Role of the 10–20-day Oscillation in Rainstorms in October 2010 over Hainan, China

Hainan, an island province of China in the northern South China Sea, experienced two sustained rainstorms in October 2010, which were the most severe autumn rainstorms during the last 60 years. From August to October 2010, the most dominant signal of Hainan rainfall was the 10–20-day oscillation. This paper examines the roles of the 10–20-day oscillation in convective activity and atmospheric circulation during the rainstorms of October 2010 over Hainan. During both rainstorms, Hainan was near the center of convective activity and under the influence of a lower-troposphere cyclonic circulation. The 10–20-day convective center was initiated in the west-central tropical Indian Ocean several days prior to the rainstorm in Hainan. The convective center first propagated eastward to the maritime continent, accompanied by the cyclonic circulation, and then moved northward to the northern South China Sea and South China, causing the rainstorms in Hainan. In addition, the westward propagation of convection from the tropical western Pacific to the southern South China Sea as well as the propagation farther northward intensified the convective activity over the northern South China Sea and South China during the first rainstorm.

The eastward propagation of the 10–20-day oscillation in the tropical Indian Ocean is similar to the convectively coupled Kelvin-Rossby wave. In addition, the westward propagation of the 10–20-day oscillation from the western Pacific is roughly similar to the equatorial Rossby wave in both its periodicity and propagation speed. Thus, the coupled Kelvin-Rossby wave and equatorial Rossby wave may have worked together to induce the rainstorms over Hainan in October 2010. We also investigated the propagation of the 30–60-day oscillation, and the results showed that there is neither eastward propagation from the Indian Ocean nor westward propagation from the western Pacific on the 30–60-day time scale.

We also examined SST anomalies in September and October 2010. The spatial distribution of the SST anomaly shows a La Niña pattern. The positive SST anomalies exist almost throughout the tropical Indian Ocean and western Pacific, forcing the active convection center and favoring the corresponding propagation from the Indian Ocean to the maritime continent. Meanwhile, during the La Niña event, the westward shift of the suppressed convection and easterly anomalies over western-central Pacific prevented the 10-20-day oscillation from penetrating farther eastward, and consequently it stayed at the maritime continent longitudes.