J. Atmos. Sci., 50, 184-199
Baozhen ZHU and Bin WANG
Department of Meteorology, School of Ocean and Earth Science and Technology
University of Hawaii at Manoa, Honolulu, Hawaii
(Manuscript received 5 December 1990, in final form 28 February 1992)
The tropical Indian and western Pacific oceans are two prominent action centers for tropical 30-60-day convective variability. When convection is enhanced over the equatorial Indian Ocean, the tropical western Pacific often experiences an abnormal dry condition (phase I), whereas the development of the convection over the tropical western Pacific tends to be accompanied by suppressed convection in the equatorial Indian Ocean (phase II). This convection seesaw is a fundamental characteristic of the tropical 30-60-day oscillation.
The seesaw is intimately associated with the activity of propagating low-frequency convective systems (LFCSs). Its formation process is season dependent. Typical boreal summer seesaw results from a time-lagged development of two systems: a western system that originates in the equatorial Indian Ocean and moves eastward and/or northward and an eastern system that develops in the western Pacific monsoon region and moves westward and/or northward. The boreal winter seesaw, on the other hand, is caused by the longitudinal dependence of the evolution of eastward-moving LFCSs that strongly amplify in the equatorial Indian Ocean, weaken and/or spilt when rapidly passing over the maritime continent, and reintensify in the South Pacific convergence zone (SPCZ).
There are two phases of the seesaw. During the first phase, the LFCSs interact with the Indian monsoon in boreal summer and Indonesian-Australian monsoon in boreal winter. Likewise, during the second phase, the LFCSs interplay with monsoon circulations over the western Pacific monsoon through in boreal summer and over the SPCZ in boreal winter. The convection seesaw activity is closely tied to the corresponding active-break monsoon cycles over the two polar regions of the seesaw.
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