Abstract
Bay of Bengal (BoB) region is a major moisture source to both Indian Summer Monsoon (ISM) and North East Monsoon (NEM) rainfall over the surrounding region. Oxygen isotopic ratios (δ18O) in the rainfall over the peripheral region show a decreasing trend towards the later months of ISM and shows minimum during NEM. This was previously explained with local factors of BoB such as isotopically depleted river discharge to BoB, increased cyclonic activity etc., however, the exact reasons remain unclear. δ18O of vapor collected from BoB when there was no cyclonic activity also shows ~2 ‰ depletion in NEM season compared to that during ISM. Here we show, easterly winds passing through a convectively active narrow zonal band (central to western equatorial Pacific and Indonesian region) bring significant amounts (median 64%: interquartile range 30%) of isotopically depleted moisture to BoB region, and results in anomalous depletion of vapor over BoB during NEM. Vapor source-tagged simulations from isotope enabled Community Atmospheric Model show easterly moisture flux from western Pacific region dominates over the BoB surface evaporation flux.
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