非持續性的南海暖流

Abstract

From the previous hydrographic and numerical studies, the finding of a year round northeastward flowing South China Sea Warm Current (SCSWC) existed on the continental shelf of northern South China Sea (SCS), even during the NE monsoon season has been claimed. However, the existence and mechanism of SCSWC is still controversial because the conclusion is mostly based on one-time hydrographic survey or very short-term moored current velocity measurements. In this study, we utilize hydrographic measurements, including CTD casts and Shipboard Acoustic Doppler Current Profiler (Sb-ADCP) current velocity, moored ADCP current velocity, the QuickSCAT/National Centers for Environmental Prediction (QSCAT/NCEP) blended ocean surface wind stress, and the Tropical Rainfall Measurement Mission Microwave Imager Sea Surface Temperature (TMI/SST) data to study the currents on the shelf in the northern SCS, especially focus on SCSWC. The composite temperature and SB-ADCP current velocity indicate that the shelf currents are toward the north, but northeastward in SW monsoon season (May – October) and northwestward in NE monsoon season (November — April). Furthermore, the present one-time hydrographic surveys and estimated geostrophic currents show that northeastward current is seen occasionally in the shelf region. The above studies are similar to previous studies in summer but has notable different in winter. Solidly and directly, a 9-month (04/29/2005 — 01/18/2006) ADCP mooring measurement on the shelf break, which is the considered range of SCSWC, shows the SCSWC is not a persistent feature. The currents were varied. The northeastward current was seen only in winter when the NE monsoon relaxes. The similar events are seen in a 5-month bottom-mounted moored ADCP velocity measurement on the shelf during 11/5/2001 — 3/21/2002. The wintertime observation reveals that this northeastward current emerges only when the NE monsoon is relatively weak, its velocity can reach 30 cm/s near surface, and the SST increased accordingly. Obviously, the SCSWC only exists after NE monsoon relaxation in winter. Its intermittent characteristic is different from previous studies. The output from the SCS model shows that the NE monsoon relaxation sets up a pressure gradient with sea level higher in the southwest than in the northeast and then causes a northeastward current, SCSWC. The results suggest that SCSWC is not a persistent feature. Wind relaxation could be the mechanism of SCSWC in winter.