The Air-sea fluxes exchange and Bulk transfer coefficients over SCS during the onset of southwest monsoon in 2002

The South China Sea (SCS) is an important but poorly monitored part of the Asia monsoon region. In 1998, the South China Sea monsoon experiment (SCSMEX) was conducted over the SCS and adjacent regions. The air-sea flux measurement at Xisha was a sub-program that aimed at measuring heat, moisture and momentum exchange at sea surface for a better understanding of air-sea interaction processes during the monsoon onset. From 1998 to 2002, three air-sea flux experiments were conducted (1998, 2000 and 2002) on air-sea flux research tower at Xisha.

The present study is based on the third SCS flux measurements made during April 24 to June 22, 2002. It is a comprehensive study of the coupled ocean-atmosphere system over SCS. Long-term fixed-point consecutive wind, cloud, precipitation, humidity, radiation, humidity and temperature fluctuation data of sea surface were obtained. Active and break periods of the monsoon, and the transitions between these phases occurred during the experiment period. Heat and moisture fluxes exchange varied with the onset of SCS monsoon, especially in short-wave radiation, net radiation budget of ocean, latent heat flux and net heat flux. The pre-onset and onset periods had significant differences in the net heating of the ocean, ranging from daily averages of +125W/m2 during the former to about 0 in the latter. Accompanying these changes in the different phases of the monsoon were distinct states of the upper ocean and the lower atmosphere.

Bulk transfer coefficients were not constant but had a simple polynomial dependence on wind speed, and a linear dependence on the air-sea temperature difference Tw-Ta. The average value of the drag coefficient CD is1.09x10-3 and reaches to the minimum in the case of 5 m/s, and increased at lower and higher wind speeds. Both CH and CE show almost similar decreasing trend with wind speed, rapidly for less than 5 m/s and gradually for larger wind speeds. When wind speed exceeds 12 m /s they tend a constant. CH and CE also show increasing trend with Tw-Ta. There is excellent agreement between the measurement of SCS-2002 and that of INDOEX ship.

Variability of heat flux can impact on SST. Variability of SST is mostly determined by surface net heat budget. Before the onset, SST increased from April to the 2nd dekad of May with increasing of net heat budget. After that SST decreased first and then increased during break of monsoon. A small positive net heat flux implies the accumulation of energy after onset, so that SST can also keep stable values at xisha areas while they decrease quickly over Arabian Sea and Bay of Bengal.