ANALYSIS OF HYDROLOGIC SECTION OFF ZHUJIANG RIVER ESTUARY IN NORTHERN SOUTH CHINA SEA DURING VARIOUS SOUTHWEST MONSOON PHASES

Based on CTD data from two cruises off Zhujiang River Estuary in northern South China Sea in July, 2000 and in May, 2001, and combining with meteorological data in corresponding period as well as historical observations and modeling results in this area, some features at observed section ZJ are discussed, including diluted water from Zhujiang River, spring layer and mixed layer, upwelling and downwelling. It was observed that these sectional hydrological features varied with different seasons: (1) with strengthening of southwest monsoon, surface layer was influenced more intensively by diluted water from Zhujiang River and stratification structure became more evident at section ZJ; and (2) there existed upwellings and downwellings at section when southwest monsoon was strong enough. Some conclusions have been reached as below. (1) Local wind and solar flux dominate temperature and depth of spring layer and mixed layer, and affect extension of diluted water from Zhujiang River in a positive way. (2) The southwest monsoon plays most important role in leading to appearance of upwellings, which transport water across shelf through Ekman effect; topography of investigated section may enhance intensity of upwellings, but can not result in upwellings when monsoon is weak. (3) The upwelling and downwelling areas coexist at section when southwest monsoon is strong. There may be two explanations on appearance of downwellings, one is weak anticyclonic eddy resulted from flows in opposite flowing directions along slope and along continent, and other is that the upwelling and downwelling coexist in need of water complement.