Study Of The Barrier Layer In The Yellow Sea And East China Sea

Based on observations by CTD, LADCP and TurboMAP-II obtained in the Yellow Sea and East China Sea during June-July 2006 and January-February 2007 aboard Dong Fang Hong R/V 2# and from the project of 973 Program undertaken by Ocean University of China,the barrier layer phenomenon in these seas is studied.Barrier layers have different vertical temperature structures, thickness, spatial distributions and formation mechanisms in summer and winter. In summer, barrier layers are about 1-2m thick and part of isothermal layers; they can be observed only in sparse regions off the mouth of the Yangtze River. The formation mechanism of barrier layers in summer is as follows: horizontally diluted water advection and local rainfall don't change the temperature of the mixed layer but facilitate a new halocline which is shallower, then a new pycnocline occurs of which density is controlled by salinity; the new pycnocline is shallower than the thermocline and a barrier layer occurs. In winter, barrier layers generally contain temperature inversions being about 17-48m thick and exist in coastal regions of the East China Sea and the central part of the Yellow Sea. During this season, the formation mechanism can be as follows: due to strong surface cooling and offshore advection by surface currents, the upper layer water becomes cold and fresh, while the water advection from the Yellow Sea warm current and Taiwan warm current in the lower layer has higher temperature and high salinity. Having temperature inversions, there exists eddy diffusive heat flux from barrier layers to upper mixed layers. This study is based on microscale observations. Dissipation rate of thermal varianceχT, dissipation rate of turbulent kinetic energyε, eddy diffusivity for heat K T and diapycnal diffusivity Kρnear to the interface between mixed layers and barrier layers are calculated. All of these four variables have large spatial variations:χT increases both southward and northward from regions off the mouth of the Yangtze River and is estimated to be 10-8～10-5°C2 s-1, on the contrary,εdecreases both southward and northward from regions off the Yangtze River mouth and is estimated to be 10-8～10-6 W/kg; K Tand Kρboth have similar distributions to that ofεand are estimated to be 10-4～10-2 m2 s-1 and 10-4～10-1 m2 s-1 respectively. Then the upward heat flux across the interface is also estimated; it averages 57 W m-2 with a maximum of 495 W m-2. The mean heat flux is capable of warming the upper mixed layer by 1.2°C per month.A simple mixed layer heat budget model in the discussion indicates that the upward diffusive heat flux is comparable to that from the horizontal advection and surface forcing, and therefore shouldn't be neglected. A power law fit to the data gives a parameterization of KT,which isand can be used as reference in further studies.