Abstract
Using tropical cyclone data along with sea surface temperature data (SST) and atmospheric circulation reanalysis data during the period of 1980–2019, the seasonal variation of tropical cyclone genesis (TCG), and the related oceanic and atmospheric environments over the Arabian Sea (AS) and Bay of Bengal (BOB) are compared and analyzed in detail. The results show that TCG in both the BOB and AS present bimodal seasonal variations, with two peak periods in the pre-monsoon and post-monsoon season, respectively. The frequencies of TCG in the BOB and AS are comparatively similar in the pre-monsoon season but significantly different in the post-monsoon season. During the post-monsoon season of October–November, the TCG frequency in the BOB is approximately 2.3 times higher than that of the AS. The vertical wind shear and relative humidity in the low- and middle-level troposphere are the two major contributing factors for TCG, and the combination of these two factors determines the bimodal seasonal cycle of TCG in both the AS and BOB. In the pre-monsoon season, an increase in the positive contribution of vertical wind shear and a decrease in the negative contribution of relative humidity are collaboratively favorable for TCG in the AS and BOB. During the monsoon season, the relative humidity factor shows a significant and positive contribution to TCG, but its positive effect is offset by the strong negative effect of vertical wind shear and potential intensity, thus resulting in very low TCG in the AS and BOB. However, the specific relative contributions of each environmental factor to the TCG variations in the AS and BOB basins are quite different, especially in the post-monsoon season. In the post-monsoon season, the primary positive contributor to TCG in the AS basin is vertical wind shear, while the combined effect of vertical wind shear and relative humidity dominates in the BOB TCG. From the analysis of environmental factors, atmospheric circulations, and genesis potential index (GPI), the BOB is found to have more favorable TCG conditions than the AS, especially in the post-monsoon season.
Highlights
The peak value of in the pre-monsoon season is higher than that part of the sub-basin, while tropical cyclone (TC) form throughout the entire Bay of Bengal (BOB) and are distributed more in the post-monsoon season but the frequency is higher in the post-monsoon season
The below 30% in all areas of the North Indian Ocean (NIO) except in the BOB near the equator, where it is close to relative humidity at 600 hPa in the NIO gradually increases in May, at which time the but over 30%
From June to September, the relative humidity conditions of the two basins improved owing to the monsoon; its positive contribution was offset by the vertical wind shear and potential intensity, resulting in a small genesis potential index (GPI) value at this time, which is not conducive to tropical cyclone genesis (TCG) activity
Summary
Further research has shown that aerosols such as black carbon can reduce the vertical wind shear in the AS by changing the atmospheric circulation, leading to an increase in storms in the AS, especially in the pre-monsoon season [17]. The frequency of TCG in the BOB and AS present similar bimodal characteristics, and previous studies have indicated that critical environmental factors such as low-level relative vorticity, vertical wind shear, potential intensity, and low- and middlelevel relative humidity, when coherently organized, create favorable conditions for TCG in the pre- and post-monsoon seasons [12,13,16]. Previous studies have shown that large-scale oceanic and atmospheric environments play important roles in TCG, including the sea surface thermal condition, low-level relative vorticity, vertical wind shear, Coriolis forcing, and humidity in the lower and middle troposphere [3,25].
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