Influence Of The Air-sea Latent Heat Flux Over The SCS On The Onset Of The SCS Summer Monsoon

The temporal and spatial distribution characteristics of the air-sea latent heat flux (LHF) and its variation mechanism in the early or late SCS summer monsoon over the South China Sea (SCS) have been analyzed in the method of diagnosis and Empirical Orthogonal Functions(EOF) analysis based on the daily RE_NCEP data of LHF, monthly NCEP/NCAR data of wind, and daily NOAA Optimum Interpolation (OI) Sea Surface Temperature (SST) from January of 1982 to December of 2001. The main conclusions are as follows:The LHF anomalies over the SCS of zonal distribution have seasonal and interannual changes. The LHF anomalies over the north SCS are opposite to the one over the south SCS in winter of the previous early or late SCS summer monsoon year. In the early (late) year, the LHF anomalies is positive (negative) over the north SCS and negative (positive) over the south SCS.Additionally, the LHF anomalies in the early year is stronger than in the late year.It is the different geographical position of the south and north SCS that triggers the different major factors to the LHF anomalies in these two regions. In the north SCS adjacent to the Chinese mainland, the LHF anomalies are attributed to the variation of the wind speed anomaly because of the influence of the cold air from the Northern China. And therefore the cold air degenerates and results in the anomalous sea surface specific humidity which is major contribution to the LHF anomalies over the south SCS. The results of quantitative analysis disclosures that wind contribution is 77%(66%) in the total positive(negative) LHF anomalies over the north SCS, while the sea surface specific humidity is 83% (67%) to the total negative (positive) LHF anomalies over the south SCS.By the combined action of the different major factors, the different anomalous wind in low level atmosphere and the different SST anomaly, the distribution of the LHF anonmalies over the SCS is opposite.The possible mechanism is as follows: In the early (late) monsoon year, the stronger(weaker) winter monsoon causes that the wind is stronger(weaker) above the 10 meters from the sea surface because of the anomalous northeast(southwest) wind from the anomalous cyclone (anticyclone) in Philippine. In the effect of the wind strength variation, the stronger (weaker) evaporation and more release heat from the sea to the atmosphere result in the positive (negative) LHF anomalies in the north SCS. In the south SCS, the sea surface specific humidity is lower (higher) because of the lower(higher) SST, which decreases(increase) the air-sea specific humidity, strengenth(weaken) the release heat fron the sea to the air, and result in the negative (positive) LHF anomalies.The correlationship is also investigated in the method of Singular Value Decoposition(SVD) between the SCS summer monsoon onset and the distribution characteristics of the LHF anomalies in the SCS in winter before its onset. In May, the LHF anomalies in winter in the SCS is positively correlated with the anomalous wind in 850hPa in the south SCS and negatively with the wind in the South China. When the LHF anomalies is positive in the north SCS and negative in the south SCS, the SCS summer monsoon breakouts early (late) and the anomalous eastern (western) wind in 850hPa appears.In summer, the LHF anomalies in the north SCS is positively correlated with the wind in 850hPa in the north SCS, which strengthen(weaken) the intensity of the wind, while it is little connected to the one in the south SCS. The possible reason of the correalation between the LHF anomalies in winter and the summer monsoon onset is as follows: the LHF anomalies impact the land-sea temporal difference and the convection by influencing the SST and the local vertical meridional circulation over the SCS. These results in the summer monsoon onset early or late.