Structure And Dynamics Of Ocean Circulation Off The East Coast Of China

The structure and dynamics of the subtidal circulation in the southwestern Yellow Sea in summer are studied based on the hydrographic data of 4 cruises in the summers of 2008 and 2009, satellite-tracked trajectory data of six Argos floats, moored current-meter data of Acoustic Doppler Profilers (ADPs) measurements, satellite scatterometer wind data,, and numerical modeling. The coastal currents off the northern Jiangsu province are shown to flow northward in summer, against the traditional view of the southward Yellow Sea Coastal Current all year round in this area. Numerical experiments suggest that the coastal currents along the Jiangsu and Lunan coasts are dominated by the southeasterly monsoonal forcing, with the baroclinic currents much weaker than the wind-driven circulation.Associated with the northward flow in the southwestern Yellow Sea, coastal upwelling currents off the Jiangsu coast and off the city of Qingdao are indicated by areas of cold sea surface temperature (SST). The upwelling off Qingdao is discovered for the first time in history. Analysis of the modeling results suggests that the upwelling off the Jiangsu coast is induced by the blocking of the surface wind-driven Ekman transport at the coast and is modulated by the tidal mixing and buoyancy effects. The onshore bottom Ekman transport associated with the northward Jiangsu coastal current brings the subsurface cold water to the nearshore area, which is elevated to the sea surface by the upwelling current. The overlapping of the surface and bottom Ekman layers over the shallow and flat Subei Bank has resulted in the upwelling to surface in an offshore belt along roughly the 30 m isobath.The upwelling currents off the Jiangsu and Lunan coasts are further shown to be an unified upwelling system in the southwestern Yellow Sea driven by the southerly winds in summer. The upwelling system is modulated by tidal mixing and by the buoyancy effect produced by the river runoff from the Sheyang River and by surface heat flux, resulting in non-homogeneous distribution of the upwelling along the coasts: South of the Subei Bank, the upwelling front surfaces along roughly the 30 m isobath due to the strong vertical mixing in the nearshore area; Over the Subei Bank, the upwelling is shut down by the buoyancy flux from the Sheyang River runoff and from surface heating; The upwelling re-occurs further north off Qingdao because the surface flow and Ekman transport push the freshwater offshore.Significant temperature inversions are found in the bottom cold water near the 40 m isobath over the west flank of the Yellow Sea trough in summer. The temperature inversions are probably produced by the relative movement between the bottom warm and saline current near center of the Yellow Sea trough and the cold and fresh water to its west. The inversions, which occur frequently to produce rich stratification structure, suggest that the vertical mixing in the Yellow Sea Bottom Cold Water (YSCWM) is weak, countering the traditional view that the YSCWM is vertically well mixed by strong tidal mixing. The trajectories of the Argos floats indicate that the Taiwan Warm Current flow northward beyond the Changjiang mouth and join the Jiangsu coastal current and the central Yellow Sea circulation in summer.Satellite images and in situ hydrography and nutrient observations are used to study the three-dimensional structure of a penetrating front off the Changjiang mouth in the East China Sea. The in situ hydrography measured during the same time as the satellite images discloses that the surface layer of the front is occupied by low salinity waters from the nearshore area, which suggests a cross-shelf jet covering a distance of about 50 km. A counter intrusion underneath the surface offshore jet is indicated by an increase of the subsurface temperature and salinity during the offshore excursion of the front, which leads to a significant temperature inversion at the head of the front. It is speculated that the penetrating front is generated because an initial instability of the coastal mean front might have provided the strongly convergent coastal currents with a conduit of offshore transport.Images of 8-day mean chlorophyll-a concentration of the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard of Aqua Satellite from 2002 to 2007 and of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) satellite from 1998 to 2007 are used to study the variability of the Cross-shelf Penetrating Fronts (CPF) in the East China Sea. The analysis suggests that the generation of the CPFs has strong seasonal-to-interannual variations, with more CPFs in summer and winter than in spring and fall. Each CPF lasts for a few days to a month. The majority of the fronts are triangle-shaped with their broad bases nearby the coast and their sharp heads penetrating into the middle shelf. There are over 40 CPFs with penetrating distances exceeding 100 km over the East China Sea during the past ten years of the satellite era, with the maximum ones reaching the Kuroshio at the edge of the continental shelf. Of all the low cloud scenes of the satellite images, more than 25% contain at least one CPF. The high frequency occurrence of the CPFs suggests that the CPFs play an important role in cross-shelf transport of heat, effluent, and land-derived materials in the East China Sea.