**3. Results**

Daily data on sea surface height and three-dimensional fields of seawater temperature, salinity, and current velocity for the year 2016 were obtained in the two numerical experiments described above. The next stages of the study compare the simulation results with observational data (validation) and analyze deepwater circulation with a focus on the continental slope region, where the most interesting features of the currents are observed. In the northeastern part of the sea, so-called undercurrents (opposite to the Black Sea surface basin-scale cyclonic gyre—the Rim Current [1–3]) are detected at a depth of 200 m for the period 9 June 2016–14 October 2016, according to ARGO data.

Validation of the model fields was performed based on temperature and salinity measurement data obtained by ARGO profiling floats [13] and R/V «Professor Vodyanitsky» Cruises 87, 89, and 91 [14] in 2016. Our validation methodology is described in [8] (Section 2.2). Root mean square errors RMSE between the model and in situ data for both experiments are presented in Table 1. The temperature RMSE in the upper layer (0–300 m) decrease in the ERA5-experiment compared to the SKIRON-experiment. The highest decrease in temperature error was observed at a depth of 0–30 m. The model salinity in the ERA5-experiment correlated more at a depth of 30–300 m. Therefore, the permanent pycnocline and seasonal thermocline layers in the ERA5-experiment are closer to the measurement data.

**Table 1.** The temperature and salinity RMSE between simulations and in situ.


A difference between the simulation results was primarily found in the velocity fields due to the strong influence of the wind on Black Sea dynamics [1,2]. The increasing wind velocity in ERA5 (Figure 1b) led to a more typical structure of the Rim Current at the end of 2016, when the basin-scale cyclonic gyre was propagated above the continental slope (Figure 2b). The Rim Current was not regenerated in winter, and mesoscale eddies were developed in the central sea part in the SKIRON-experiment (Figure 2a) due to insufficient kinetic energy inflow from the wind [8].

The model circulation in the upper layer was generally cyclonic for both experiments. At the same time, the most significant difference of the current velocity fields was detected below the permanent pycnocline core. Thus, at deepwater horizons, in the ERA5-experiment (Figure 2d), the current field was more intense, and maximal velocity was higher than in the SKIRON one (Figure 2c).

Analysis of ARGO float ID6901833 trajectory data [13] revealed a change in the direction of the alongshore subpycnocline current from the northwestern (cyclonic) to the northeastern (anticyclonic) near the northeastern continental slope. Thus, from 6 September to 14 October, 2016 the float drifted anticyclonicaly at its parking depth of 200 m (Figure 3a, red arrows). Such behavior of the alongshore current was not modeled in the SKIRONexperiment (Figure 3b), but was clearly reconstructed in the ERA5-experiment (Figure 3c). Averaged over the period of anticyclonical movement of the float, the model velocity of the undercurrent riches 0.03–0.05 m/s with instant value up to 0.08 m/s. The undercurrent generation near the Black Sea continental slope is probably associated with the intense mesoscale variability under the permanent pycnocline in the ERA5-experiment (Figure 3c). Here, some eddies were observed along the continental slope. The undercurrents that form

near the northeastern slope of the Black Sea seem to be of an anticyclonic nature, similar to the undercurrents formed by anticyclones in the western part of the Bay of Bengal [15].

**Figure 2.** Monthly mean current fields in December 2016 at a depth of 50 m (**<sup>a</sup>**,**b**) and at a depth of 500 m (**<sup>c</sup>**,**d**) according to the SKIRON-experiment (**<sup>a</sup>**,**<sup>c</sup>**) and the ERA5-experiment (**b**,**d**).

**Figure 3.** (**a**) ARGO float ID6901833 trajectory at parking depth of 200 m. Model current velocity at 200 m time-averaged for 6 September–14 October 2016 by the SKIRON-experiment (**b**) and the ERA5- experiment (**c**). Blue arrows illustrate the northwestern alongshore current, red arrows correspond to the southeastern current (undercurrent).

The structure of the circulation is inextricably linked with the spatiotemporal variability of seawater thermohaline characteristics [3,7]. The model temperature and salinity fields on the zonal cross-section along 44◦N averaged over the period of existing undercurrent are shown in Figure 4. As seen in temperature fields (Figure 4a,b), the upper mixed layer reached a depth of 20–25 m in both experiments, but in the ERA5-experiment its thickness was larger near the eastern coast (up to 25–30 m), and its temperature was higher here as well. The mesoscale anticyclones shown in Figure 3c led to the deepening of isotherms and isohalines near the eastern coast and the formation of an undercurrent along the slope.

There is a downward deflection of the isotherms at zone of 38.3–39.0◦ N in Figure 4b that corresponds to the anticyclonic current. A similar deflection is also visible in the salinity field (Figure 4d). Thus, the distribution of temperature and salinity in the ERA5-experiment is consistent with the anticyclonic current near the continental slope detected in the ARGO float ID6901833 data [13].

**Figure 4.** Zonal cross-section along 44◦ N of the model temperature (**<sup>a</sup>**,**b**) and salinity (**<sup>c</sup>**,**d**) fields timeaveraged for 6 September–14 October 2016 by the SKIRON-experiment (**<sup>a</sup>**,**<sup>c</sup>**) and the ERA5-experiment (**b**,**d**).
