A study on anomalous activities of East Asian summer monsoon during 1999

Abstract

Using the NCEP-NCAR reanalysis datasets, the anomalous activities of East Asian summer monsoon for 1999 have been studied, with the following characteristic features identified: 1) the South China Sea (SCS) summer monsoon began at the end of May, which was much later than the normal year (mid-May), with three active and two break phases observed, 2) the northward advance of the summer monsoonal southwesterlies or southerlies in East China was markedly weak with the northernmost position of 32° N, and the monsoonal airflow could not establish itself in the areas north of the Yangtze River, thus leading to a very weak moisture transport and the condition of prolonged drought in North China, 3) correspondingly, the stationary position of seasonal heavy rainfall-belt in East China was considerably farther south than normal, also with the northernmost position being near 32° N, never going beyond the Yangtze River for the whole summer, 4) the low-level southwesterly (SW) airflow from the Bay of Bengal and the tropical Indian Ocean played a very important role in the abnormal activities of the SCS summer monsoon. Three active periods of the SCS monsoon were almost concurrent with the anomalous eastward extension of this airflow, and 5) after the beginning of July, as the Western North Pacific subtropical high (WNPH) (110-150° E, 10-30° N) rapidly shifted eastward up to the region east of Japan, the low-level southwesterly airflow-assumed a marked eastward migration with its leading front reaching the longitudes of 160-170° E, thus bringing about the major moisture transport belt to shift eastward, and anomalous heavy rainfalls to occur around Japan for a long time. The reasons behind these anomalous activities of the 1999 summer monsoon have been examined. From the viewpoint of thermal conditions, the late onset of the summer monsoon this year was likely to be related to late development and reduced intensity of heat source over the Tibetan Plateau, which resulted in the smaller thermal contrast between the atmosphere over the plateau and the atmosphere over its surrounding areas. This could lead to prevalence of the anomalous easterlies in the eastern and southeastern flanks of the plateau, which was not favorable to the normal development of the East Asian summer monsoon (EASM). The continuous northward advance of summer monsoonal southwesterlies was blocked by the anomalous easterlies, so that the SW airflow was confined in lower latitudes for a long time. ©2001, Meteorological Society of Japan.