Effects of atmospheric oscillations at different time scales on persistent autumn rainstorms in Hainan, China

In the autumn of 2008, 2010, and 2011, Hainan experienced anomalously persistent rainstorms. The objective of this study was to examine the roles of atmospheric oscillations at different time scales on these three rainstorm events. A wavelet analysis of daily rainfall data revealed that the 3-10-day synoptic-scale oscillations were the dominant mode in 2008 and 2011 and the 8-15-day quasi-biweekly oscillations were dominant in 2010. These three autumn rainstorm events can be divided into two major periods. Both the synoptic-scale and quasi-biweekly mode influenced the rainstorms, but their oscillation centers and propagation directions were completely different. There is a good corresponding relationship between the two heavy rain periods and the synoptic-scale oscillation cyclones and convection during the autumn of 2008 and 2011. During the first period, moisture mainly came from vapor convergence in the South China Sea (SCS), but during the second period, moisture came from the equatorial Indian Ocean. The major source of the synoptic-scale oscillation was the western North Pacific. The synoptic-scale oscillation cyclones and convection moved northwest to Hainan, resulting in heavy rainfall. Moisture during the first period mainly came from water vapor convergence in the SCS during the autumn of 2010, but moisture in the second period came from western North Pacific. The quasi-biweekly oscillation convection was generated and developed mainly over the equatorial Indian Ocean. Strong convection accompanying oscillation cyclones spread toward the northeast, triggering heavy precipitation in the Hainan region.