Impact of environmental variables on the North Indian Ocean tropical cyclones radial parameters

The present study elucidates the correspondence between the variability in TC radial parameters and related meteorological factors over the North Indian Ocean (NIO) basin by using the scatterometer data, ERA5 (5th Generation ECMWF Reanalysis), and NCEP (National Centers for Environmental Prediction, USA) reanalyses datasets and the environmental datasets from the National Oceanic and Atmospheric Administration, USA and Indian Space Research Organisation and TC best track datasets from India Meteorological Department and Joint Typhoon Warning Center (JTWC), Japan. The study illustrates their annual and seasonal variation and isolates the possible relationships with various environmental and storm-related features during 1982–2019 at different time frames depending upon the availability of datasets. The years 1982 and 2019 experienced the highest number of highly intensified TCs (wind speed > 46 m/s). Although the bi-modal characteristics of NIO TC seasons are perceived to be during pre- and post-monsoon months, the monthly occurrences during 1982–2019 suggest that May, June, October, and November are the higher breading months. Considering the TC cases during 2001–2019, it is observed that the size parameters exhibit an increasing trend, whereas Rmax (radius of maximum wind) shows a decreasing tendency. On performing the Mann Kendall test, an increasing trend is realized in the case of R50 (radius of 50 knots wind), but not for other parameters. An increasing trend is visible for both PDI (power dissipative index) and ACE (accumulated cyclone energy) with the TC size, but they were found to be decreasing in the case of Rmax. This positive tendency supported the increase in TC intensity and significantly impacted ACE, PDI, and TC size parameters during 2004–2019. A positive or increasing trend for JTWC and scatterometer estimated TC size with the sea surface and lower tropospheric temperature values is observed, while a decreasing trend for sea level pressure and lower stratosphere temperature is seen. Stronger and positive feedback of mid-tropospheric relative humidity (RH) for governing the variability of TC size is found, but a weaker correlation is realized with the near-surface air temperature and no or negligible correspondence with the low-level RH and vertical wind shear. Changes in outgoing long-wave radiation and near-surface omega appear to positively correlate with TC size parameters, while sea surface salinity (SSS) and mid-tropospheric omega variations have a negligible impact after 2005. There is a modest association between SSS and Rmax change. Comparatively larger TC sizes and Rmax are realized in the La Niña period during 2001–2019, but the corresponding ranges are nearly analogous during both positive and negative Indian Ocean Dipole (IOD) scenarios. However, IOD has a substantial role in determining TC radial characteristics across the NIO basin after 2005 compared to El Niño-Southern Oscillation (ENSO).