Northeasterly Cold Surges and Near-Equatorial Disturbances over the Winter MONEX Area during December 1974. Part I: Synoptic Aspects

Abstract

This is a pre-Winter Monsoon Experiment (Winter MONEX) pilot study to examine the possible interactions between the northeasterly cold surges off the Asia continent and the convective disturbances in the near equatorial region. Based on surface and 850 mb wind and temperature analyses, satellite data, and synoptic weather charts of the Hong Kong Royal Observatory, a sequence of synoptic events associated with two cold air surges and near-equatorial disturbances over the Winter MONEX area of South China Sea and its vicinity during December 1974 is discussed. The results lead to the tentative conclusions that due to the varying degree of air-sea interactions between cold air originating from the southeastern China coast, Taiwan and Luzon Straits, and that originating from the South China coast, the near-equatorial latitudes of the South China Sea will experience a freshening of the low-level northeasterly monsoon Winds prior to a decrease in surface temperature which, if it occurs, is confined to the Western portion of the South China Sea. This allows a near-equatorial disturbance (which may have originated from the semi-stationary near-equatorial trough over the coast of North Borneo or from a westward propagating wave in the western Pacific) to be intensified at an early state of the surge by enhanced low-level convergence and organised deep cumulus convention. Afterward it may be weakened by either the cold air incursion along the Vietnam coast or a slackening of the northeasterlies.