Abstract
A step-like increase in the 850mb zonal mean westerlies over the equatorial Southern Hemisphere (3.75°-11.25°S, 90°E-180°) occurred between 22-30 December 1978. This event was used to subjectively define the onset for the 1978-79 Southern Hemisphere monsoon. During the onset period, widespread increases in 850mb cross-equatorial northerlies occurred over the western Pacific between 120°E and the dateline, reflecting the intensified low-level Hadley circulation after the onset. Over the maritime continent (Sumatra, Borneo, New Guinea), the large-scale apparent heating rate (Q1) decreased considerably from the preto post-onset phase. By contrast, drastic intensification in Q1 heat sources took place near Cape York Peninsula and the equatorial South Pacific east of Australia, with a maximum heating rate of about 4°C per day at 447mb. The spatial distribution of the large-scale apparent moisture sources (Q2) exhibits a parallel feature to that of the Q1 pattern for both the pre- and post-onset phases. An approximate inverse relationship exists between the large-scale apparent vorticity sources (Z) at 850 and 200mb over the Southern Hemisphere monsoon region. For example, near Cape, York Peninsula, large positive Z values at 850mb are contrasted with pronounced negative Z values at 200mb after the onset.
