Large-scale Aspects of the 1978-79 Winter Circulation over the Greater WMONEX Region

Part I: Monthly and Season Mean Fields1

Abstract

An objective analysis scheme based on an extension of the correction method was applied to the unprecedented volume of FGGE/WMONEX Level IIa and IIb wind data at 850 and 200mb over the greater WMONEX region (45°N-30°S, 40°E-100°W) for the 1978-79 winter (1 December 1978 to 28 February 1979). Some of the characteristic features of the 1978-79 winter circulation were investigated by examining the monthly and season mean zonal and meridional winds, and streamfunction (vorticity) and velocity potential (divergence) fields.
The season mean 850mb wind field revealed a well-defined zone of equatorial westerlies which extended between the equator and 10°S, from the Indian Ocean (80°E) to the central South Pacific (170°E). These westerlies probably denote the extent of the Southern Hemisphere summer monsoon region during this period. In addition, the eastern end of the nearequatorial trough at around 10°S, 170°E may be a major heat source region for the Southern Hemisphere monsoon, as it was dominated by a pronounced low-(upper-) level convergent (divergent) center.
Over the Indonesian Seas-western South Pacific area between 5° and 15°S, monthly mean near-equatorial westerlies at 850mb exhibited a substantial increase from December to January. However, this intensification of the Southern Hemisphere near-equatorial westerlies was not due to 850mb northerly surges over the Sumatra-Borneo-New Guinea region, as these cross-equatorial flows from the Northern Hemisphere were of the same general intensity throughout the season. In contrast, significant month-to-month changes were noted in the 850mb cross-equatorial flows near the dateline.
At 200mb, strong upper-level westerlies over the Australian continent in December were replaced by easterlies as a weak anticyclonic cell developed over the region in January. Interestingly, this upper-level anticyclonic cell (20°S, 130°E) was located apporximately 40° to the west of a major 200mb divergent center (°S, 170°E) in January.