Case Studies Of Mechanisms For Quasi-biweekly Rainfall Oscillation During Spring And During Summer Over Southern China

The satellite-based TRMM rainfall and the JRA55 atmospheric reanalysis datasets are used to investigate the intraseasonal variability of rainfall over southern China (SC) during spring and summer seasons by means of case studies, revealing the physical mechanisms responsible for such intraseasonal variability in terms of the structure and evolution of intraseasonal extratropical atmospheric circulations. The primary conclusions are as follows.For the representative case of the 2006 spring during which the total rainfall amount was anomalously more than normal over SC, it is found that the temporal evolution of the SC rainfall in intensity exhibited a distinct intraseasonal variation characterized by significant quasi-biweekly oscillation (QBWO) with a period of 10-20 days. Composite analyses show that the wet (dry) phases of the QBWO were associated with a huge anomalous cyclone (anticyclone) encircling the Tibetan Plateau (TP) in the lower troposphere, with anomalous low-level southwesterlies(northeasterlies) prevailing over SC. In the middle and upper troposphere,a coherent wave train was observed with a series of anomalous cyclones and anticyclones alternating over Eurasian continent. With the wave train propagating southeastward,the anomalous cold (warm) air intruded into SC,which induced anomalous ascending(descending) motion and consequently positive (negative) rainfall anomalies over SC.The dominant factors leading to the local change of intraseasonal temperature and related anomalous vertical motion were the intraseasonal temperature horizontal advection by seasonal mean winds and the seasonal mean temperature advection by intraseasonal winds. The wet episode of the QBWO provides a favorable circulation background for synoptic-scale disturbances, conducive to generating successive extreme heavy rain events. Significantly negative correlations were found in the Western Equatorial Pacific (WEP) when the stream function anomalies led the SC rainfall anomalies by 5 days, which implies that the quasi-biweekly variation of the circulation over WEP is indicative to forecast the intraseasonal rainfall over SC.Wavelet and power spectra demonstrate that the intraseasonal variability of the SC rainfall during the 2002 summer was characterized by significantly QBWO with a period of 10-30 days. Composite analyses show that such a QBWO of the SC rainfall was not only related to the alternately occurrence of an anomalous anticyclone(cyclone) in the lower troposphere over the coastal region of SC that modulate the western North Pacific subtropical high going into (out of) the South China Sea, but also dependent largely on the extratropical atmospheric oscillation in the middle and upper troposphere. The QBWO in the upper troposphere was characterized by a southeastward-propagating wave train consisting of a series of anomalous cyclones and anticyclones, which were aligned in a northwest-southeast direction over Eurasian continent. When the anomalous anticyclone and cyclone were respectively located over the Tibetan Plateau and northern China as the wave train moved, the upper tropospheric divergence between this dipole anomaly pattern was favorable for the development of ascending motion over SC, leading to above-normal rainfall;Conversely, the below-normal rainfall occurred over SC. For the transition between dry and wet episodes of the QBWO, both the tropospheric horizontal temperature advection anomalies and upper tropospheric absolute vorticity anomalies in the subtropics due to southward migration of the wave train are the dominant factors leading to alternation between ascending and descending motions over SC as well as reversals of the resultantly coupled meridional circulations,with local anomalous sensible heating having some contributions to a certain extent. Significantly positive correlations were found when the 500-hPa geopotential height anomalies over the Tibetan Plateau led the SC rainfall anomalies by 4 days (that is,a phase lag of 1/4 period of the QBWO), which imply that the 500-hPa geopotential height anomalies around the Tibetan Plateau may be a potential predictor to forecast the intraseasonal variation of rainfall over SC.