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On the Epochal Variability in the Frequency of Cyclones during the Pre-Onset and Onset Phases of the Monsoon over the North Indian Ocean

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Abstract

In this paper we examine the epochal changes in the frequency of cyclones over the North Indian Ocean during the pre-onset and onset phases of the monsoon. We consider three epochs; namely, the early (1955–74), middle (1975–94) and recent (1995–2014) epochs. It is found that the number of cyclones in the Bay of Bengal (BOB) decreases throughout the three epochs. Over the Arabian Sea (ARB), however, there is a decrease in the early epoch, before then reaching a minimum in the middle epoch followed by an increase in the recent epoch, thus exhibiting epochal variability. Dynamic and thermodynamic parameters along with Genesis Potential Index (GPI) are examined to understand the frequency variation in cyclogenesis over the ARB and BOB. Over the ARB, thermodynamic factors such as mid-level moisture, surface latent heat flux and sensible heat flux, and dynamic parameters such as lower-level convergence and upper-level divergence, are favorable during the early and recent epochs but unfavorable during the middle epoch, and these results are found to be consistent with the observed epochal variability in the frequency of cyclogenesis. However, all these influential parameters are found to have decreased over the BOB during the entire 60-year period.

摘要

本文研究了季风建立前和开始阶段,北印度洋地区热带气旋频次在不同年代的变化特征。我们选择了三个时间段:早期(1955-74)、中期(1975-94)和近代(1995-2014)。研究发现,孟加拉湾地区的气旋数量在三个时期均呈减少的趋势。然而,在阿拉伯海地区,气旋数量表现出明显的年代变化:在早期减少,到中期减至最少,随后在近代有所增多。为了理解这两个地区气旋生成频次的变化,我们对潜在生成指数(GPI)对应的动力和热力学参数进行了诊断。在阿拉伯海地区,热力因子(中层湿度、地表潜热和感热通量)和动力因子(低层辐合和高层辐散),有利于早期和近代,但不利于中期的气旋生成。这与该地区在观测上不同时期气旋生成频次的变化相一致。相对而言,在孟加拉湾地区,所有这些因子在整个60年均对应减弱。

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Acknowledgements

The authors are grateful to NCEP-NCAR, WHOI OAFlux, the UK Met Office Hadley Centre and the IMD for providing datasets used in this study. S. ABHILASH thanks UGC BSR for providing facilities and support. We thank C. S ABHIRAM for helping with analyzing the data.

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Correspondence to S. Abhilash.

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Article Highlights

• Distinct background climate states favor epochal variability in cyclone frequency during the onset phase of the monsoon over the Arabian Sea and a decreasing trend over the Bay of Bengal.

• The distinct variability is closely linked with the variability in the favorable cyclogenesis parameters over the Arabian Sea and the Bay of Bengal.

• The thermodynamic and dynamic background basic states corroborate well the frequency variation in cyclones during the different epochs.

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Baburaj, P.P., Abhilash, S., Mohankumar, K. et al. On the Epochal Variability in the Frequency of Cyclones during the Pre-Onset and Onset Phases of the Monsoon over the North Indian Ocean. Adv. Atmos. Sci. 37, 634–651 (2020). https://doi.org/10.1007/s00376-020-9070-5

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  • DOI: https://doi.org/10.1007/s00376-020-9070-5

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