The summer Bohai Sea hydrological environment analysis

The Bohai Sea is a nearly closed shallow sea which goes deep into china mainland, it has a supper shallow water shelf properties. The research about the Bohai Sea environment must consider the wind, tide, temperature and salinity. The fixed point observed temperature, salinity and current data were analysed. The results show that the Bohai Sea is regular half-day tidal current trend, and the tidal current trend is the substantially reciprocating flow properties. The reciprocating flow direction within the bay is trend towards the bay’s direction; the rest is trend towards the sea shore. During the observation, the current direction is stable and the current rate is small at all fixed points, with a good correlation with wind. Thereinto the Liaodong Bay has a clockwise circulation and the Laizhou Bay flows substantially clockwise either, which consistent with the historical trend observation. One fixed point in the Bohai Bay flows different trend compared with the historical observation. The Bohai Sea surface temperature is slightly lower than that in August of the calendar year; salinity has a decreased trend compared to historical statistics.


Foreword
Bohai Sea is a nearly closed shallow sea which penetrate deeply into China mainland [1], it belongs to ultra-shallow continental shelf properties, it's response forcing factor is relatively short, the circulation dynamics studies about the Bohai Sea must consider the combined effects of wind、tide、heat and salt [2]. In September 2003, State Oceanic Administration organized and implemented a marine synthesized survey and evaluation [3]. Its investigation ranges includes the Bohai Sea. This paper use this survey data to analyze the summer Bohai Sea hydrological environment.

Data Source
Data this article refer to are derived from marine synthesized survey and evaluation, summer physical oceanographic survey. Survey area is the Bohai Sea west of the Bohai Strait.
Observation includes section observation and fixed moorings observation. Section observation contain 188 stations, time ranges from July 2006 to August 2006, and observation elements include station information、 water depth, temperature, salinity and turbidity. The interlayer spacing is 1meter. There are 4 fixed moorings stations, time ranges from July to September 2006, observation elements mainly are currents, and the interlayer spacing is 1 meter. Figure 1 is observation station distribution image.

Current Characteristics
Fixed mooring station referred to as No.1-No.5 stations. The following table (Table 1) shows the frequency primary current direction appears at fixed mooring stations, including primary rising trend and frequency; primary falling trend and frequency. Wherein the primary rising trend of No.1 station point to the Liaodong Bay seashore, the primary falling trend point to estuary as shown in Figure 2. No.2 station is similar to No.1 station, rising trend point to the Bohai Bay seashore while falling trend point to estuary (see Figure 3). As shown in Figure 4

Frequency of each current speed levels in fixed mooring station
The following table (Table 2) lists the highest frequency current speed in every fixed mooring station. From the table we can know the current speed dimension is not large. The highest frequency current speed at deeper layers is larger than surface layer at some stations.

Fixed mooring station tidal current character
Tidal current usually refers to the seawater flow caused by the rise and fall of the astronomical tide. Tidal current character is divided by the following formula: K value is shown in the following table (Table 3). These five fixed mooring station in addition to 5th station is irregular semidiurnal tidal current character, the rest are all regular semidiurnal tidal current properties.

Fixed mooring station tidal current trend form
Tidal current trend form depends on the ellipse elements of main partial tides in the sea. By the tidal current character analysis showed above we know that the trend of five fixed mooring stations are semidiurnal tidal current character, so the trend form of main semidiurnal tide (M 2 and S 2 ) could represent the tidal current trend form of this sea. The parameter reflecting the trend form of tidal current is rotating rate (also known as ellipticity), its value is the ratio of minor axis and major axis of the ellipse of this partial tide, which sign has the difference of "+", "-", positive sign denotes that the partial tidal current rotates counterclockwise, negative sign means clockwise rotation.
As shown in Figure 7 and Rotation ellipse of partial tide M 2 and S 2 manifest strong reciprocating current properties, major axis of these two partial tides are consistent. Wherein the rising tidal current direction of No 1 station point to seashore, falling tidal current direction point to Liaodong Bay mouth; Rising tidal current direction of No.2 station also point to seashore and falling tidal current direction point to Bohai Bay mouth; Since No.3、4 and 5 stations are not located within the Bay, and close to the shore, so the direction of the reciprocating current parallel the seashore. Compare to section 4.1 current direction frequency, semidiurnal partial tide rotation ellipse represent the same direction with primary current direction. This shows that tidal current plays important role in current in Bohai Sea.     1 station) corroborates the viewpoint of Zhao BR [5], who considered that it is clockwise residual current in Liaodong Bay.

Residual current analysis in observation period
Jiang WSH [4,6] launched artificial jellyfish in Bohai Bay and the whole Bohai Sea in summer 1993 and summer 2000, the analysis concluded that the bottom residual current in Bohai is basically less than 5cm/s, the results of this paper show that the bottom residual current in Bohai Sea is mostly about 5cm/s or larger.
Jiang WSH considered that there was an anticlockwise circulation in north Liaodong Bay, and there was a clockwise circulation in Bohai Bay. However, further analysis revealed that the migration of bottom artificial jellyfish didn't fully reflect the direction of circulation, even reversed the trend in some stations. The result of this paper is different with his result, residual current shows up clockwise trend in Liaodong Bay, and there exists an anticlockwise residual current in Bohai Bay.
The residual current direction within Bohai Bay (No.2 station) is anticlockwise, which is different with one clockwise trend of the double current loop structure analysed by Zhao BR [5], we infer it is maybe the influence of wind.
Laizhou Bay (No.5 station) residual current flows substantially in a clockwise trend, which supports the viewpoint of Zhao BR [5].

Seawater temperature plane distribution characteristic
As shown in Figure 10, the surface temperature distribution situation of the Bohai Sea in summer is that temperature is high along the coast, there present a low-temperature region near the Bohai Strait, and it extends to the direction of the Bohai Bay. With the depth increase, temperature decrease, and the scope of the cold water zone gradually expand. The average surface temperature is 23.58℃. Sea surface temperature is lower consider to August plane temperature distribution over the years analysed by He XM [7].
The temperature distribution at 5 meter layer 、10 meter layer (see Figure 11) and bottom layer is consistent with the temperature distribution measured in summer 2006 which is analysed by Song WP [8].

Salinity plane distribution characteristic
The surface salinity in summer as shown in Figure 12 has three high salinity areas and three low salinity areas, they cross each others, the Bohai Bay, middle of Liaodong Bay and the Bohai Strait are high salinity areas, the estuary of Liaodong Bay and middle of Bohai sea are low salinity areas. The high salinity areas under 10 meter (see Figure 13) represent herringbone. The salinity characteristic is consonant with that analyzed by Song WP [8].
The mean surface salinity is 30.93, lower than the mean surface salinity after 90 age analyzed by Bi CC [9].

Conclusion
The fixed point observed temperature and salinity, and current data which were quality controlled were analyzed. The results show that：  The Bohai Sea is the regular half-day tidal current trend, and the tidal current trend is the substantially reciprocating flow properties.  The reciprocating flow direction within the Bay is trend towards the Bay's direction, the rest is trend towards the sea shore. During the observation, the current direction is stable and the current rate is small all fixed points, with a good correlation with wind. Thereinto the Liaodong Bay has a clockwise circulation, and the Laizhou Bay flows substantially clockwise either, which consistent with the historical trend observation. One fixed point in the Bohai Bay flows different trend compare with the historical observation.  The Bohai Sea surface temperature is slightly lower than the in August of the calendar year, salinity has a decreased trend compared to historical statistics.