Upper-Level Features Associated with Winter Monsoon Surges over South China

Abstract

The upper-level features associated with the two kinds of winter monsoon surges over south China are studied: the easterly surge (ES) and the northerly surge (NS). The study is similar to that used by Wu and Chan, except that a broader region (0°?60°N, 70°?160°E) is considered. The outbreak of an NS is associated with a breakdown of the Siberia?Mongolia high. The upper-level features suggest that the evolution of the Siberia?Mongolia high in an NS can be related to an eastward passage of a short-wave trough and the polar jet based on the quasigeostrophic theory. The intensification of the Siberia?Mongolia high appears likely to be caused mainly by the adiabatic cooling due to rising motion associated with the trough and the polar jet passages. After the passage of the trough and the jet, the Siberia?Mongolia high breaks down in response to the sinking motion upstream of the trough, causing a surge of the northerly winds over the south China coast. For the ES, the passage of an upper ridge is observed. The zonal index increases in an ES but the subtropical jet weakens. The high pressure center responsible for the ES (the Dahingganling high) is found to be a split cell from the Siberia?Mongolia high, and the splitting is related to the ridge passage. A temperature inversion is only observed when the high is not far from the parent high (i.e., Siberia?Mongolia high). Unlike the NS, an ES is found not to be associated with a strong north?south thermal contrast. Significant differences are observed when comparing the features of the ES and NS. On the whole, the results from this study and those from Wu and Chan suggest that on the synoptic scale a clear distinction exists between the ES and NS on the synoptic scale both at upper levels and the surface.