Cold Water Anomalies In The Middle Layer Of The Northern Taiwan Strait In Spring:A Numerical Approach

Taiwan Strait is a long and narrow channel located between Fujian and Taiwan,which plays an important role in water exchange of South China Sea and East China Sea.The composition of the water mass in the strait is complex and currents inside are various with different seasons,which can seriously affect the aspects of ecology and economy along the coast.In 2015 spring,measured data from the shared voyage of Taiwan Strait of national fund showed that the intermediate cold water phenomenon observed in the northern strait,which performed the colder water occurred in middle level than the surface and bottom.We used the Taiwan Strait Nowcast-Forecast System(TFOR)based on the ROMS(Regional Ocean Modeling System)to simulate the hydrological condition of Taiwan Strait,then we modulated the temperature simulation via adjusting the heat flux to ensure the accuracy of the model result,and the applicable consequence was used to the research of intermediate cold water.The TFOR has simulated the period during the voyage,and the intermediate cold water anomaly phenomenon was also observed in the model result of the northern strait that indicated the reliability for simulation of temperature.We analyzed the aspects of the wind field?temperature field?velocity field and thermal diagnostics,then we found the intermediate cold water phenomenon happened with the change of the wind direction,namely,the northeaster transformed to southwester,the stratification of water got started with the mixture of Zhe-Min coastal water becoming weak,at the same time,the southwester forced warm water to transport from south to north.The cold water was remained in offshore area of the northern strait,Ekman transport occurred in the surface to force the cold water flowing offshore and upwelling formed in the bottom due to climbing along the terrain of South China Sea warm current,which led to the warming of nearshore cold water in the bottom.During the spring,the solar radiation became stronger and transmitted the heat to deeper area,the warming of surface water was more obvious than deeper water,moreover,the warming of intermediate water caused by upwelling was also relatively weaker than bottom water,in conclusion,the intermediate water maintained the relatively lower temperature.Finally,the thermal diagnostics confirmed the opinions above,the rise of temperature in bottom was affected by the horizontal convection,which contained the effects of flow along section and flow across section,that is the upwelling and the northward warm flow bottom forced water temperature to rise together,the weaker effect of the former to the intermediate water was the reason that the intermediate water retained relatively lower temperature;the rise of surface temperature was mainly caused by the vertical diffusion,which was more obvious in 0-10m layer approximately and related to solar radiation.In spring,the water temperature began to rise in the northern strait,the Zhe-Min coastal water stayed in the nearshore area,the effect of bottom upwelling?southern warm current and solar radiation resulted in the intermediate cold water.