Exploring the Factors Influencing the Strength and Variability of Convectively Coupled Mixed Rossby–Gravity Waves

Abstract

The three-dimensional structure, horizontal and vertical propagation characteristics, and convection–circulation coupling of the convectively coupled westward-propagating mixed Rossby–gravity (MRG) waves are examined by classifying the waves based on their amplitude. Convective signals of the MRG waves were identified and isolated using empirical orthogonal function analysis of wavenumber–frequency-filtered outgoing longwave radiation (OLR) data. It was found that about 50% of the MRG waves occur during the August–November months, and this strong seasonality was considered while characterizing the MRG waves. Five strong and five weak MRG wave seasons were identified during 1979–2019, based on seasonal wave amplitude, and through this classification, significant differences in the strength of convection–circulation coupling, zonal scale of circulation, vertical structure, and propagation characteristics of MRG waves were brought out. It was also found that the seasonal mean background state is significantly different during strong and weak MRG wave seasons. While a La Niña–like background state was found to favor enhanced MRG wave activity, the MRG wave activity is mostly suppressed during an El Niño–like background state. The presence of extratropical wave intrusions is another factor that distinguishes the strong MRG wave seasons from the weak ones. Eastward- and northeastward-propagating extratropical wave trains from the South Atlantic to the east Indian Ocean were observed during strong MRG wave seasons.