Moisture sources for the genesis of tropical cyclones over the Bay of Bengal

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Moisture sources for the genesis of tropical cyclones over the Bay of Bengal using the lagrangian FLEXPART model - Climate Dynamics

Sources of moisture-feeding for the incipient tropical cyclones (TCs) over the Bay of Bengal (BoB) are least explored. Using a three-dimensional Lagrangian model running with NCEP-FNL data, the present work identified various source locations and major transport routes that contributed moisture to pre- and post-monsoon TCs genesis (TCG) over the BoB. The clustered mean trajectories identified using the k-means method demonstrated that moisture transport towards the TCG region during pre-monsoon is dominated by intense low-level westerly/south-westerly trajectories propagating across the North Indian Ocean covering the Somalia coast and the Arabian Sea. Meanwhile, during post-monsoon, north-easterly/easterly trajectories are more prevalent and bring moisture to the BoB TCG region from the South China Sea and North-West Pacific. Moreover, the study highlights the spatiotemporal variations of moisture uptake (MU) pattern and its contribution and demonstrates that robust MU with a contribution of 20–90% occurs locally before three days of TCG. Meanwhile, remote sources contribute a moisture amount of ≤ 20%. Furthermore, the composite analysis of different diagnostic parameters confirms that identified MU at the local scale is attributed to an evaporation process in association with a dynamic uplifting mechanism caused by an anomalous cyclonic circulation, which provides moisture for TCG, while the horizontal advection is the mechanism, which gradually builds moisture required for the TCG from the remote sources.

Undoubtedly, the availability of water vapor, i.e., in terms of the amount of moisture present in the atmosphere and especially at the mid-troposphere, has a vital role in tropical cyclone genesis (TCG). Nevertheless, it remains unclear where this moisture is coming from a particular region (source) to support the TCG over the Bay of Bengal (BoB) region.

The current study attempts to reveal various moisture sources and their contribution percentages promoting TCG activity over the BoB during different seasons (pre- & post-monsoon), using a three-dimensional Lagrangian model running with NCEP-FNL data.

Interestingly, the study identifies important moisture sources (local and remote) regions as well as moisture pathways through which moisture is transported towards the BoB-TCG region using the k-means method. Pre-monsoon (Mar-Apr-May) TCG activity is dominated by intense low-level westerly/southwesterly trajectories propagating across the North Indian Ocean covering the Somalia coast and the Arabian Sea. Meanwhile, during post-monsoon (Oct-Nov-Dec), northeasterly/easterly trajectories are more prevalent and bring moisture to the BoB TCG region from the South China Sea and North-West Pacific.

Furthermore, the study demonstrates relevant mechanisms, such as an evaporation process in association with a dynamic uplifting caused by an anomalous cyclonic circulation and horizontal advection, which is responsible for the spatiotemporal variation in quantitative contributions of each source region.

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