ENSO-phase dependent TD and MRG wave activity in the western North Pacific

Abstract

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 Nino and La Nina summers are investigated based on observational and reanalysis data. A clear MRG-to-TD transition was observed during El Nino summers while such a transition is unclear during La Nina summers. The vertical structure of the TD-MRG waves appears equivalent barotropic during El Nino but becomes tilted eastward with height during La Nina. The diagnosis of barotropic energy conversion shows that both the rotational and divergent components of the background flow change associated with E1 Nino-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 Nino and La Nina while its phase speed does. A faster (slower) westward propagation speed during La Nina (El Nina) 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 Nina than during El Nino. 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 Nino (La Nina) 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.