Variations in Temperature and Salinity at Different Survey Periods in the Central and Eastern Beibu Gulf and Their Relationship with Circulation Patterns

Abstract

Utilizing data from the four survey voyages in the Beibu Gulf from 2021 to 2022, this study investigates the variations in water temperature and salinity at different survey periods across its central and eastern regions, analyzing their horizontal and vertical distribution patterns. The research further explores the relationship between these distributions and the gulf’s circulation dynamics, elucidating their underlying formation mechanisms. The results indicate that the variations of water temperature and salinity are obviously different in different periods. Horizontally, spring and summer survey voyages exhibit a transition from high-temperature (max 31.90 °C), low-salinity (min 29.54 PSU) waters in the north to low-temperature (min 21.10 °C), high-salinity (max 34.19 PSU) waters in the south, while autumn and winter survey voyages show low-temperature (min 18.57 °C), low-salinity (min 29.54 PSU) waters in the north shifting to medium-to-low-temperature (21.52–28.20 °C), high-salinity (max 34.19 PSU) waters in the south. Vertically, the spring survey period displays distinct stratification with multiple thermoclines and haloclines in different regions. The summer survey period exhibits the strongest thermohaline gradients, with intensified thermoclines and haloclines accompanied by regional up- and downwelling. In the autumn survey period, vertical mixing homogenizes temperature and salinity, while the winter survey period shows near-complete mixing, effectively eliminating stratification. These patterns are driven by a combination of oceanic dynamics and regional climatic factors, including monsoon forcing, solar radiation, coastal runoff, horizontal circulation, and up-/downwelling processes.

1. Introduction

The Beibu Gulf, situated in the northwestern South China Sea, is a semi-enclosed gulf bordered by China’s Leizhou Peninsula and Hainan Island to the east and Vietnam to the west. Covering approximately 128,000 km² with an average depth of 46 m and a maximum depth of 100 m [1,2], it connects to the South China Sea via the Qiongzhou Strait and its southern mouth. The Beibu Gulf is dominated by northeast winds in winter and southwest winds in summer. It is often hit by typhoons, mostly occurring from June to October [3]. These winds have a significant impact on evaporation rates and surface currents within the gulf [4]. The annual precipitation mainly occurs in summer [5]. River discharges contribute significantly to the hydrology of the gulf, with coastal runoff transporting fresh water and nutrients, affecting local marine ecosystems [6]. The gulf’s complex hydrometeorological conditions, driven by monsoons, typhoons, runoff, westward flow through the Qiongzhou Strait, and South China Sea water intrusion, result in intricate circulation patterns and distinct water mass distributions.

Field surveys are important research methods in obtaining measured data. In 1962, China and Vietnam jointly carried out a comprehensive marine survey of the Beibu Gulf, filling the historical gap in marine scientific research of the gulf. Subsequently, various surveys and studies on this gulf have been conducted for many years, including the comprehensive coastal zone survey during the 1980s–1990s, the comprehensive offshore environment survey during 2005–2011, the phaeocystis survey in the northern waters of the gulf during 2016–2017 [7], the large-area observation of the eastern waters of the gulf during 2018–2019 [8], and the recent Sino-Vietnamese cooperation project “Demonstration Study on Ecological Protection and Management in Typical Bays: Seasonal Survey of Beibu Gulf” during 2021–2022. Most of the coverage of these field surveys is limited to the eastern waters of the Beibu Gulf. Based on the field survey data, combined with satellite remote sensing data and various numerical simulation techniques, scholars have carried out research on the complex and changeable hydrological environment of the gulf, focusing on the distribution of and variation in thermohaline water masses in different seasons in the gulf [7,8,9,10], as well as its circulation structure and characteristics [11,12,13,14,15,16,17,18,19,20,21,22,23].

Cao et al. [7] analyzed the distribution and seasonal variation in water masses in the study area using the K-mean dynamic cluster analysis method, based on hydrological data such as the temperature, salinity, and density of nine voyages of the Brown cystis survey in the Beibu Gulf from September 2016 to August 2017. According to the thermohaline data, the seawater types in the study area are divided into low-temperature and high-salinity shelf bottom water masses, shelf-surface-mixed water masses, and low-salinity coastal water masses, with thermohaline distribution characteristics revealing the existence of an overall counterclockwise circulation on the eastern slope of the Beibu Gulf. Li et al. [8] analyzed the characteristics of water masses and flow fields in the northeast of the gulf based on the large-area observation data east of 108° E and north of 18° N in the gulf from July 2018 to February 2019. The results show clear seasonal variations in water characteristics, with the flow field generally exhibiting a counterclockwise circulation structure and seasonal variations in flow velocity. Based on the summer voyage data of a Sino-Vietnamese joint survey of the Beibu Gulf in 2022, Zeng et al. [9] analyzed the horizontal and vertical distribution characteristics of temperature and salinity in the central and eastern waters of the gulf, further interpreted the internal laws forming the distribution characteristics combined with the calculation results of numerical models, and expounded the circulation characteristics of the gulf, indicating that the Qiongzhou Strait water flux is a key factor influencing the circulation pattern of the gulf.

In the research on the circulation of the gulf, remarkable progress has been made over the years with the continuous enrichment of survey data and the development and improvement of numerical simulation technology. Xu et al. [12] analyzed dynamic height data from more than 6000 stations spanning 50 years (1921–1970), showing that the Beibu Gulf exhibits an annual cyclonic circulation pattern, though the southern circulation presents an anticyclonic structure under the influence of strong southwest monsoons in summer. Zhuang et al. [13] analyzed historical data and proposed a dominant cyclonic circulation in winter, forming a smaller counterclockwise eddy at 19° N. The research of Xia et al. and other scholars supports this cyclonic structure [14,15,16,17], especially in winter, while acknowledging more complex circulation patterns in summer. Various explanations exist for the summer circulation in the Beibu Gulf, with some studies believing that there is a double vortex structure, supported by research from Yang et al. [18] and Bao et al. [19]. Su et al. [20] argued that the gulf is dominated by cyclonic eddies, with one cyclonic vortex in autumn and winter and two cyclonic vortices in spring and summer. Using current measured data combined with satellite remote sensing and HYCOM reanalysis data, Chen et al. [21] analyzed the seasonal variation in the circulation in the northern part of the gulf, showing an overall counterclockwise circulation in winter, though a clockwise circulation appears in offshore waters from the east of Fangchenggang City to Beihai City. Gao et al. [22] demonstrated through POM model simulations that the northeastern part of the Beibu Gulf circulation in the winter semi-annual is jointly influenced by local wind fields and westward flow through the Qiongzhou Strait. The southern gulf was occupied by an anticyclonic eddy, whereas the northern gulf was dominated by a cyclonic gyre [6]. Chen et al. [23] reached similar conclusions using the FVCOM model, calculating multi-year monthly averaged, multi-layer circulations and presenting interannual and monthly variations in the intensity and extent of these opposing vortices. In addition, the relationship between the water flux of the Qiongzhou Strait and the circulation of the gulf is discussed, and it is pointed out that the water flux of the Qiongzhou Strait significantly influences the circulation structure of the gulf. Thus, most studies agree on cyclonic winter circulation, but summer circulation remains debated, with different interpretations such as cyclonic vortices, double-vortex structures, and local anticyclonic vortices. It can be seen that the circulation of the gulf is affected by multiple factors such as water and wind in the South China Sea as well as water in the Qiongzhou Strait and presents complex and changeable characteristics.

In previous studies, research work was mostly limited to analyzing changes in temperature and salinity and less established the correlation analysis with circulation. In addition, more studies mainly focused on the study of circulation characteristics, or the research was limited to variable analysis in a single voyage, as in reference [9], or the study area was smaller, as in reference [7], which was limited to the northeastern region of the Beibu Gulf. This study used more voyages and a larger range of field survey data as data sources and attempts to link variations of water temperature and salinity with circulation patterns. In this study, we aimed to analyze the variation characteristics of water temperature and salinity in different periods in the central–eastern waters of the gulf, expound the variation laws of water temperature and salinity in each period, and, combined with the circulation characteristics of the gulf, explore the relationship between water temperature and salinity distribution and circulation as well as the formation mechanism. The structure of this paper is as follows: Section 2 analyzes the distribution characteristics of water temperature and salinity based on survey data; Section 3 discusses the relationship between water temperature, salinity, and circulation; Section 4 explores the formation mechanism of the distribution characteristics of temperature and salinity and discusses the impact of circulation in the Beibu Gulf on the ecosystem; and Section 5 gives the conclusions.

2. Analysis of Water Temperature and Salinity Distribution Characteristics

2.1. Data

Data for this study were obtained from the 2021–2022 four-voyage surveys of the Beibu Gulf, conducted under the Asian Cooperation Special Fund Project “Demonstration Study on Ecological Protection and Management of Typical Bays”. The four survey periods were located in four seasons, respectively. The survey area spanned approximately from 107 to 111° E and from 18.5 to 21.5° N, comprising seven east–west transects with 6–10 evenly distributed stations, totaling 51 stations (Figure 1). Survey dates are listed in

Table 1. Field survey date information.

Survey Voyage	Survey Date
1. Voyage I (in spring)	20 May 2021 to 26 May 2021
2. Voyage II (in winter)	26 December 2021 to 4 January 2022
3. Voyage III (in summer)	8 August 2022 to 15 August 2022
4. Voyage IV (in autumn)	24 November 2022 to 29 November 2022

. The measuring equipment utilized was the SBE911plus CTD, manufactured by Seabird (Bellevue, WA, USA). This instrument is equipped with dual temperature and conductivity sensors, featuring a temperature accuracy of 0.001 °C and a conductivity accuracy of 0.0003 S/m. Instrument calibration includes methods such as pressure correction, heat flux correction, and hysteresis calibration. For poor data processing, it is necessary to exclude the data when the instrument is in the air and manually correct the data based on the vertical distribution map of each physical quantity. On vertical binning, average by depth of 1 m.

2.2. Statistical Characteristics of Water Temperature and Salinity

Statistical analysis of water temperature and salinity data from four voyages, stratified by integer-meter depth intervals, is presented in

Table 2. Characteristic value statistics of water temperature and salinity (units: water depth: m; temperature: °C; salinity: PSU).

Item		Depth	Temperature	Salinity
count		7949	7949	7949
mean	spring	/	28.15	33.11
	winter	/	21.94	33.40
	summer	/	28.56	32.80
	autumn	/	26.23	32.93
	total	24.43	26.25	33.06
std		17.06	3.20	0.71
min		1.00	18.57	29.54
33%		14.00	25.15	32.96
50%		21.00	26.38	33.25
67%		31.00	28.54	33.47
max		100.00	31.90	34.19

. These data show a water temperature range of 18.57 to 31.90 °C (difference: 13.33 °C) and a salinity range of 29.54 to 34.19 (difference: 4.65) for all four voyages, with standard deviations of 3.20 for temperature and 0.71 for salinity. These findings indicate that water temperature exhibits a greater variability than salinity in the survey periods. The mean water temperature was highest at 28.56 in summer and lowest at 21.94 in winter, while the mean salinity was lowest at 32.80 in summer and highest at 21.94 in winter.

To quantitatively analyze the distribution patterns of high and low water temperature and salinity, the values of water temperature and salinity were sorted from low to high, as the 1/3 and 2/3 quantile values were taken as boundaries to define low temperature and low salinity, medium temperature and medium salinity, and high temperature and high salinity, respectively. The statistical results show that the 1/3 and 2/3 quantile values of water temperature are 25.15 and 28.54 °C, while those of water salinity are 32.96 and 33.47.

The water temperature and salinity values of the four-voyage survey were plotted on temperature-salinity (T-S) scatter diagrams, as shown in Figure 2. It can be seen from this figure that in the spring voyage, the salinity of the S1 to S7 section from north to south varies spatially from fast to stable and finally stabilizes between 33.0 and 33.5, while temperature is mainly distributed between 28 and 32 °C before dropping sharply to 22 °C, forming an inverted L-shape in the T-S plot. Summer voyage exhibits similar patterns but with more obvious cross-sectional variations and a tighter clustering of data points. The salinity increases rapidly from 30 to 33 and then gradually to 34, whereas the temperature remains stable at 30–32 °C before dropping abruptly to 22 °C. Both spring and summer voyages show a general transition from high-temperature, low-salinity waters in the north to low-temperature, high-salinity waters in the south.

The temperature and salinity distribution patterns in winter and autumn voyages differ entirely from those in spring and summer voyages. Along the S1 to S7 section from north to south in the winter voyage, salinity uniformly increases from 32.5 to 33.8, while temperature increases uniformly from 19 to 24 °C, with distinct cross-sectional variations in the temperature and salinity of water. There is a positive correlation trend on the T-S plot. The autumn voyage shows patterns similar to those of the winter voyage, but with larger fluctuation ranges of the temperature and salinity of water, as salinity gradually increases from 31 to 33.5, and temperature increases gradually from 25 to 28 °C. In winter and autumn voyages, there is a general change from low-temperature, low-salinity waters in the north to medium- and low-temperature, high-salinity waters in the south.

To quantify seasonal variations in water mass properties, we performed k-means clustering on temperature and salinity data. K-means clustering was conducted with k = 4, assuming alignment with the four seasons. Cluster quality was evaluated using the silhouette score, and seasonal correspondence was assessed via cross-tabulation.

Table 3. Cross-tabulation of cluster versus season.

Cluster	Autumn (%)	Spring (%)	Summer (%)	Winter (%)
0	49.32	19.65	31.03	0.00
1	0.00	12.75	10.26	76.99
2	35.09	39.69	25.22	0.00
3	7.02	28.20	64.79	0.00

shows the results of cross-tabulation of clusters versus seasons. It can be seen that cluster 1 was predominantly winter (76.99%), cluster 3 was dominated by summer (64.79%), cluster 0 included 49.32% autumn and 31.03% summer, and cluster 2 was distributed with 39.69% spring, 35.09% autumn, and 25.22% summer. K-means clustering applied to the temperature–salinity dataset yields a silhouette score of 0.4652, indicating moderate cluster separation, thus displaying moderate seasonal differences. This suggests that while the clusters are distinguishable, there is some overlap or variability that prevents perfect seasonal grouping. This is related to the time period selected for the survey and also to the length of time. More representative time periods and longer survey periods will show more significant seasonal differences. Nevertheless, it is still necessary to analyze the data of the four voyage surveys to obtain the variations in water temperature and salinity during different survey periods.

2.3. Horizontal Distribution of Water Temperature and Salinity

The horizontal distributions of surface and bottom water temperatures in the Beibu Gulf across four survey voyages are presented in Figure 3. The surface and bottom layers refer to 1 m below the water surface and 1 m above the seabed, respectively. In the spring voyage, surface and bottom water temperatures increase distinctly from west to east, increasing more steeply in the bottom layer. High-temperature zones are located near the western coasts of Hainan Island and the Leizhou Peninsula. Surface temperatures range from 28.6 to 32.0 °C, exhibiting a relatively uniform distribution, while bottom temperatures range from 21.0 to 30.6 °C, transitioning from high-temperature waters near the Leizhou Peninsula to low-temperature waters in the central gulf. In the winter voyage, water temperature decreases clearly from southwest to northeast, with both surface and bottom layers ranging from 18.4 to 24.4 °C, predominantly low-temperature waters. The surface and bottom temperature distributions are highly consistent. Water masses from the gulf mouth advance northeastward, accelerating along the deep channel, forming tongue-shaped isotherms that decrease northeastward. Northward-moving water from the gulf mouth converges with westward-flowing water from the Qiongzhou Strait at approximately 109° E, forming an approximate strip-shaped zone about 40 km wide with a temperature around 21 °C.

In the summer voyage, surface and bottom temperatures are lower in the southwest and higher in the northeast. Surface temperatures range from 27.4 to 31.4 °C, primarily high-temperature waters, while bottom temperatures range from 22.5 to 31.5 °C, transitioning from low- to high-temperature waters from southwest to northeast, predominately medium- and high-temperature waters. At the surface, a tongue-shaped cooler water mass from the gulf mouth advances northward along Hainan Island’s southwestern coast, converging with high-temperature water from the western Qiongzhou Strait at about 19.5° N in the central Beibu Gulf, forming a weak east–west-trending front. At the bottom, a cooler water mass at the gulf mouth advances northeastward along the central deep channel, meeting high-temperature water from the Qiongzhou Strait at about 109.2° E, forming a north–south-trending front. The autumn voyage resembles the winter voyage, with temperatures higher in the southwest and lower in the northeast; surface temperatures range from 25.0 to 28.4 °C, and bottom temperatures from 24.6 to 28.0 °C, predominantly medium-temperature waters, with low-temperature waters limited to the northwestern area near Guangxi.

The horizontal distributions of surface and bottom salinity across four survey voyages are shown in Figure 4. In the spring voyage, surface salinity is lowest in the central study area, approximately 30.6, gradually increasing to 34.0 eastward and southward. In the north, the lowest salinity is about 30.7. The salinity gradient strength is latitudinally 0.012/km. While bottom salinity rises from 31.8 in the north to 34.2 southward and southwestward, with a salinity gradient strength of 0.009/km. The central surface waters are predominantly low-salinity, influenced by heavy rainfall during the survey period, while bottom waters are mainly medium- and high-salinity, with some low-salinity waters near Guangxi. In the winter voyage, surface and bottom salinity are higher in the southwest and lower in the northeast, with relatively small decrease ranges from 32.5 to 33.9. The salinity gradient strength is latitudinally 0.005/km. Notably, similar to the horizontal temperature distribution, surface and bottom salinity exhibit highly consistent horizontal distributions.

In the summer voyage, surface salinity gradually increases from 29.5 in the north to 33.8 in the south, predominantly low-salinity waters, with a small amount of medium- and high-salinity waters off the western coast of Hainan Island. The salinity gradient strength is latitudinally 0.016/km. Bottom salinity transitions from high-salinity waters in the southwest to low-salinity waters in the northeast, ranging from 30.6 to 34.2, with a salinity gradient strength of 0.013/km. At the bottom layer, a north–south-trending front forms at approximately 109.2° E, and a saline tongue from the gulf mouth advances northward along the deep channel, converging with nearshore low-salinity water at about 20.8° N to form a second east–west-trending front. In the autumn voyage, surface and bottom salinity distributions are generally consistent, higher in the southwest and lower in the northeast, ranging from 31.2 to 33.7, with a salinity gradient strength of 0.009/km. Surface waters are mainly low-salinity, with some medium-salinity waters in the south, while bottom waters transition from high- to low-salinity from southwest to northeast.

From the salinity gradient results of the four voyages, it can be seen that the salinity gradient of the summer voyage is the largest, followed by the spring voyage, and the surface salinity gradient is greater than the bottom salinity gradient. The winter voyage has the smallest salinity gradient among the four voyages.

2.4. Cross-Sectional Distribution of Water Temperature and Salinity

Cross-sectional distributions of water temperature and salinity for each survey voyage are presented in Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9, revealing the following characteristics.

The northern S1 transect, located near Guangxi’s inshore waters, has a depth of 15–22 m and relatively flat topography. Distinct variations in water temperature and salinity in different survey periods are observed along the transect. In the spring survey period, pronounced stratification occurs, with higher temperatures and salinities in the surface layer and lower values below. In the winter survey period, the water column exhibits uniform vertical mixing with minimal stratification. During the summer survey period, mid and surface layers show homogeneous temperature and salinity, while a weak thermocline and strong halocline exist at approximately 15 m in depth. Notably, near 108°30′ E, isotherms and isohalines significantly sink downward, indicating the presence of downwelling. In the autumn survey period, isotherms generally shift eastward from the surface to the lower layer, whereas isohalines remain vertical, suggesting a uniform vertical salinity distribution.

The S2 transect has a water depth decreasing from 40 m in the west to 20 m in the east, presenting a uniform gentle upslope trend. In the spring survey period, pronounced water temperature and salinity stratification occur. Thermoclines and haloclines are notable in the 15–20 m depth range during the spring and summer survey periods, with a weaker intensity in the spring survey period primarily located in the deeper western area, while the thermohaline in the summer survey period extends eastward to approximately 108°54′ E. In spring and summer survey periods, the upper water consists of high-temperature, low-salinity water, while the lower layer features low-temperature, high-salinity water. In autumn and winter survey periods, waters are predominantly in a state of vertical mixing.

The S3 transect has a water depth decreasing from nearly 60 m in the west to 20 m in the east, with an average depth of approximately 50 m in the western half and 20 m in the eastern half. The water temperature and salinity distributions in the spring, autumn, and winter survey periods are similar to those of transect S2. In the summer survey period, influenced by the shoaling coastal topography, distinct upwelling occurs, causing isotherms and isohalines to exhibit a pronounced upward tilting along the topography. Near 108°40′ E, downwelling is observed, with isotherms and isohalines significantly sinking downward.

The S4 transect is the longest, extending from the central Beibu Gulf to the middle of the Qiongzhou Strait, and features high topography in the middle and low topography on both sides, forming a submarine hill with a peak depth of approximately 15 m and maximum depths exceeding 60 m on both sides. In spring and summer survey periods, pronounced water temperature and salinity stratification occur on the left side of the hill, with thermoclines and haloclines emerging at around 30 m in depth. Distinct up- and downwelling are observed here, forming a compensatory pattern: upwelling ascends along the left slope of the hill, while downwelling occurs near 108°30′ E. In autumn and winter survey periods, the water exhibits primarily vertical mixing. On the right side of the hill, vertical mixing is uniform across all seasons, with minimal stratification, likely due to water accumulation from the eastern Qiongzhou Strait blocked by the subsurface hill.

The S5, S6, and S7 transects are located in the southern survey area, featuring similar topography and comparable lengths, and the water depth decreases gradually from approximately 60 m in the central Beibu Gulf toward Hainan Island across all three transects, accompanied by analogous water temperature and salinity distributions. To minimize figure length, cross-sectional maps of water temperature and salinity for all survey voyages are presented for transect S7 (Figure 9). As shown in this figure, a distinct thermocline and halocline exist near 40 m in water depth in the spring survey period, with a pronounced low-temperature zone, where the water temperature is below 24.5 °C, and in the deep water below 50 m. All transects exhibit homogeneous vertical mixing in the winter survey period, while summer and autumn survey periods are characterized by significant water temperature and salinity stratification. A notable thermohaline front forms near 20 m in depth, with reduced intensity in the autumn survey period compared to that in the summer survey period. Meanwhile, thermohaline isopleths show a distinct upward tilting along the continental slope, indicating the presence of upwelling.

Based on the above analyses, the spring survey period is characterized by pronounced water temperature and salinity stratification accompanied by thermoclines and haloclines. The summer survey period exhibits the most distinct water temperature and salinity cross-sectional features and the most significant differential variations among the four survey voyages. Compared to spring, summer has stronger thermohaline clines. Additionally, summer is accompanied by the occurrence of upwelling and downwelling. The autumn survey period is dominated by the predominantly homogeneous vertical mixing of temperature and salinity, representing a transitional state toward the winter survey period, whereas the winter survey period shows even more uniform vertical mixing with minimal stratification.

3. Relationship Between Water Temperature, Salinity, and Circulation

3.1. Circulation Structure in Beibu Gulf

The circulation in the Beibu Gulf is primarily formed by the westward water transport through the Qiongzhou Strait, wind fields, river runoff discharging into the gulf, and topography and tidal residual currents. According to the calculations of long-term mean circulation in the gulf by Chen et al. (2024) [23], the surface mean circulation in autumn, winter, and March–April in spring exhibits a cyclonic pattern, while summer and May in spring are characterized by a multi-vortex structure. The cyclonic circulation center is located at the deepest part of the gulf, specifically along the 60 m isobath of the deep channel west of Hainan Island. The current flows westward from the Qiongzhou Strait, crosses the coastal waters of Guangxi, and turns southward upon reaching the eastern coast of Vietnam. Nearshore waters along the western coast of Hainan Island flow northward, turning westward near Yangpu to form the eastern margin of this circulation. The anticyclonic circulation is generally smaller in scale, concentrated mainly in western Hainan Island. The bottom mean circulation forms an arc-shaped cyclonic circulation approximately parallel to the western coastline of Hainan Island in November in autumn and winter. In summer, South China Sea water from the gulf mouth enters the Beibu Gulf west of the central gulf, with a cyclonic vortex west of the main current and an anticyclonic vortex east thereof. Except for January and February in winter, South China Sea water from the gulf mouth can reach the northern Beibu Gulf directly, with northward bottom currents widely existing in the northern gulf. In the northeastern Beibu Gulf, offshore water from the gulf mouth meets inflow from the Qiongzhou Strait, forming complex vortex and frontal structures. Here, seawater converges and sinks, constituting an important hydrological feature of the Beibu Gulf. The seawater converges and sinks here, constituting an important hydrographic feature of the gulf.

3.2. Relationship Between Circulation and Horizontal Distribution of Temperature and Salinity

The distribution of water temperature and salinity in the Beibu Gulf is closely linked to water transport, particularly salinity. Salinity is primarily governed by the advance and retreat of low-salinity water flowing westward through the Qiongzhou Strait and high-salinity water from the southern gulf mouth, with nearshore areas influenced by river runoff.

In the spring survey period, water temperature in the survey area is lower in the west and higher in the east, while salinity is lower in the north and higher in the south. Driven by anticyclonic vortexes, high-salinity water in the deep sea from offshore areas can move northward along the western coast of Hainan Island to approximately 20.5° N, which is associated with the high-salinity water west of Yangpu Port. By contrast, low-salinity water in the Guangxi coastal area is linked to river runoff discharging into the sea. In the winter survey period, both water temperature and salinity are higher in the southwest and lower in the northeast. Low-temperature and low-salinity water from the western Guangdong coast enters the Beibu Gulf through the Qiongzhou Strait, converging with low-temperature and low-salinity water along the Guangxi coast. Under the blocking effect of relatively high-temperature and high-salinity water from the gulf mouth and the driving force of westward inflow from the Qiongzhou Strait, the converged water mass diffuses westward and northwestward. Seawater from the gulf mouth moves northward along the western coast of Hainan Island, meets the inflow from the Qiongzhou Strait in the northern Beibu Gulf, and then turns cyclonically westward, diffusing southwestward from the central gulf and refluxing to the open sea. In the summer survey period, the water temperature is lower in the southwest and higher in the northeast, while salinity shows the opposite pattern. During this season, the westward flow of the Qiongzhou Strait is relatively weak, and the low-temperature and high-salinity water at the mouth of the gulf pushes northeast along the deep trough to the northeast of the Beibu Gulf, where it meets and mixes with the water from the Qiongzhou Strait and the western nearshore water of the Leizhou Peninsula, forming more than one temperature and salinity front. Affected by the increase in runoff caused by summer rainfall, the low-salinity area of the coastal surface expands significantly, and its value drops to the lowest in a year. In the autumn survey period, both water temperature and salinity are higher in the southwest and lower in the northeast. The surface and bottom mean circulations exhibit a cyclonic pattern, and driven by this circulation, the high-salinity water at the mouth of the gulf advances northeastward along the western coast of Hainan Island, reaching the vicinity of Section S4 and the northwestern area of Yangpu. Meanwhile, during its northward progression, this high-salinity water continuously diffuses toward the gulf center through cyclonic vortexes. The distribution of isohalines in both surface and bottom layers aligns with the circulation directions, especially in the bottom layer, where the arc-shaped circulation causes the isohalines to exhibit an arcuate advancing trend.

3.3. Relationship Between Circulation and Cross-Sectional Distribution of Temperature and Salinity

Cross-sectional distributions of water temperature and salinity are influenced by water flow, reflecting vertical water movement. As all survey transects are east–west-oriented, these distributions are shaped by the east–west component of horizontal circulation and vertical water movements. As analyzed in Section 2.4, it is evident that except for the overall state of water temperature and salinity vertical mixing in the winter survey period and the basically vertical thermohaline isopleths, the thermohaline isopleths in other survey periods often bulge to one side, accompanied by water temperature and salinity stratification. The bulging direction of thermohaline isopleths can indicate the water movement trend, as shown by the dashed path with arrows in Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9. Conversely, water movement shapes the cross-sectional distribution characteristics of temperature and salinity. For example, in Section S1, the bottom thermohaline isotherms bulge rightward from Stations S1 to S4 in the autumn survey period (Figure 5); in Sections S2 and S3, the bottom thermohaline isopleths exhibit clear rightward bulges in spring and summer survey periods (Figure 6 and Figure 7); in Section S4, the bottom thermohaline isopleths bulge rightward from Stations P21 to P25 in spring and summer survey periods (Figure 8); and in Section S7, the bottom thermohaline isopleths bulge rightward in summer and autumn survey periods. These all indicate the presence of eastward-directed circulations in the region (Figure 9). In Section S1, the surface thermohaline isotherms bulge leftward from Stations S6 and S7 in the autumn survey period, and in Section S7, the surface thermohaline isopleths bulge leftward in the autumn survey period. These indicate the presence of westward-directed circulations in the region.

The presence of cyclonic and anticyclonic vortices in the waters of the Beibu Gulf leads to the occurrence of up- and downwelling. In summer, the South China Sea water enters the gulf from the midwestern part of the gulf mouth, with anticyclonic vortices forming east of the main current. The convergent sinking of water at the center of the anticyclonic circulation creates downwelling, which is the primary cause of thermohaline isopleths sagging deeply near 108°30′ E in Section S4 (Figure 8). In Sections S1 and S3, a current convergence zone, where water converges and sinks, exists between 108°30′ E and 109° E, which is the main dynamic factor for downwelling in Sections S1 and S3, causing isotherms and isohalines to sink in this region (Figure 5 and Figure 7).

Influenced by topography, the onshore climbing flow of part of the bottom water inevitably induces nearshore upwelling. In addition, the southward flow in the nearshore bottom generates onshore bottom Ekman transport, which also leads to the occurrence of upwelling. In Section S2, where water depth decreases from 40 to 20 m from west to east, the eastward circulation in spring and summer survey periods ascends along the slope, inducing upwelling that causes the bottom isopleths of temperature and salinity to bend toward shallow waters (Figure 6); similarly, in Section S3, the distributions of temperature and salinity in spring and summer survey periods are influenced by both horizontal circulation and upwelling (Figure 7); and in Figure 8, for Section S4 in the spring survey period, the bottom low-temperature water isotherms of 23 to 24 °C in the western uphill segment follow the seafloor topography, bulging upward to the right from Station P21 eastward near Station P24, while the bottom isohalines of 33.3 to 33.5 extend eastward from Station P21 to the vicinity of Station P24 along the seafloor topography, indicating that the low-temperature and high-salinity water in the deep area flows eastward and forms upwelling as it ascends the topography. In the summer survey period, isotherms and isohalines on the western side are densely distributed near the 30 m water depth. In addition, the upper high-temperature and low-salinity water and the bottom low-temperature and high-salinity water form a distinct thermohaline pycnocline, with the bottom thermohaline isopleths bulging upward to the right along the upslope following the topography, likewise indicating the presence of upwelling, which drives the bottom water deeper than 40 m to move upward to the depth of about 20 m. Similarly, in Section S7, the distributions of water temperature and salinity in the summer and autumn survey periods are influenced by the eastward circulation and clear upwelling (Figure 9).

4. Discussion

4.1. Formation Mechanism of Distribution Characteristics of Temperature and Salinity in Different Periods

The Beibu Gulf has two channels for water exchange with the open sea—one is the Qiongzhou Strait in the east, and the other is the mouth of the gulf in the south—with the distribution of water temperature and salinity of the gulf controlled by multiple oceanic dynamic processes and regional climatic factors. In general, the formation of these distribution characteristics is fundamentally the result of the combined effects of monsoon forcing, solar radiation, terrestrial runoff, ocean circulation, and water mass exchange. Figure 10 shows the ocean dynamic and climatic factors that affect the distribution of water temperature and salinity in the Beibu Gulf.

In winter, the Beibu Gulf is under the dominance of the northeast monsoon, which enhances vertical water mixing, inhibits the formation of stratification, and promotes the sinking of low-temperature and high-salinity water, resulting in vertical homogenization characteristics in water temperature and salinity profiles. With weak solar radiation and low air temperature in winter, significant sea surface heat loss occurs, leading to relatively low water temperature. Additionally, the inshore waters have shallow depths and are strongly influenced by land, causing faster cooling and generally lower temperatures in inshore waters compared to the open sea. In summer, the southwest monsoon prevails, driving the warm surface water northeastward, leading to the accumulation of warm water on the western side of the Qiongzhou Strait. Meanwhile, inshore shallow waters are affected in summer by solar radiation heating, causing the water temperature to exhibit a pattern of being higher in the north and lower in the south. Vertically, due to strong solar radiation, the surface water temperature rises, and the warm surface water is less likely to exchange with the lower layers, forming a distinct thermocline.

The Beibu Gulf is surrounded by numerous rivers, with abundant rainfall in summer; the rivers enter their high-flow period, and a large amount of freshwater is discharged into the inshore waters, leading to a decrease in salinity in inshore areas, an increase in salinity gradient, and significantly lower salinity in inshore areas than in offshore regions. In winter, the runoff decreases, the freshwater input weakens, and the salinity rises, but the surface distribution of the open sea is still lower nearshore and higher in the open sea due to the influence of high-salinity water.

The Beibu Gulf generally features counterclockwise circulations, with the intensity and extent of the circulations varying across seasons. The counterclockwise circulation existing throughout the year drives high-salinity water northward along the western coast of Hainan Island, forming a distribution pattern where isohalines lift northward and isotherms bulge southward in spring and summer and northward in autumn and winter. In summer, the circulations intensify, accelerating the northward transport of warm water and combining with the effect of solar radiation in shallow waters, leading to the water temperature at the northern Beibu Gulf being higher than that in the southern part. The westward residual currents through the Qiongzhou Strait, as part of the circulations, continuously transport low-salinity coastal water, forming a water temperature and salinity front with the high-salinity water in the gulf, which also influences the water temperature and salinity distribution in local areas. Additionally, up- and downwelling cause changes in water temperature and salinity in the vertical direction. Coastal upwelling brings bottom low-temperature and high-salinity water to the surface, affecting the temperature and salinity distribution in local sea areas, such as the upwelling in the western part of Hainan Island and the coastal coast of Guangxi as shown in Figure 10, which makes the water temperature relatively low and the salinity relatively high.

The exchange of water masses is also an important factor in the change in water temperature and salinity distribution in the Beibu Gulf. As shown in Figure 10, the transport of water and heat through the two channels of the mouth of the gulf and Qiongzhou Strait in different seasons affects the seasonal variations in water temperature and salinity in the gulf. In winter, spring, and autumn, high-salinity water from the South China Sea invades northward through the mouth of the gulf, and the high-salinity water (salinity > 33.4) can reach as far as the western entrance of the Qiongzhou Strait in the spring survey period. In summer, the coastal current from western Guangdong carries the Pearl River plume through the Qiongzhou Strait into the Beibu Gulf, leading to a decrease in salinity in the northeastern part of the gulf.

4.2. The Impact of the Circulation in the Beibu Gulf on the Ecosystem

The Beibu Gulf, rich in marine biological resources, is a hotspot for biodiversity research. Water temperature and salinity distributions significantly influence the gulf’s marine ecosystems, affecting nutrient cycling [24,25], biological distribution and reproduction [26,27], biological adaptation [28], ecosystem stability [23], and water mass formation and mixing. For instance, in terms of biological adaptation, hermatypic corals forming coral reefs have strict requirements for water temperature, with the suitable water temperature range for most hermatypic corals being 20–30 °C. Coral reefs in the waters of northwestern Hainan Island and Weizhou Island, Guangxi, in the eastern Beibu Gulf experienced “bleaching events” in 1998, 2003, 2004, 2005, 2008, and 2020 [23], all of which are associated with abnormal changes in environmental water temperature.

The distribution of water temperature and salinity in the Beibu Gulf is ultimately related to the circulation of the gulf. Beibu Gulf circulation governs spatial distribution and seasonal variations in nutrients. In winter, the Qiongzhou Strait brings inshore water from western Guangdong with high nutrients into the Beibu Gulf, leading to a massive phytoplankton bloom, while the waters west of the Qiongzhou Strait exhibit characteristics of high chlorophyll and high nutrients. In summer, the nutrients in the gulf are primarily sourced from terrestrial nutrients carried by river runoff [25]. Additionally, upwelling induced by circulation brings bottom nutrients to the surface, promoting phytoplankton growth and thereby maintaining diverse zooplankton, fish, and other marine organisms. In the northeastern Beibu Gulf, the massive northward movement of bottom water forms numerous upwelling areas along the Guangxi coast. During summer, the season with the weakest vertical convective mixing, upwelling brings abundant nutrients, stimulating the reproduction and growth of phytoplankton and zooplankton, making it an area with an excellent ecological environment in the Beibu Gulf. The waters in the northeastern Beibu Gulf constitute a current convergence zone of open-sea water from the mouth of the gulf and inflow from the Qiongzhou Strait. Here, the frontal structure of the convergence zone and upwelling areas provides abundant food resources for fish and other marine organisms, making it a renowned fishing ground in China.

5. Conclusions

This study examines the variations in water temperature and salinity at different survey periods in the central and eastern Beibu Gulf, analyzes their horizontal and sectional distribution patterns, elucidates the mechanisms by which Beibu Gulf circulation influences these distributions, and evaluates the ecological impacts. The main conclusions are as follows:

(1)

Water temperature and salinity in the study area exhibit pronounced variations at different survey periods. In terms of horizontal distribution, spring and summer survey voyages are generally characterized by a gradient from high-temperature, low-salinity waters in the north to low-temperature, high-salinity waters in the south, with the transition from low-temperature, low-salinity waters in the north to medium- and low-temperature, high-salinity waters in the south in autumn and winter survey voyages. In terms of sectional distribution, water temperature and salinity show clear stratification in spring and summer survey periods, with pronounced thermoclines and haloclines in deep-water areas, and with stronger gradients in the summer survey period than in the spring survey period. In the autumn survey period, the intensity of thermoclines and haloclines decreases, and vertical distributions generally tend to be uniform. In the winter survey period, thermoclines and haloclines disappear, with uniform vertical mixing and minimal stratification.

(2)

The motion patterns of water in the Beibu Gulf affect the distribution of water temperature and salinity. Horizontal circulation, upwelling, and downwelling cause changes in the distribution of temperature and salinity in both horizontal and vertical directions. Apart from the significant influence of river runoff on inshore salinity in summer, salinity is mainly influenced by the advance and retreat of low-salinity water from the Qiongzhou Strait and high-salinity water from the southern gulf mouth. In general, the distribution patterns and variations in water temperature and salinity at different survey periods in the Beibu Gulf are formed by the combined effects of monsoon-driven forces, solar radiation, river runoff into the sea, oceanic circulation, and water mass exchange.

(3)

The water temperature and salinity distributions in the gulf significantly impact the marine ecosystem. The complex hydrodynamic conditions and seasonal variations in circulation in the Beibu Gulf foster the rich marine biodiversity in the region.

(4)

It should be pointed out that the observation data used in this study are from relatively short survey periods in each season. The results reflect the trends and differences in water temperature and salinity during different survey periods in the study area but cannot yet present more comprehensive intra- and inter-seasonal temperature and salinity change processes and circulation characteristics. It is recommended to conduct further research when more data are available in order to more finely characterize the changes in water temperature and salinity within seasons and the differences between seasons.