Bin WANG and Xiaosu XIE
Department of Meteorology, University of Hawaii, Honolulu, Hawaii
Over the warm pool of the equatorial Indian and wester Pacific Ocean, both the climatological mean state and the processes of atmosphere-ocean interaction differ fundamentally from their counterparts over the cold tongue of the equatorial eastern Pacific. A model suitable for studying the coupled instability in both the warm pool and cold tongue regimes is advanced. The model emphasizes ocean mixed-layer physics and thermodynamical coupling that are essential for the warm pool regimes. Different coupled unstable modes are found under each regime.
In contrast to the cold tongue basic state which favors coupled unstable low-frequency SST mode, the warm pool regime (moderate mean surface westerlies and deep thermocline) is conducive for high-frequency (intraseasonal time scale) coupled unstable modes. The wind-mixed layer interaction through entrainment/evaporation plays a central role in the warm pool instability. The cloud-radiation feedback enhances the instability, whereas the ocean wave dynamics have little impact. The thermodynamic coupling between the atmosphere and ocean mixed layer results in positive SST anomaly leading convection, which provides eddy available potential energy for growing coupled mode. The relatively slow mixed-layer response to atmospheric forcing favors the growth of planetary-scale coupled modes. The presence of mean westerlies suppresses the low-frequency SST modes.
The characteristics of the eastward propagating coupled mode of the warm pool system compares favorably with the large scale features of the observed Madden-Julian Oscillation (MJO). This suggest that, in addition to atmospheric internal dynamic instability, the ocean mixed layer thermodynamic processes interacting with the atmosphere may play an active part in sustaining the MJO by (a) destabilizing atmospheric moist Kelvin waves, (b) providing a long-wave selection mechanism, and © slowing down phase propagation and setting up the 40-50 day time scale.
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