10-30-day Oscillation Of Persistent Heavy Rainfall Over South China During May-August

This study investigates the relationship between 10-30-day low-frequency variations of the circulation and sea surface temperature (SST) over South China-East Asian coastal region (EACR), accompanied with persistent heavy rainfall (PHR) events over South China during May-August through statistical analysis. The distribution and evolution of the anomalous diabatic heating during the PHR events and its effects on the circulation are also analyzed.Based on the intensity threshold and duration criterion of the daily rainfall, a total of 63 May-June (MJ) and 59 July-August (JA) PHR events are selected for South China region from 1979-2011. The lower-level circulation anomalies on 10-30-day time-scale exhibit an anomalously cyclonic circulation over South China and an anomalously anticyclonic circulation shaped like a tongue over the South China Sea (SCS) during the PHR events for MJ group. The anomalously cyclonic circulation over South China in MJ originates from low-value systems in the mid-high latitudes before the rainfall. The anomalously anticyclonic circulation over the SCS is due to the westward extension of the western Pacific subtropical high (WPSH) and the southeastward propagation of the anomalously anticyclonic circulation from South China before the rainfall. For JA group, the lower-level anomalous circulation pattern is similar to that for MJ over South China-EACR, however, with different features of propagation. The anomalously anticyclonic circulation is also related with the westward stretch of the WPSH; while the anomalously cyclonic circulation is traced back to the anomalous circulation over the Philippine Sea several days before the rainfall events.The positive SST anomaly is located over the SCS and the Philippine Sea during the MJ PHR events on 10-30-day time-scale. It is closely related with the variation of anomalously anticyclonic circulation. In contrast, the negative SST anomaly occupies the South China coastal region for JA group, and it is driven by the anomalously cyclonic circulation which propagates northwestward from the Philippine Sea. The 10-30-day positive (negative) SST anomalies are generated via the local processes of above (below)-normal incident solar radiation and below (above)-normal latent heat fluxes. The possible role of the low-frequency SSTA on the local convective instability is also analyzed in this study.The condensational heating is found to dominate the diabatic heating over the East Asian summer monsoon region. The evolution of the anomalous 10-30-day low-frequency diabatic heating during the PHR is similar with the anomalous circulation. The diabatic heating anomalies propagate southward from north of 30°N to South China during the MJ PHR events, while the anomalous diabatic heating propagates northwestward to South China from the SCS and Philippine Sea during the JA PHR events. Anomalous circulation pattern could decide the position and intensity of the rainfall and condensational heating, however, the condensational heating could give a feedback to anomalous circulation by affecting the vertical vorticity tendency. For two groups of events over South China, when the rainfall is developing, the significant positive vorticity tendency anomalies on 10-30-day time-scale is over South China under the influence of both the anomalous diabatic heating and the advection of the vorticity. The vertical nonuniform of the diabatic heating have an important influence on the vorticity tendency, and the vorticity advection is closely related with the mean state of the wind fields. When the rainfall is dying, the vertical nonuniform of the diabatic heating is not notable, the negative vorticity tendency anomaly is over South China which is favorable for the ending of the anomalously cyclonic circulation. The WPSH exists zonal oscillation during the JA events, which may be closely related with the β-effect.