Factors for Interannual Variations of September-October Rainfall in Hainan, China

The present study investigates the year-to-year variations of September-October rainfall in Hainan, China, for the period 1965-2010. The dominant circulation anomalies feature a cyclone (an anticyclone) over the Indochina Peninsula and northern South China Sea, an anticyclone (a cyclone) over subtropical western North Pacific and lower-level convergence (divergence) over the Maritime Continent in the wet (dry) years. These circulation anomalies are responses to an east-west sea surface temperature (SST) anomaly pattern with negative (positive) SST anomalies in the equatorial central Pacific and positive (negative) SST anomalies around the Maritime Continent in the wet (dry) years. Although the SST anomaly pattern is similar (but with opposite anomaly), the SST signal in the equatorial central Pacific is more significant in the dry years than in the wet years. This difference indicates a larger case-to-case variability in the wet years than in the dry years. The large variability in the wet years is attributed to contributions of tropical cyclones (TCs) and intraseasonal oscillations (ISOs). There are more TCs impinging on Hainan and the TC tracks are closer to the island in the wet years than in the dry years. The rainfall shows large intraseasonal variations with periods of 10-20 and 30-60 days during September-October in the wet years. The 10-20-day ISO originates from the Maritime Continent, whereas the 30-60-day ISO develops over tropical Indian Ocean and propagates northeastward to northern South China Sea. In contrast, the ISO signal is much weaker in the dry years.