Coastal Upwelling In The Zhoushan And Its Adjacent Seas Detected By Satellite Measurements

Sea surface temperature derived from NOAA/AVHRR and Chlorophyll-a derived from MODIS & SeaWiFS as well as wind vectors data from QuikSCAT during summer (from May to October) in 2004 have been used to investigate the short-term variability of upwelling events in the Zhoushan Islands and its adjacent seas. SST monthly averaged data derived from the AVHRR Pathfinder during the period from May to October in 1985~2005, Chlorophyll-a monthly averaged data from May to October in 2002~2006 as well as wind vectors data between May and October from 1992 to 2006 have been used to investigate the annual variability of upwelling in the study domain.Short-term observations of satellite images during summer in 2004 have revealed that two upwelling zones in the study area. The upwelling near Zhoushan Islands was located at 29°~31°N, 121.5°~123°E, with the upwelling core near 30°N, and the upwelling near Yangtze River estuary was located at 31°~32°N,121.5°~123°E, with the upwelling core near 31.5°N. These areas are noted for their high fisheries production. Upwelling looks like a long tongue in shape in the Zhejiang coastal zones, the sea surface thermal front exists obviously in its southeastern edge. The low temperature in the study area was about 23~27℃with a mean value of 25℃; the temperature difference between the upwelling and non-upwelling areas was about 3~4℃.The seasonal variability was obvious found in 2004, upwelling started to appear in June and got stronger in July and August, it assumed to become weaken until vanished in September. Chlorophyll-a data of MODIS-derived and SeaWiFS-derived showed that high Chl_a areas coincided with low SST areas in terms of location, time and shape. The relationship between upwelling intensity and sea surface wind field were analyzed by using AVHRR SST and QuikSCAT wind field, the result showed that the variations of sea surface wind speed as well as wind direction was coincided with the variations of upwelling, which indicated the southwest wind alongshore off Zhejiang coastal is an important factor for the variations of upwelling.Lone-time observation of satellite images have revealed that the upwelling zones occurred every year in the study area for summer months, which characterized with the distinct low water temperature and high Chl_a. The size of the upwelling has been found to be smaller during summer in 2001; The strength of upwelling has been found to be stronger during summer from 1997 to 2000.The difference of SST among upwelling and the surrounding waters is about 1.4℃. Chl_a images derived from MODIS sensor show high Chl_a in the upwelling zone, which coincide with low SST in terms of location, shape, and time. These high Chl_a in the upwelling was considered to be related with the upwelling waters that bring nutrients from bottom to surface. The wind forcing exerts positive effects on upwelling. The upwelling began to occur when the direction of wind changed from north to south between May and June, and was getting weaker during August and September as the direction of wind has been changed from south to north during that time. The study area has been found to be dominated by the southwest monsoon that blows strongly across Zhejiang coastal area from late June to early August, and the upwelling reached to the top during that time. The southwest monsoon exerts important influence on the upwelling.