ENSO-phase dependent TD and MRG wave activity in the western North Pacific
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The three-dimensional structure and evolution characteristics of tropical depression (TD) and mixed Rossby-gravity wave (MRG) type disturbances in the tropical western North Pacific during El Niño and La Niña summers are investigated based on observational and reanalysis data. A clear MRG-to-TD transition was observed during El Niño summers while such a transition is unclear during La Niña summers. The vertical structure of the TD-MRG waves appears equivalent barotropic during El Niño but becomes tilted eastward with height during La Niña. The diagnosis of barotropic energy conversion shows that both the rotational and divergent components of the background flow change associated with E1 Niño-Southern Oscillation (ENSO) are responsible for energy conversion from the mean flow to the TD-MRG perturbations. A further examination of the pure MRG mode shows that its intensity does not vary between El Niño and La Niña while its phase speed does. A faster (slower) westward propagation speed during La Niña (El Niña) is attributed to enhanced (reduced) mean easterlies in the western equatorial Pacific. The heating associated with the MRG wave appears more anti-symmetric during La Niña than during El Niño. In contrast to the MRG waves, the amplitude of the TD waves depends greatly on the ENSO phase. The enhanced (suppressed) TD disturbances during El Niño (La Niña) is attributed to greater (less) barotropic energy conversion associated with the background flow change. The vertical structure of the TD waves appears quasi-barotropic in the geopotential height field but baroclinic in the divergence field.
KeywordsENSO MRG waves TD waves Western North Pacific
This work is jointly supported by the National Natural Science Foundation of China Grant 41205052, 41230527, and 41175076, the Knowledge Innovation Program of the Chinese Academy of Sciences Grant KZCX2-EW-QN204, and the Special Scientific Research Project for Public Interest Grant GYHY201006021. TL was supported by ONR Grants N000140810256 and N000141210450 and JAMSTEC/NOAA/NASA. This is SOEST contribution number 8914 and IPRC contribution number 974.
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