Major Factors Governing the Trends and Interannual Variability in the Occurrences of Mixed Rossby-Gravity Wave Events

Mixed Rossby-Gravity (MRG) Waves are westward propagating synoptic scale equatorial disturbances which play a crucial role in the formation of tropical cyclones and tropical depressions, and they constitute a significant part of various modes of tropical variability such as the Madden-Julian Oscillation (MJO) and Quasi-Biennial Oscillation (QBO). In this study, we have investigated the trends and Inter Annual Variability (IAV) in the occurrence of upper tropospheric MRG events using ERA-I reanalysis data for the period 1979-2018. The MRG events are identified by projecting the upper tropospheric meridional winds onto the theoretical spatial structure of MRG waves. A steady increasing trend is observed in the occurrence of MRG events which is contributed by the MRG events associated with the intrusion of extratropical disturbances. The possible factors that govern the observed trends and IAV in the occurrences of MRG events are El Nino Southern Oscillation (ENSO), MJO and extratropical forcing. The MRG events over the central and Eastern Pacific contribute maximum to the IAV. ENSO explains about 25% of the IAV. It exhibits a positive correlation with non-intrusion MRG events and a negative correlation with intrusion MRG events. These observations have been investigated by exploring the strength and the extent of the westerly duct at 200 hPa and the Outgoing Longwave Radiation (OLR) during El Nino and La Nina years over the central-Eastern Pacific ocean. The convectively active state of the MJO over the Western Pacific explains 20% of the IAV over the central-Eastern Pacific. Besides ENSO, MJO exhibits a diametrically opposite correlation with intrusion and non-intrusion MRG events. The antisymmetric heating with respect to the equator, associated with the MJO, enhances non-intrusion MRG events. The subtropical easterlies forbid the intrusion of extratropical disturbances, thereby lowering the occurrences of intrusion MRG events. The increasing trend in the intrusion of extratropical disturbances explains the observed trend in the upper tropospheric MRG events. Such an increasing trend is not observed in the strength or extent of the upper tropospheric westerly duct over the central-Eastern Pacific.