Precipitation and Moisture Balance of the Asian Summer Monsoon in 1991

Part II: Moisture Transport and Moisture Balance

Abstract

Features of the moisture transport and the moisture balance within the Asian summer monsoon region in 1991 are studied utilizing the 24-hour prediction data by a global weather prediction model in relation to changes of precipitation and the major circulation systems (CSs) defined in Part I of the present paper.
The change during May, June and July is characterized by a shift of the intense rainfall areas, with the strong moisture sink from the equatorial zone over the Indian Ocean and Indonesia, to the subtropical area of the Indian subcontinent and East Asia. The southwesterly moisture transport in 30-80°E is mainly due to CS-3 (the clockwise circulation over the Indian Ocean), whereas the southwesterly moisture transport in 80-110°E and 110-140°E is mainly due to CS-4 (clockwise circulation around Indonesia) and CS-5 (circulation around the Pacific subtropical anticyclone), respectively. The confluence between the adjacent CSs yields strong southerly moisture transport into the monsoon rainfall areas and large moisture-flux convergence.
It is an important fact that the seasonal variation of the moisture flux crossing the boundary of a large domain, which is bounded by 10°S, 40°N, 35°E and 140°E, and the variation of cross equatorial moisture transport from the southern hemisphere, are significantly smaller than the variations of the moisture transport within the domain. The moisture balance calculation also indicates that the essential feature of the summer monsoon is the formation of a pair of moisture source region, and the adjacent moisture sink region, within the large domain. The moisture source regions form under the subsidence and convective stable stratification. The moisture sink regions appear under the ascent motion and convective unstable stratification.
Several parameters such as “moisture influx ratio” and “rainfall production ratio” are defined to discern the characteristics of the moisture balance. The temporal and spatial variations of these parameters depict well the features of moisture balance within the Asia summer monsoon region.