Tropical Circulations Associated with Southwest Monsoon Onset and Westerly Surges over the South China Sea
Chen, George T.-J.
MetadataShow full item record
The earliest onset of the Asian summer monsoon occurs in early to middle May over the South China Sea. This onset is signified by the development of low-level westerlies and leads to heavy convective rainfall over southern China (pre-Mei-Yu). In June, low-level westerly surges over the northern South China Sea are associated with the Mei-Yu rainfall system in the Yangtze region and southern Japan. In this work, the ECMWF data for 1981-86 are used to study the tropical circulations associated with the development of low-level westerlies during both events. Composites of horizontal wind, geopotential height, moisture, and vertical velocity during six May onsets and nine lune surges, respectively, indicate that both events occur with the approach of a midlatitude trough-front system. The possible triggering of the South China Sea summer monsoon onset by the midlatitude system may explain why the South China Sea onset occurs prior to other regions of the Asian monsoon. During boreal spring, this is the only Asian monsoon region where midlatitude fronts can move into the Tropics without having to overcome significant terrain barriers. Following the two events, opposite teleconnection-like patterns develop in the Tropics in both hemispheres. During the May onsets, the arrival of the midlatitude trough/front appears to lead to a southwestward extension of a cyclogenesis zone into the equatorial Indian Ocean. Along this zone, cyclonic vortices develop over the Andaman Sea, the Bay of Bengal, and perhaps the southern equatorial Indian Ocean, and increased deep convection is indicated by the OLR composites. During the June surges, a pair of anticyclones develop straddling the equator at the longitudes of Indochina. This anticyclonic couplet is associated with decreased deep convection and propagates westward to dominate the flow changes over the Bay of Bengal and the southern Indian Ocean. The steady 4-5 m S-I westward speed and near-perfect symmetry with respect to the equator indicate the possibility of an equatorial Rossby wave generation in conjunction with the June westerly surges in the northern South China Sea.
Showing items related by title, author, creator and subject.
The influence of Large-scale 200 mb tropical divergence events on the midlatitude zonal flow over the Asia-Pacific region during the 1983-84 winter Keng-Gaik, Lum (1985-06);Global band analysis grid point data produced by FNOC for the 1983/1984 winter season are used to study the daily variation of the mid-latitude 200 mb zonal wind in relation to the divergence field. It is found that the ...
Chang, C.-P.; Li, Tim (2000-07-15);The key questions of how the tropospheric biennial oscillation (TBO) maintains the same phase from northern summer in South Asia to southern summer in Australia, and how the reversed phase can last through three locally ...
Southern Hemisphere Application of the Systematic Approach to Tropical Cyclone Track Forecasting. Part IV : sources of large track errors by dynamical models Reader, Grahame; Boothe, Mark A.; Elsberry, Russell L.; Carr, Lester E. (Monterey, California. Naval Postgraduate School, 2000-08); NPS-MR-00-004Sources of 72-h track errors > 300 n mi by four dynamical model tropical cyclone predictions in the Southern Hemisphere during the 1997-98 and 1998-99 seasons are studied using conceptual models Carr and Elsberry have ...