Atmospheric mixed Rossby-gravity waves over the tropical Pacific during the austral summer

Abstract

Atmospheric mixed Rossby-gravity wave (MRGW) activity during the austral summer months (December-January-February) is examined by means of observational analyses for the 1991-2020 period. The main objective of the study is to explore the relationship between tropical circulations at upper- and lower-tropospheric levels and tropical convective activity. Using an empirical orthogonal function (EOF) analysis of the high-frequency meridional component anomalies of the wind at 200 hPa, for zonal wavenumbers 4-6, episodes of intense MRGW activity are detected. Composite analyses based on an EOF analysis show a quadrature phase over the central eastern equatorial Pacific between the MRGW structure in the upper and lower troposphere. Lagged correlations between the first two EOFs' principal components, as well as the wind field and outgoing longwave radiation (OLR), show that MRGWs are laterally forced at upper-tropospheric levels over the westerly duct region and later propagate westward and downward. Once the MRGW reaches the lower-tropospheric levels, it induces zones of moisture convergence that modulate convective activity. Tropical convection develops in the MRGW moisture convergence region at 700 hPa and the divergent region of the wave at 200 hPa. Since the MRGW phase tilts eastward with height, moisture convergence at lower-tropospheric levels tends to coincide with divergence at upper levels favoring intense convective activity which results in the antisymmetric outgoing longwave radiation anomalies observed off the Equator near the MRGW. Therefore, the occurrence of MRGWs over the eastern Pacific is a form of tropical-extratropical interaction that generates tropical convection anomalies by means of induced lower-tropospheric moisture convergence and divergence anomalies.