Summer Monsoon Experiment—A Review

Abstract

This paper presents a short summary of the Summer Monsoon Experiment (MONEX). The review is largely based on those papers that have made use of the summer MONEX observations during 1979. 0bservational aspects of this study emphasize the annual march of the monsoon rainfall belt from Indonesia to the foothills of the Himalayas, from the northern winter to the northern summer season and a reverse motion thereafter. The excellent FGGE/MONEX data sets have provided a detailed definition of the divergent wind; these are summarized with reference to the Hadley and the Walker circulations. The manner in which monsoonal circulations respond to the evolving differential heating fields are presented via the mutual interactions among the rotational and divergent wind components. Specific examples of heat sources from the studies of Luo and Yanai highlight their contrast over different regions of the monsoon including the Tibetan Plateau. A problem of considerable interest in this context is the cooling of the Arabian Sea. A summary of results pertaining to this problem?especially the distribution of the wind stress curl?is highlighted. The planetary boundary layer is another area of investigation which has drawn much interest, especially over the western Arabian Sea where the Somali jet exhibits interesting properties during summer monsoon. These studies cover modeling, theoretical and observational areas. The onset and active monsoons were monitored by a large array of ship and research aircraft during MONEX. Studies in this area place an emphasis on observational, theoretical stability analysis and numerical weather prediction. The major results with respect to medium range prediction of the onset of monsoon and the formation and motion of a monsoon depression are summarized in the review. A component of the MONEX observational program that is examined is the structure and maintenance of desert heat lows. A summary of these results includes the structure of the mixed layer, the day-night differences in the vertical motion profiles and the thermodynamic heat budget. The final section of this review includes studies on low frequency modes?especially on the time scale of 30 to 50 days. It is becoming apparent that modulations of active and inactive spells of the monsoon are related to wave motions on this time scale. These MONEX data sets provide a strong signal for monitoring these waves. These wave motions on the planetary scale move eastward; on a more regional scale they move northward over the monsoon region. Their behavior is illustrated with respect to the onset, active and break monsoons.