Mechanism Study Of Upwelling Evolvement In The Yangtze River Estuary Adjacent Waters In Summer

Upwelling is one of the most important dynamic processes in the offshore oceanwaters, which has an important impact on vertical thermohaline structure and nutrientdistribution by vertical transport processes, and thus upwelling provides conditions formarine fisheries formation and red tide outbreak. There are upwelling regions in summernear the waters off Yangtze River Estuary (YRE), including the upwelling off YRE andZhejiang coasts. Therefore, the study of the intensity of the upwelling and its spatialdistribution is of great significance for fishery industry and harmful algae bloom prediction.We utilize the high resolution satellite product, NGSST sea surface temperature (SST)and CCMP wind field data, EOF method as well as numerical ocean model (ROMS) tostudy the evolvement mechanism of the intensity of the upwelling and its spatialdistribution as well as its influence on SST anomaly in this specific region.The results from EOF analysis show that the intensity and scope of the upwellingareas in summer exhibit evident variations, which is the crucial factor for the SST anomalyin this region. Forcing from atmospheric temperature is the most important factor affectingSST anomalies, which makes SST anomalies show homologous over whole area. However,the variable ratio of SST anomalies is uneven in space on account of the Taiwan WarmCurrent invasion and topographic variation. Upwelling caused by wind field is a key factorto affect SST short-term changes; the plume front caused by the mixing between YangtzeRiver freshwater and seawater induce the Yangtze River estuary upwelling, which can notbe ignored for SST anomalies in the region; upwelling induced by tides also can cause theSST anomaly, but the impact is relatively limited. The modal results also show that thedistribution pattern of SST anomalies is closely linked with topographic variations.ROMS model reproduces the upwelling near the waters off YRE, including theupwelling off YRE underwater valley and Zhejiang coasts as well as the upwelling nearLüsi. Southwest wind and the positive wind stress curl in summer strengthen the intensityof upwelling, and are crucial factors for short-period evolution of local upwelling. Furthermore the contribution of wind stress curl is on a par with the component of windstress along the coast. Tidal movements work on formation and evolution of upwelling notonly by its barotropic and baroclinic effects, but also by influencing the direction changeand spreading paths of the diluted water; tidal mixing affects changes of upwelling near theYRE valley and Qidong, and makes them show an astronomical monthly period. Theinfluence of tide is relatively small for Zhejiang coastal upwelling. The topographicvariation determines the center of the upwelling, which could be induced at the stronginflow flank above the steep and narrow heave, and a corresponding downflow occursbehind the heave. The broad and mild slope does not favor the formation of upwelling.