West coast India's rainfall is becoming more convective

The study investigates whether rainfall over India’s west coast has been shifting from predominantly stratiform to a more convective character in recent decades, motivated by extreme events such as the 2018–2019 Kerala floods and broader evidence of Indian Ocean warming. Background literature indicates global warming alters tropical precipitation distributions and extremes, with the Indian Ocean—especially the Arabian Sea—warming rapidly since the mid-20th century. Competing hypotheses exist for long-term ISMR changes, including SST gradient changes, aerosols, land-use change, and greenhouse gas–forced uniform Indian Ocean warming that boosts atmospheric moisture. Recent observations suggest a post-2002 strengthening of ISMR linked to enhanced land–sea thermal gradients and ITCZ dynamics. Given the socio-economic stakes, the paper examines whether, during JJAS, cloud and rainfall characteristics over 73–77°E, 8–20°N have evolved toward deeper, more convective systems post-2000, potentially associated with Arabian Sea warming, increased instability, and intensified monsoon circulation.

Study region and epochs: Focus on the west coast of India (73–77°E, 8–20°N) and the eastern Arabian Sea (65–70°E, 5–20°N). Two epochs are compared: pre-2000 (1980–1999) and post-2000 (2000–2019) for JJAS.

Reanalysis data: ERA-5 monthly means at 0.25° resolution (1980–2019, JJAS) for dynamics (horizontal and vertical winds, Omega), thermodynamics (air temperature, specific humidity), sea surface temperature (SST), precipitation, outgoing longwave radiation (OLR), and cloud microphysics (specific cloud ice water, SCIW, and specific cloud liquid water, SCLW) with 50 hPa vertical resolution. Analysis includes spatial composites, anomaly maps (post−pre), and a vertical–meridional cross-section averaged over 73–77°E. Significance testing uses 95% or 99% confidence thresholds (stippled in figures).

Statistical analysis: Interannual time series of SST (eastern Arabian Sea) and lower-tropospheric temperature (LTT; 1000–700 hPa mean over the west coast) correlations; rainfall–OLR interannual regression and comparison to trend regression slopes. Linear trends estimated via least squares; correlations assessed with significance tests.

Observational datasets: Radiosonde CAPE at Mangalore (12.9°N, 74.8°E), twice daily at 0000/1200 UTC during 1980–2019 JJAS; summarized via box plots comparing pre- and post-2000 distributions. Satellite datasets include ISCCP-H deep convective cloud cover (1° grid; JJAS trends 1998–2014), CERES-MODIS cloud-top pressure (1° grid; JJAS, 2000–2019; box plots for 2000–2009 vs 2010–2019), and TRMM 3A25v7 convective rainfall (0.5°×0.5°; JJAS trends 1998–2014). Gauge dataset: IMD daily gridded rainfall (0.25°×0.25°) used to construct PDFs of daily JJAS rainfall (0–400 mm day−1, 50 mm day−1 bins) for pre- vs post-2000.

Comparative diagnostics: Mapped anomalies of SST and low-level winds (850 hPa), and OLR; vertical-meridional sections of Omega and meridional wind, and SCIW/SCLW climatology and anomalies to infer changes in cloud depth and vertical structure. All analyses emphasize differences between epochs to diagnose shifts in convective characteristics beyond changes in mean rainfall amount.

• SST–LTT linkage: JJAS SST anomalies in the eastern Arabian Sea and LTT over the west coast are in phase with a strong correlation r = 0.90 (95% confidence), and both show positive 40-year trends, indicative of forced warming.
• Rainfall and OLR trends: Over the west coast, ERA-5 rainfall shows an upward but not 95% significant trend, while OLR shows a significant downward trend. Rainfall and OLR are highly anti-correlated interannually (r = −0.82), with an interannual regression slope of −3.8 W m−2 per mm day−1. However, the regression of their secular trends yields a much steeper slope of −13.3 W m−2 per mm day−1, suggesting cloud tops are rising and/or cloud shields expanding beyond what would be expected from rainfall increases alone.
• Spatial changes: Post-2000 JJAS SST anomalies are positive over most of the Indian Ocean with low-level southerly wind anomalies in the Arabian Sea strengthening onshore flow toward the west coast. OLR anomalies are strongly negative north of the northwest Indian coast and significantly negative over parts of southern peninsular India and the central Bay of Bengal, consistent with deeper, colder cloud tops.
• Monsoon circulation: Vertical–meridional sections (73–77°E) reveal stronger ascent north of 10°N post-2000, with increases most pronounced above 500 hPa up to ~150 hPa (15–20°N), indicating a more top-heavy monsoon overturning.
• Cloud microphysics structure: ERA-5 SCIW shows two positive anomaly cores (8–12°N and 15–25°N) aligned with mid-level SCLW anomalies, implying enhanced melting layer ice/hydrometeors. Near-surface SCLW decreases in the same columns, indicating a more vertically extended, top-heavy cloud mass consistent with deeper convection.
• Instability: Radiosonde-derived CAPE at Mangalore increased significantly post-2000, with median values shifting from ~700 to ~1100 J kg−1, signaling greater convective potential energy.
• Satellite cloud and rain: ISCCP-H shows statistically significant positive trends (1998–2014) in deep convective cloud cover along the west coast, with climatological deep convective cloud fractions of ~8–16%. TRMM 3A25v7 indicates positive trends in convective rainfall along the coast, though more spatially confined than cloud trends, supporting a deepening of convective processes rather than just more frequent convection.
• Gauge rainfall extremes: IMD gridded daily rainfall PDFs show increased frequency of extremely heavy rain events (>244.5 mm day−1) post-2000 along the west coast. Overall, multiple lines of evidence indicate that west coast India monsoon rainfall has become more convective (deeper, ice-rich, top-heavy) since 2000, concomitant with Arabian Sea warming and intensified monsoon ascent.

Findings suggest a secular shift toward deeper, more convective cloud systems over India’s west coast during JJAS in the post-2000 era. The steepening of the rainfall–OLR trend relationship relative to interannual variability indicates rising cloud-top altitudes and/or expanded cold cloud shields beyond what would be expected from rainfall increases, corroborated by ERA-5 SCIW/SCLW vertical structure, strengthened top-heavy ascent, higher CAPE, enhanced deep convective cloud cover (ISCCP), and increased extreme rainfall frequencies (IMD). These changes are plausibly linked to Indian Ocean warming—particularly in the eastern Arabian Sea—enhanced land–sea thermal gradients, and changes in zonal/meridional SST gradients that modulate large-scale circulation and onshore moisture transport. If these trends reflect a forced climate signal, further intensification of convective characteristics and extreme precipitation is likely, with significant hydrological, ecological, and socio-economic implications for a region already prone to flood disasters. Nonetheless, the inference relies partly on reanalysis and aggregated datasets, underscoring the need for higher-temporal-resolution observations and modeling to resolve convective processes and attribute mechanisms more definitively.

The study provides convergent evidence that monsoon rainfall over India’s west coast has become more convective since 2000, characterized by deeper clouds (lower OLR, higher SCIW), intensified and top-heavy ascent, increased CAPE, greater deep convective cloud cover, and more frequent extreme rainfall. These changes are consistent with warming of the eastern Arabian Sea and evolving SST gradients that favor stronger moisture transport and instability. The work contributes an early, multi-dataset assessment linking oceanic warming and monsoon circulation changes to shifts in convective rain characteristics. Future research should: (1) employ instantaneous, high-resolution satellite, radar, and in situ observations to directly quantify convective depth, organization, and microphysics; (2) use convection-permitting and coupled ocean–atmosphere modeling to attribute causes and project future changes; (3) assess regional impacts on hydrology, hazards, and infrastructure; and (4) examine the roles of aerosols, land-use change, and remote SST gradients in modulating convective regimes.

• Reliance on monthly/seasonal ERA-5 reanalysis limits direct interpretation of convective morphology; cloud property inferences (e.g., from OLR, SCIW/SCLW) are indirect and subject to reanalysis model/assimilation biases.
• Statistical significance for some metrics (e.g., rainfall trend) is marginal; CAPE distributions are skewed and clipped, complicating formal significance testing.
• Satellite datasets span relatively short periods (ISCCP-H 1998–2014; TRMM 1998–2014; CERES-MODIS 2000–2019) and may include instrument/calibration drifts.
• Spatial and temporal coverage constraints (single radiosonde site for CAPE; gridded gauge uncertainties) can affect generalizability.
• Attribution among competing drivers (SST gradients, aerosols, land-use changes, circulation variability) is not fully resolved and requires targeted modeling studies.