Variation Characteristics Of The Winter Quasi-stationary Front In South China And Its Relationship With Precipitation

The winter quasi-stationary front in South China is believed largely responsible forabnormal precipitation over South China, but it has rarely been elaborated in previousliterature, especially from a climatological perspective. The present study defines an index tomeasure the strength of the South China quasi-stationary front (SCSF), and then investigatesthe structure and characteristic circulation of strong SCSFs in relation to winter precipitationover South China based on the NCEP global final analysis (FNL) data in2000－2011and theNCEP/NCAR reanalysis data in1951－2011. The climatological characteristics of the winterSCSF and its interannual variability are also analyzed by composite analysis.The results show that strong SCSFs are almost west-east oriented, more common inJanuary and February, and its frequency of occurrence presents a clear upward trend in thepast12winters. The frontal zone of a strong SCSF is indicated by a dense band of potentialpseudo-equivalent temperature in the lower troposphere, slanting northward with altitude,with obvious differences across the band in temperature rather than in moisture. In addition,the frontal zone is often associated with great temperature inversion, positive relative vorticity,and moisture flux convergence of north and south winds. Updrafts mainly appear above theslanted frontal zone, and disturbance zonal circulations with sub-frontal scale updrafts anddowndrafts exist at about850hPa below the upper-level unanimous westerlies.Strong SCSFs can be classified into three types according to the wind convergencepattern in the frontal zone at850hPa. Among the three styles, the northerly convergence typeis characterized with stronger cold air from the north and less precipitation in South China;the southerly convergence type is featured with strong south winds, a deeper India-Burmatrough, and more precipitation in South China; the northerly and southerly convergence typefalls in between. Winter rainfall in South China is closely related to winter SCSF. In thepresence of strong SCSFs, stronger moisture flux convergence and enhanced ascendingmotion appear in the lower frontal zone over South China, in accompany with a deeperIndia-Burma trough in the mid troposphere to the west of the SCSF, leading to increasedrainfall in South China.No clear linear trend is found with the variations of Iscsfover the period1951－2011, butinterannual variability is markedly significant. Anomalies in the atmospheric circulations andlocal precipitation in strong and weak SCSF years are analyzed to further delineate theimpacts of the SCSF on winter climate in South China. Composite analysis reveals that stronger cold air from stronger Siberian cold high and stronger East Asia trough oftenaccompanies a stronger SCSF. More importantly, lower SLP in the South China Sea,intensified low-level cyclonic circulation, increased moisture flux convergence, and largervertical motion occur over South China during strong SCSF years. These cause moreprecipitation in South China; and vice versa.Further analysis illustrates that the variations of SCSF intensity in winter may be relatedto the East Asian trough and the SST around the South China Sea. Stronger East Asian troughprovides stronger downward flow in the north while higher SST around the South China Seafacilitates formation of a stronger upward flow in the south, both conducive to the verticalcirculation of the SCSF.