Water Mass Characteristics and Stratification at the Shallow Sunda Shelf of Southern South China Sea

Abstract

CTD data obtained from two oceanographic cruises during June and October 2012 were used to define the water mass characteristics and degree of stratification at the shallow Sunda shelf located at the southern South China Sea. The water masses during both cruises showed characteristics similar (southwest monsoon) to those observed in the adjacent regions. A clustering method was used in which three water masses were classified from the composite dataset. There are WM1 (T 29˚C - 31˚C, S 32 - 33.5 psu, & σT, ~19.5 - 20.7 kg/m3), WM2 (T 25˚C - 29˚C, S 32.8 - 33.8 psu, & σT, ~22.3 - 23 kg/m3) and WM3 (T 23˚C - 25˚C, S 33.5 - 34.0, & σT ~22.3 - 23.3 kg/m3). Even though the water masses found were characterized under southwest monsoon characteristics, the degrees of stratification obtained varied between these cruises. The stability of the stratification also plays roles in the distribution of the water masses in the water column. WM2 was found in thermocline layer and most dominant in June compared to in October. In June, WM3 was found under the thermocline layer and absent in October. The stable thermocline caused the denser WM3 cannot mixed with WM2. Higher temperatures and weaker winds during June may have caused the strong stratification, while decreasing temperature and stronger winds of an upcoming northeast monsoon enhanced the vertical mixing during October.