Transition from the Southern to Northern Hemisphere Summer Monsoon

Abstract

During the period from 15 February to 20 June 1979, the northward migration of the zones of low outgoing longwave radiation was most clearly defined over the convectively active continental regions of equatorial Africa, the maritime continent, and equatorial South America. Regionally, the withdrawal of the Southern Hemisphere summer monsoon over Indonesia and Australia was followed by the first establishment of the summer monsoon over the Malaysian Peninsula during the midtransition around 15 April, which is about two months earlier than the monsoon onset over central India. In investigating the wind changes during the transition period, smoothed time-series data were constructed by adding three components of winds; viz., the annual mean winds, which are approximately symmetric about the equator; and the yearly and half-yearly wind harmonics that are asymmetric about the equator with a distinct seasonal character. The 200 mb wind patterns during the midtransition exhibit new symmetric character with pairs of anticyclones straddling the equator over Africa, the maritime continent, and South America, respectively. Of particular interest is the first establishment of the Northern Hemisphere summer monsoon anticyclone near southern Indochina as a response to symmetric equatorial heat sources over the Sumatra-Borneo-New Guinea region during the midtransition. The yearly component of the winds. which merely reflects the seasonal variations of the Hadley circulation and its associated zonal winds, contributes little to the formation of the Indochina anticyclone. The half-yearly harmonics are primarily responsible for the establishment of the anticyclone, which propagates northward through Indochina, finally reaching the southern periphery of the Tibetan Plateau by midsummer. This study also confirmed that the character of long-period (30?50 day) oscillations changes drastically during the transition period. Prior to about 15 April, the 30?50 day modes are prominent over the Southern Hemisphere tropics (5?15°S). They propagate eastward along the equator over the Indian Ocean. After 15 April, the 30?50 day perturbations exhibit the Northern Hemisphere summer character of systematic eastward propagation and large amplitude over the global tropics between the equator and 20°N.