Atmospheric Mixed Rossby Gravity Waves over Tropical Pacific during the Austral Summer

Abstract. Atmospheric Mixed Rossby-Gravity Wave (MRGW) activity during the austral summer months (Dec-Jan-Feb) 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 wavenumber 5–6, 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, and the wind field and 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 modulates convective activity. Tropical convection develops in the divergent region of the MRGW at 200 hPa and in the MRGW moisture convergence region at 700 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 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.