Suppression of pre-monsoon tropical cyclones over the north Indian Ocean

Abstract

Abstract

Pre-monsoon (April-June) tropical cyclones (TC) over the North Indian Ocean (NIO) pose severe coastal hazards. Surprisingly, in recent years like in 2025 and 2026 pre monsoon season there was not a single named TC reported over NIO. In this study, we explored the drivers for such a suppressed activity. We considered 46 years (1980-2025) of India Meteorological Department (IMD) best-track data and found that such situation occurred in 12 specific years during the analysis period. We considered those 12 years as non-TC years and to examine the contrast, we identified 10 TC-active years, where at least one TC occurred in both Bay of Bengal (BoB) and Arabian sea (AS) basins. Genesis Potential Parameter (GPP) composites reveal a basin-wide suppression during non-TC years, with a statistically significant difference of 1.0 × 10⁻⁵ concentrated over the BoB and AS. Logarithmic decomposition of the GPP shows that low-level vorticity is the primary driver of enhanced GPP over both basins during TC years, while reduced vertical wind shear provides additional positive support over the BoB. In general, the conditions are less favourable for the TC formation over the AS and are favourable over the BoB. But, during TC years, warmer SST, higher mid-level moisture and strong low-level positive vorticity supported the TC activity over the AS. Enhanced atmospheric moisture transport (IVT) and stronger large-scale upper-tropospheric diabatic heating during TC years reflect a pre-existing convective environment driven by organized large-scale forcing rather than TC-induced latent heat release. We further identify the MJO as the dominant large-scale modulator of pre-monsoon cyclogenesis, with TC activity systematically phase-locked to long duration and high-amplitude MJO phases 2-4 during active years, while suppressed years are characterized by weak or inactive MJO convective envelopes (RMM amplitude < 1.0). Wavenumber-frequency spectral analysis exhibits enhanced power in equatorial Rossby, Kelvin, and MRG waves alongside active MJO during TC years. Analysis of ENSO and IOD states reveals no systematic difference between TC and non-TC years. These findings suggest role of intraseasonal variability in modulating the large-scale kinematic forcing, alongside local thermodynamic conditions behind the cyclogenesis activity over the NIO.