A statistical study of winter monsoon cold surges over the South China Sea and the large-scale equatorial divergence

Abstract

A 14-winter (1974/75-1987/88, November-March) tropical global band data set is used to study the relationship between the occurrence of cold surges over the South China Sea and the variation of the 200mb and surface tropical divergence from the western Pacific to the eastern Indian Ocean. The data are truncated by 12 empirical orthogonal function modes. A canonical correlation analysis (CCA) between the surface meridional winds over the South China sea between the surge onset and 48h later is used to define a basic cold surge mode. This mode represents the spatial- and time-coherent variations of cold surge events and is constructed from 8 CCA modes which contain approximately half of the total variance of the surface northerly winds. Lag correlations between this mode and the surface meridional velocity, 200mb velocity potential and surface velocity potential, respectively, are computed to examine the effect of the surges. The results show that, within 72h of the surge onset, the anomalous surface northerlies reach 5m/s centered in the northern South China Sea, and the anomalous southerlies reach 2m/s over Burma. They represent about 70% and 30% of the total variance of the local surface meridional winds in the two areas, respectively. The cold surge-related 200mb tropical divergence variations have two centers. The primary one is located in the equatorial South China Sea between the Malay Peninsula, Sumatra and Borneo, which is about 50° to the west of the long-term winter mean divergence center in the equatorial western Pacific around 155°E. The secondary center is over the Bay of Bengal. Divergence around both centers increases after the surge onset, with the primary one accounting for >25% of the local variance after 5 days, and increasing to 40-50% if the surge lasts 7-8 days. The magnitude of the increased divergence in the equatorial South China Sea is positively correlated with the surge intensity only during the first three days, while that in the Bay of Bengal is correlated with the surge intensity throughout the surge period. At the surface, anomalous tropical convergence increases during a surge event and develops into two centers. The first one is over northwestern Borneo near the 200mb anomalous divergence center. It probably reflects the importance of the diurnal Borneo convection system. The other surface center is in the eastern Indian Ocean south of the Bay of Bengal. At both centers the surge-related fractional variances are approximately 40% two days after the onset, and become >50% five days after the onset. There is no indication of cold surge influences on the long-term mean divergence center in the equatorial western Pacific.