In the dynamic and complex environment of the Bay of Bengal, tropical cyclones are more than just destructive weather events—they are key drivers of intricate ocean-atmosphere interactions that shape the physical and biological fabric of the upper ocean. A groundbreaking study recently published in Ocean-Land-Atmosphere Research delves deep into the contrasting behaviors of pre-monsoon and post-monsoon cyclones, revealing seasonally distinct oceanic responses that have significant implications for cyclone intensity forecasting and regional climate modeling.
Tropical cyclones originating in the Bay of Bengal pose a unique challenge to oceanographers and meteorologists alike, largely due to the Bay’s intricate monsoonal influences which create divergent oceanic conditions across seasons. Pre-monsoon cyclones, forming between March and May, and post-monsoon cyclones, developing from October to December, manifest starkly different thermal and salinity profiles that influence their physical structure and progression. Understanding these differences is pivotal in decoding the mechanisms behind cyclone-ocean feedbacks.
Central to the study is the observation that pre-monsoon cyclones evolve over warmer sea surface temperatures (SST), fluctuating between 29°C and 31°C. These elevated temperatures enhance the upper ocean’s thermal stratification and drive significant freshwater inflow from coastal rivers and tributaries during this season, leading to coastal freshening—an observable reduction in sea surface salinity. This freshening plays a crucial role in modulating mixed layer depth and subsequently affects the transfer of heat and momentum from the atmosphere to the ocean.
In stark contrast, the post-monsoon cyclones are found to develop over relatively cooler oceans, with the SST stabilizing between 28°C and 29°C. During this phase, the dominant force appears to shift from ocean surface temperature to atmospheric dynamics such as intensified rainfall and convection. The study highlights that post-monsoon cyclones are associated with heavier precipitation events which increase chlorophyll-a concentrations within the mixed layer. Elevated chlorophyll-a levels between 9.9 and 14.4 mg/m³ point to enhanced biological productivity triggered by nutrient upwelling, which is in turn stimulated by cyclone-induced Ekman transport mechanisms.
A defining feature of both cyclone types is their influence on the mixed layer depth (MLD). Pre-monsoon cyclones, with their higher SST and freshwater influx, tend to create shallower mixed layers near coastal zones. This stratified environment impedes vertical mixing and contributes to heat accumulation in the upper ocean, potentially amplifying storm intensity. Post-monsoon storms, conversely, generate deeper mixed layers due to increased wind-driven turbulence and nutrient entrainment, which have significant repercussions on marine ecosystems and biogeochemical cycles.
The authors pay particular attention to the role of sea surface salinity (SSS) variability. Pre-monsoon cyclones are linked to a stronger freshwater influence, as the runoff from monsoonal rivers dilutes the coastal salinity and influences buoyancy fluxes. This freshwater layer acts to stabilize the upper ocean, modifying the heat content available to cyclones. Post-monsoon cyclones, subjected to intense rainfall rather than freshwater inflow, experience surface freshening patterns with distinct spatial and temporal signatures, demanding nuanced observation approaches.
Another intriguing aspect illuminated by the study is the sea level pressure (SLP) distribution associated with these cyclones. Post-monsoon cyclones demonstrate lower central pressures and stronger wind fields, perhaps a consequence of sustained atmospheric convection and ocean-atmosphere coupling specific to the cooler SST regime. This interplay between ocean surface conditions and atmospheric forcing not only governs cyclone lifespan but also their potential for intensification or dissipation.
The research team underscores the temporal lag in oceanic responses to cyclone events. While some variables, like SST and SLP, display immediate changes, others such as biological productivity and nutrient distribution respond over longer time scales. This delay highlights the necessity for comprehensive, high-resolution ocean monitoring both during and after cyclone passage to fully capture the cascade of physical and biogeochemical processes at play.
Despite the robustness of these findings, the authors acknowledge limitations inherent to the study. The analysis is constrained by a relatively small sample size and focuses on a limited temporal window, which may not account for interannual variability or anomalies. Nevertheless, the results lay a solid foundation for future expansive research endeavors integrating real-time satellite observations and advanced ocean-atmosphere coupled models.
Foreseeing the implications of their work, the researchers advocate for the development of predictive tools that can utilize these seasonal oceanic signatures to enhance cyclone forecasting accuracy. Early warning systems, informed by differential ocean responses, could improve disaster preparedness not only in the Bay of Bengal but also in similar tropical cyclone-prone regions with marked seasonal monsoonal cycles.
This study exemplifies the critical need to integrate physical oceanography and atmospheric science to unravel the complexities of cyclone behavior in monsoon-dominated environments. The intricate feedbacks between ocean surface conditions and atmospheric processes are essential parameters that determine cyclone genesis, evolution, and potential destructiveness.
Contributors to this impactful research hail from the Bangladesh Oceanographic Research Institute’s Physical, Space, and Biological Oceanography Divisions as well as the Department of Oceanography at the University of Chittagong. Their collaborative efforts highlight the importance of regional expertise in addressing climate challenges relevant to vulnerable coastal communities.
In conclusion, the nuanced differentiation between pre- and post-monsoon cyclone interactions with the upper ocean unveiled by this study advances our understanding of cyclone dynamics in the Bay of Bengal. It also informs broader climate and oceanographic models that underpin global efforts to predict and mitigate the impacts of tropical cyclones in a changing climate, emphasizing the critical role that seasonality plays in modulating oceanic and atmospheric processes.
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Subject of Research: Not applicable
Article Title: Upper Ocean Response Mechanisms to Pre-Monsoon and Post-Monsoon Cyclones in the Bay of Bengal
News Publication Date: 25-Aug-2025
Web References: http://dx.doi.org/10.34133/olar.0105
References: Chowdhury, S. U. M. B., Karmakar, A., Hoque, M. E., Hoque, M. M., Tahsin, T. H., & Chowdhury, S. (2025). Upper Ocean Response Mechanisms to Pre-Monsoon and Post-Monsoon Cyclones in the Bay of Bengal. Ocean-Land-Atmosphere Research.
Image Credits: Chowdhury, S. U. M. B., Karmakar, A., Hoque, M. E., Hoque, M. M., Tahsin, T. H., & Chowdhury, S. (2025).
Keywords
Air sea interactions, Ocean circulation, Meteorology
Tags: Bay of Bengal cyclonescoastal freshening phenomenacyclone-ocean feedback mechanismsfreshwater inflow effectsmonsoonal influences on cyclonesocean-atmosphere interactionspost-monsoon cyclone dynamicspre-monsoon cyclone behaviorsea surface temperature impactsseasonal climate modelingtropical cyclone intensity forecastingupper ocean physical changes
