Investigating the Effect of Tropical Cyclones on Atmospheric Chemistry in the Upper Troposphere

Tropical cyclones (TCs) containing widespread and intense convection, play a dominant role in stratosphere-troposphere exchange (STE) processes in the upper troposphere and lower stratosphere (UTLS) region. Here we examine the variation of meteorological and chemical fields associated with two different pre-monsoon tropical cyclones: MORA and FANI, by combining satellite-based observations from AIRS (The Atmospheric Infrared Sounder ) and different model reanalysis datasets from ERA5 (fifth generation of ECMWF atmospheric reanalyses), CAMS (Copernicus Atmosphere Monitoring Service), MERRA-2 (The Modern-Era Retrospective analysis for Research and Applications, Version 2), and NCEP (National Centers for Environmental Prediction). An increase in the upper-tropospheric ozone (O₃) by 15– 30 ppbv is observed over the Bay of Bengal during the high phase of MORA cyclone. Intrusion of O₃ from lower stratosphere to upper troposphere is clearly observed from 50 to 300 hPa during the cyclonic period, contributing enhancement in the upper tropospheric O₃. There are no such indication of enhanced O₃ values before and after the dissipation of MORA cyclone. General behavior of intrusion associated with severe MORA cyclone is well captured by all the models and satellite, however some differences are seen in the intensity and structure of the STE events. Strong updrafts and downdrafts present in the vicinity of tropopause during TC passage weakened the stability of tropopause layer. The low tropopause temperature with enhanced potential vorticity (PV) feature extended vertically downward from lower stratosphere to troposphere confirms the stratosphere to tropospheric intrusion during the cyclonic period. Concurrently, low relative humidity (RH) along with negative RH-O₃ correlation during the overhead cyclone further supports the intrusion. Contrarily, satellite and model results revealed no significant variation in O₃ mixing ratio in the lower stratosphere down to the tropopause level during the high phase of extremely severe FANI cyclone. Strong convective activity during the passage of FANI confirms the upward propagation of CO rich (O₃ poor) air masses from surface to the mid/upper troposphere. The air masses are then trapped by anticyclone around the tropopause levels. This study clearly reveals that tropical cyclones play major role in exchanges of mass and energy between the stratosphere and troposphere (and vice versa) besides being general weather phenomena.