Trends in Atmos. Sci., 1, 349-373
Department of Meteorology, School of Ocean and Earth Science and Technology
University of Hawaii at Manoa, Honolulu, Hawaii ABSTRACT
Madden-Julian Oscillation (MJO) is hypothesized to be a result of collective activity of planetary-scale tropical intraseasonal systems (TIS). This paper reviews theoretical studies of MJO, discusses various dynamic aspects of TIS, and presents some outstanding issues that require further investigations. In particular, a unified theoretical framework is proposed for study of the basic dynamics of TIS with which an Ekman-wave-CISK theory is advanced to explain the origin of TIS.
The Ekman-wave-CISK is a low-frequency instability with a typical growth rate on an order of 10-6 s-1. The unstable mode has a multi-scale wave packet structure: Global-scale circulation couples with a large-scale (several thousand km) precipitation complex. The latter in turn consists of a few synoptic-scale (one thousand km) precipitation cells that are accompanied by boundary-layer equatorial westerly surges. The wave packet moves eastward with a group speed slower than the phase speed of individual cells. The theory provides new physical insights into some observed key features of TIS, such as their low-frequency development/decay, multi-scale structure, slow eastward movement, longitude-dependent life cycle, and seasonality.
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