High-frequency variability of latent-heat flux in the South China Sea

Synoptic-scale disturbances of latent-heat flux in the South China Sea are examined using Xisha automatic weather station measurements and satellite-derived daily latent heat-flux datasets. The power spectra of observations suggest a synoptic feature with a pronounced energy peak at a period of 4-8 days, comparable with the timescales of the atmospheric synoptic variabilities in the region. The characteristics of the synoptic latent-heat flux variations in summer are remarkably different from those in winter. The amplitudes of significant synoptic oscillations are about 20 and 40W.m(-2) during summer and winter monsoons, respectively. An active monsoon is clearly correlated with positive and negative phases of the synoptic latent-heat flux oscillations in the South China Sea. Using a composite analysis, some fundamental features of the synoptic latent-heat flux variabilities were revealed. In summer, the synoptic-scale latent-heat flux fluctuations are highly correlated with wind speed. While the synoptic variations of winds were removed during summer, the latent-heat anomalies precede about 1 day. In winter, however, they are primarily associated with winds and near-surface air humidity. When the synoptic oscillations of near-surface air humidity are removed, the amplitude of the composite latent-heat flux is reduced by 70%.