Observational Study On Spatial-seasonal Variation Of Temperature,Salinity And Currents In The Zhanjiang Bay

A bay plays crucial roles in the sea-land hydrological cycle and material transport.As the second largest bay in South China,Zhanjiang Bay is also an important aquaculture,shipping hub and military hub in China.Revealing the hydrological characteristics in the Zhanjiang Bay is of great significance for the activities in the bay and the future development.Here we use in-situ hydrographic observations from January to December 2017 and that in March,June,July,September,October,and December 2018,to investigates the spatial distribution and seasonal variation of hydrological characteristics such as temperature,salinity and current in the Zhanjiang Bay.The mechanism and factors influencing the hydrographic features are also analyzed.The results are shown as follows:(1)The annual mean temperature,salinity,potential density and buoyancy frequency(N²)in the Zhanjiang Bay are 23-27?,19-27,11-17 kg/m³,and 10^-5-10^-2 S^-2 respectively.The vertical structure and horizontal distribution of N² are similar to those of the temperature,while the distributions of density and salinity are almost the same.The temperature has distinct seasonal variation with a summer-winter difference reaching 15?,while the seasonal mean salinity varies slightly.For the horizontal distribution,the temperature decreases and the salinity increases from the Bay head,the Mid Bay,the Bank region,the Shoal region to the Bay mouth.The average temperature difference between the Bay head and the Bay mouth is 2.3?,and the salinity difference is 2.7.The temperature-salinity(?-S)diagram shows a banding distribution with one end denoting the low-temperature high-salinity water in the Bay mouth,the other end denoting the high-temperature low-salinity water in the Bay head.Different bands are shown in different seasons.Runoff,tidal current,precipitation,human activities and geostrophic effect are factors affecting the distribution of temperature and salinity in Zhanjiang Bay.(2)The currents in the Zhanjiang Bay are dominated by tides.It flows northwestward from the Bay mouth to the Bay head during flood tide,with an average velocity of 0.22 m/s,and reverses to be southeastward.During ebb tide,with an average velocity of 0.28 m/s.The annual-mean ebb tidal velocity is faster than that during the flood tide.For the vertical distribution,the velocity decreases with depth,of which the maximum value is in the surface layer(0.26 m/s),and the minimum value is near bottom(0.19 m/s).The flow direction changes little with depth,and the difference between the surface and bottom layers is generally less than 20°.The velocity vector rotates counterclockwise from surface to bottom at most stations,while it keeps the same direction in shallow waters,accounting for about 25%of the total stations.Mechanism analysis shows that Ekman bottom friction may be the cause of current deflection.(3)The tidal analysis from the continuous observations shows that the tidal current amplitude of the semidiurnal tidal constituent M₂(0.45 m/s)is greater than that of the diurnal tidal constituent K₁(0.15 m/s),while that of the shallow water constituents M₄is equivalent to that of the diurnal tidal constituent K₁.The M₂ tidal current at the Bay mouth is generally larger than that at the Mid Bay.The tidal current amplitude in summer is larger than that in autumn during the neap tide.The spring tide,amplitude is smaller in summer and winter than in spring and autumn.The inclination and minor axis of the M₂ tidal current ellipse change little with depth,while the major axis gradually decreases.The tidal ellipse rotates counterclockwise at the Mid Bay,but clockwise at the Bay mouth.Generally,the volume flux at the Bay mouth is larger than that in the Bay head,and the spring-tide flux is obviously larger than that during the neap tide.The seasonal variation of residual current may be related to the seasonal change of wind field.