the routes of our ships using the data of our echo sounders(Kongsberg Maritime, Kongsberg Viken, Norway).

> 88 03.02.1994 24°37.6' S, 34°11.9' W 89 04.02.1994 24°14.2' S, 33°51.8' W 90 04.02.1994 23°50.8' S, 33°30.9' W 91 04.02.1994 23°27.5' S, 33°10.3' W 92 04.02.1994 23°04.0' S, 32°49.4' W

1441 02.11.2003 26°00.0′ S, 35°15.0′ W 1442 02.11.2003 26°14.9′ S, 34°53.0′ W 1443 02.11.2003 26°27.5′ S, 34°35.8′ W 1444 03.11.2003 26°40.9′ S, 34°15.4′ W 1445 03.11.2003 26°43.2′ S, 34°12.1′ W 1446 03.11.2003 26°44.4′ S, 34°10.8′ W 1454 09.11.2003 27°05.5′ S, 35°54.7′ W R/V Akademik Ioffe 2079 18.04.2009 26°42.9′ S, 34°12.2′ W 2080 18.04.2009 26°01.9′ S, 33°58.0′ W 2437 06.11.2010 26°39.9′ S, 34°16.8′ W 2438 06.11.2010 26°42.2′ S, 34°13.8′ W 2439 06.11.2010 26°43.4′ S, 34°12.1′ W R/V Akademik Sergey Vavilov 2494 05.11.2012 26°36.8′ S, 33°59.3′ W 2495 05.11.2012 26°35.9′ S, 34°10.2′ W 2496 06.11.2012 26°31.1′ S, 34°03.3′ W 2497 06.11.2012 26°35.8′ S, 33°51.7′ W 2521 18.10.2013 26°20.5′ S, 32°00.1′ W 2522 18.10.2013 26°27.7′ S, 32°53.1′ W 2523 18.10.2013 26°23.0′ S, 32°53.1′ W 2525 19.10.2013 26°18.6′ S, 33°11.5′ W 2526 20.10.2013 26°35.7′ S, 33°51.6′ W

R/V Akademik Sergey Vavilov (only CTD)


**Table 1.** CTD/LADCP stations at the exit from the Vema Channel in 1994–2022.


**Table 1.** *Cont.*

Our analysis is based on our CTD/LADCP measurements in 2003, 2004, 2009, 2010, 2012, 2013, 2018, 2019, 2020, and 2022. The casts reached depths approximately 5 m above the bottom. We also used the data from the WOCE A17 section (WHP, 2002) and the A09 section in 2009 and 2018 along 24◦ S (http://cchdo.ucsd.edu (accessed on 27 October 2022)). The data from several stations from the WOD18 database were also used. Velocity measurements were corrected by removing the currents of the barotropic tide using the TPXO 9 model [16].

#### **3. Results**

The CTD/LADCP measurements from the Vema Channel over sections in 2020 and earlier show the presence of the two cold AABW flows of Weddell Sea Deep Water (with potential temperatures below 0.2 ◦C). Both cold cores of the currents were observed in the sections at latitudes 26◦300–27◦000 S, measured in 2020 (Figure 3) [3,12]. Let us follow each jet and consider them separately. Each jet was identified based on the bottom topography (a channel deeper than the surroundings by 50–100 m), lower potential temperature, and velocities of the current.

The best-known AABW jet flows along the deep bed of the Vema Channel [5,17]. The deep core of the current is located between 4400 and 4700 m. Let us consider this deep outflowing current of AABW using the data gathered at our CTD/LADCP stations in 2003–2020. The stations were located in the region 26◦300–26◦500 S, 33◦300–34◦200 W (Figure 4). In 2003, the minimum measured potential temperature in this region of the northern part of the Vema Channel (Vema extension at 26◦43.2<sup>0</sup> S, 34◦12.1<sup>0</sup> W) was θ = −0.094 ◦C. In 2020, the potential temperature at the same point increased to θ = −0.067 ◦C. We found previously that the continuation of the coldest AABW flow splits into two jets [10,11]. These near-bottom jets rapidly warm and decay over a distance of about 100 km at latitude 25◦300 S (Figure 4).

Another AABW branch was detected over the northwestern slope of the Vema Channel. This flow is elevated by 600 m over the deepest branch of the AABW current [12]. The highest velocities in the core of this branch were recorded at depths of 4100–4200 m (150 m above the bottom). Such an isolated core of Weddell Sea Deep Water over the western terrace was repeatedly observed (in a section along 31◦120 S) from 1984 to 2020. The potential temperature of this branch has been increasing over time. The measurements of velocities within 30–36 cm/s using LADCP over the repeated section confirm the stable existence of this branch [3,18]. The continuation of this jet was found at the outflow of the Vema Channel in 2020 at 26◦250 S, 34◦420 W above a depth of 4420 m (Figure 3, western section). Then, this jet continues to the north, approaches the upper part of a zonal canyon at 24◦300 S (Figure 4), and flows down the canyon to depths deeper than 4800 m [12].

In 2009, scientists from Great Britain occupied a WOCE CTD section (A09) along 24◦ S. They repeated experiments on this section in 2018 (https://cchdo.ucsd.edu, accessed on 27 October 2022). In 2009, two cores of cold water were detected in this section: the bottom potential temperatures at longitude 31◦500 W (at depths of approximately 4650–4750 m) ranged from −0.04 ◦C to −0.05 ◦C, while at 33◦50<sup>0</sup> W (in the depth range of 5000–5200 m), potential temperatures ranged from −0.03 ◦C to −0.04 ◦C. These cores were slightly displaced in 2018. Potential temperatures in the cold cores in 2018 were close to −0.03 ◦C. Sections of potential temperature along 24◦ S in 2009 and 2018 are shown in Figure 5. No measurements of currents over this section are available.

Figure 5. Sections of potential temperatures deeper than 4000 m in 2009 (a) and 2018 (b) (WOCE line A09 along 24° S). The coldest bottom potential temperatures are indicated over the background of the bottom. **Figure 5.** Sections of potential temperatures deeper than 4000 m in 2009 (**A**) and 2018 (**B**) (WOCE line A09 along 24◦ S). The coldest bottom potential temperatures are indicated over the background of the bottom.

In Figure 5, two cores of cold water are observed at depths of 4600–4800 m and 5100 m. Previously, it was found that two jets of bottom current became warmer and decayed at 25°30′ S at depths of ~4700 m [12]. Thus, they could not transport very cold water to latitude 24° S. The jet that descended the canyon could have been recorded at 24° S at a depth of 5100 m. However, a cold core at 33°–34° W at depths of 4700 m could appear only if this water has been transported by another jet. Finding this jet is the goal of this research. Below, the existence of this jet is considered in more detail. The potential temperature of cold water detected at 24°00′ S, 33°42′ W in 2009 at depths of ~4600 m was In Figure 5, two cores of cold water are observed at depths of 4600–4800 m and 5100 m. Previously, it was found that two jets of bottom current became warmer and decayed at 25◦300 S at depths of ~4700 m [12]. Thus, they could not transport very cold water to latitude 24◦ S. The jet that descended the canyon could have been recorded at 24◦ S at a depth of 5100 m. However, a cold core at 33◦–34◦ W at depths of 4700 m could appear only if this water has been transported by another jet. Finding this jet is the goal of this research. Below, the existence of this jet is considered in more detail. The potential temperature of cold water detected at 24◦00<sup>0</sup> S, 33◦42<sup>0</sup> W in 2009 at depths of ~4600 m was −0.049 ◦C. By 2018, it had become warmer (−0.029 ◦C).

−0.049°C. By 2018, it had become warmer (−0.029°C). There is no other source of such cold water than the upper branch of the current from the Vema Channel. The section at 24° S that was occupied in 2009 and 2018 is located approximately 300 km north of the Vema Channel. The continental slope of South America here is not at all steep: the slope is estimated as 1/(100–120). A moving flow along the isobaths can keep its path above such a slope due to the balance of the gravity force directed down the slope and the Coriolis force that is directed to the left of the flow upslope. Small channels on the continental slope with a depth of about 50–100 m over a section at 24° S were found exactly coinciding with the cold potential temperatures above the bottom. Thus, these flows could have eroded them in the sediments. In our expeditions after 2018, we tried to find the pathway of the continuation of the upper jet north of the Vema Channel. The criterion was the existence of low bottom potential temperatures There is no other source of such cold water than the upper branch of the current from the Vema Channel. The section at 24◦ S that was occupied in 2009 and 2018 is located approximately 300 km north of the Vema Channel. The continental slope of South America here is not at all steep: the slope is estimated as 1/(100–120). A moving flow along the isobaths can keep its path above such a slope due to the balance of the gravity force directed down the slope and the Coriolis force that is directed to the left of the flow upslope. Small channels on the continental slope with a depth of about 50–100 m over a section at 24◦ S were found exactly coinciding with the cold potential temperatures above the bottom. Thus, these flows could have eroded them in the sediments. In our expeditions after 2018, we tried to find the pathway of the continuation of the upper jet north of the Vema Channel. The criterion was the existence of low bottom potential temperatures and a general northward direction of velocity, with possible deviations along the isobaths to the east.

and a general northward direction of velocity, with possible deviations along the isobaths

to the east.

The upper branch of the AABW current was traced on the basis of low potential temperatures and the northern-northeastern direction of velocity vectors at the bottom. These vectors are shown by the yellow arrows in Figure 4. In 2020, we found that this bottom branch descends into an underwater abyssal canyon (Figure 4), which is at approximately 24◦250–24◦400 S [12]. At longitude 33◦100 W, this canyon becomes wider. One can see from the historical bottom temperature distribution that this canyon diverts part of this cold AABW jet downslope. The branch that descends the canyon was detected in 2020 based on low potential temperatures and vectors of velocities at the bottom at five locations between 26◦250 S and 24◦550 S. This branch forms the cold core at a depth of 5100 m at 24◦ S.

At latitude 24◦ S, one can see another cold core at depths of 4600 at longitudes 33◦–34◦ W. This core could have been formed by a jet located shallower than the one that descended the canyon. Therefore, there is a third jet at depths of ~4600 m that has been detected from low potential temperatures over the section. This branch has been traced from low temperatures based on CTD casts in 1994 and 2003; however, no LADCP measurements were made in 1994 or 2003. In 1994–2003, the bottom potential temperatures were much lower than now. This jet is drawn in Figure 4 with a brown line connecting the points, with cold potential temperatures at the bottom. When plotting this line, we tried to connect measurement points with low temperatures. In addition, the pathway was drawn so that over steep slopes the flow was descending more rapidly than over flatter slopes. The potential temperature of the bottom flow has been strongly increasing since 1994. The very cold temperatures that were measured in 1994 and 2003 have not since been recorded near the bottom. Thus, we detected a third branch of AABW flow directed to the north, which is shown in Figure 5 as a cold core at 24◦ S at depths of 4600 m. The potential temperature of this upper branch at a depth of 4600 m is below zero at 24◦ S. Based on the measurements in the databases north of 24◦ S, this branch spreads further to the north. The continuation of this jet north of 24◦ S is presumably the flow of AABW described in [19]. Thus, on the basis of our targeted measurements and historical data, we have constructed a scheme of pathways for the continuation of AABW flow (Figure 4).

To detect this jet at the outflow from the Vema Channel, we constructed a section in 2022 to cross all possible flows from the Vema Channel. Before measurements were taken in 2022, a CTD section (without LADCP) was constructed in 2003 of only five stations (1441–1446) (Figures 6 and 7) approximately along the same line (inset in Figure 1). Section 2003 does not allow us to resolve three jets of currents because of a small number of stations and lack of LADCP measurements. Because of the general warming of abyssal waters in the Atlantic, potential temperatures increased significantly [8].

The section of 13 stations occupied in 2022 revealed three branches of AABW current from the Vema Channel, which were presumably detected from previous measurements. These jets are observed both on the temperature section and on the sections of the current components along the meridian and normal to the section. Jets of currents on a gentle slope are observed at longitudes of 35.1◦ W and 34.7◦ W.

Three different components of currents across the section north of the Vema Channel are shown in Figure 8. The currents are generally directed to the east-northeast. High current velocities are observed at the bottom in the channel bed closer to the western wall: up to 21 cm/s with an eastern-northeastern direction at station 7415. At the neighboring station 7417, the current accelerates to 28 cm/s and acquires a pronounced northern direction near the bottom. A fairly wide flow with velocities exceeding 10 cm/s and a predominant direction to the east-northeast was noted on a gentle slope. The maximum velocities (20 cm/s) near the bottom were simultaneously recorded at station 7424 (35◦060 W) in the same place where the third jet with the minimum potential temperature was observed. Even higher current velocities, up to 29 cm/s, were noted in the depth range of 3800–3900 m at stations 7420, 7424, and 7426. Our previous numerous LADCP casts that returned profiles of bottom currents in the Vema Channel suggest that the maximum velocities were found not at the very bottom but 100–150 m above it, while the water with the coldest potential temperatures was detected exactly at the bottom.

Water 2022, 14, x FOR PEER REVIEW 10 of 12

Figure 6. Potential temperature section north of the Vema Channel in 2003. Contour lines of density (~46 units) are shown. **Figure 6.** Potential temperature section north of the Vema Channel in 2003. Contour lines of density (~46 units) are shown. components along the meridian and normal to the section. Jets of currents on a gentle slope are observed at longitudes of 35.1° W and 34.7° W.

Figure 7. Potential temperature section of AABW outflow from the Vema Channel in 2022. The location of the section was slightly different from its position in 2003 (Figure 1). Contour lines of density (~46 units) are shown. **Figure 7.** Potential temperature section of AABW outflow from the Vema Channel in 2022. The location of the section was slightly different from its position in 2003 (Figure 1). Contour lines of density (~46 units) are shown.

Figure 7. Potential temperature section of AABW outflow from the Vema Channel in 2022. The location of the section was slightly different from its position in 2003 (Figure 1). Contour lines of

Three different components of currents across the section north of the Vema Channel are shown in Figure 8. The currents are generally directed to the east-northeast. High current velocities are observed at the bottom in the channel bed closer to the western wall: up to 21 cm/s with an eastern-northeastern direction at station 7415. At the neighboring station 7417, the current accelerates to 28 cm/s and acquires a pronounced northern direction near the bottom. A fairly wide flow with velocities exceeding 10 cm/s and a

Three different components of currents across the section north of the Vema Channel are shown in Figure 8. The currents are generally directed to the east-northeast. High current velocities are observed at the bottom in the channel bed closer to the western wall: up to 21 cm/s with an eastern-northeastern direction at station 7415. At the neighboring station 7417, the current accelerates to 28 cm/s and acquires a pronounced north-

density (~46 units) are shown.

Figure 8. Sections of the velocity components at the outflow of the AABW from the Vema Channel in 2022. Top panel (A): positive direction to the east; middle panel (B): positive direction to the north; bottom panel (C): positive direction to the northeast, normal to the section. **Figure 8.** Sections of the velocity components at the outflow of the AABW from the Vema Channel in 2022. Top panel (**A**): positive direction to the east; middle panel (**B**): positive direction to the north; bottom panel (**C**): positive direction to the northeast, normal to the section.

predominant direction to the east-northeast was noted on a gentle slope. The maximum velocities (20 cm/s) near the bottom were simultaneously recorded at station 7424 (35°06′ W) in the same place where the third jet with the minimum potential temperature was observed. Even higher current velocities, up to 29 cm/s, were noted in the depth range of 3800–3900 m at stations 7420, 7424, and 7426. Our previous numerous LADCP casts that returned profiles of bottom currents in the Vema Channel suggest that the maximum velocities were found not at the very bottom but 100–150 m above it, while the water with

the coldest potential temperatures was detected exactly at the bottom.

Thus, our analysis reveals the existence of three jets of bottom flow that continue the spreading of AABW in the Vema Channel. Previously, the existence of a third jet before measurements were taken in 2022 could only be judged from CTD data. Presumably, the upper jet, which initially flows above the western wall of the Vema Channel, splits into two branches of continuation, and both of them spread further north than the lower jet Thus, our analysis reveals the existence of three jets of bottom flow that continue the spreading of AABW in the Vema Channel. Previously, the existence of a third jet before measurements were taken in 2022 could only be judged from CTD data. Presumably, the upper jet, which initially flows above the western wall of the Vema Channel, splits into two branches of continuation, and both of them spread further north than the lower jet from the deep bottom of the Vema Channel. Therefore, the upper jet is the current that transports AABW further north and is the source of abyssal bottom water in the deep basins in the North Atlantic. The shallower part of this upper flow (the third jet in our context) transports cold water to 24◦ S and is observed as the cold core at depths of 4600 m. The deeper part of the upper jet descends the canyon and fills the deepest abyssal regions of the Atlantic.

**Author Contributions:** E.G.M.: conceptualization and original draft preparation; D.I.F., V.A.K. and O.A.Z.: field data, original draft preparation, writing, and figures. All authors have read and agreed to the published version of the manuscript.

**Funding:** The work was supported by the Russian Science Foundation grant 21-77-20004.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** No studies involving humans and animals were performed.

**Data Availability Statement:** Data are available upon request.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

