Re-Visiting Monsoon Intra-Seasonal Variability from the perspective of Large-Scale Atmospheric Energetics

Abstract In this pilot study, an attempt has been made to revisit the problem of monsoon intra-seasonal variability (MISV) of the southwest monsoon from the perspective of large-scale atmospheric energetics. For that, daily values of two energy generation terms and four energy conversion terms for the period from 1 May to 30 September for the years 1979–2017, obtained from Dutta et al. (2019), have been used. These generation and conversion terms were computed using high-resolution (0.125° × 0.125° ) daily re-analysed ERA-5 data of air temperature (T) and three components of wind () at standard isobaric levels, viz., 1000, 925,850, 700, 600, 500, 400, 300, 250, 200, 150, 100 hPa levels at 00 UTC over the domain 5°N–35°N, 65°E–95°E, during the above-mentioned period. Results of this study suggest that the variability of atmospheric energetics of the Indian Summer Monsoon is fundamentally dominated by synoptic-scale variability (2–8 days) across all rainfall categories. At the same time, the occurrence, persistence, and intensity of lower-frequency oscillations (QBW, EQBW, MISO) vary systematically with monsoon strength. Energy generation terms are predominantly governed by synoptic disturbances, highlighting the primary role of monsoon depressions, low-pressure systems, and associated mesoscale weather systems in modulating the monsoon energy cycle. Energy conversion terms likewise exhibit statistically significant synoptic-scale variability in every ISMR category, suggesting that rapid baroclinic and barotropic energy conversions constitute the dominant mode of monsoon energetics irrespective of seasonal rainfall. Overall, the results suggest that while synoptic disturbances dominate the monsoon energy cycle across all ISMR categories, the persistence and organization of quasi-biweekly and MISO-scale variability largely determine the efficiency and longevity of atmospheric energy generation and conversion in stronger monsoon years.