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2 2021

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Role and response of ocean–atmosphere interactions during Amphan (2020) super cyclone

Vissa Naresh Krishna, Anandh P. C., Gulakaram Venkata Sai, Konda Gopinadh

Acta Geophysica

2021

Vol. 69, no. 5

1997--2010

Cyclone Amphan (May 2020) is one of the strongest cyclones in the recent decade during premonsoon season in the North Indian Ocean. Satellite, observational and reanalysis data products were analysed before, during and after the passage of Amphan to understand the role of ocean–atmosphere interactions for the rapid intensification, recurvature and upper ocean response. To examine the pre-oceanic conditions and rapid intensifcation of Amphan in the North Indian Ocean, a twolayer reduced gravity model is used to derive the upper ocean thermal profle to estimate the tropical cyclone heat potential (TCHP). Results reveal that prior to the passage of Amphan, unusual high TCHP anomalies (>25 kJcm−2) and SST anomalies (>1 °C) are evident. Time-longitude, sea level anomalies suggest that high TCHP and SST are associated with propagation of downwelling Kelvin and Rossby waves in the Equatorial Indian Ocean and the Bay of Bengal, respectively. Before rapid intensification, Amphan changes its path from north-northwestward to north-northeastward direction. Amphan produced significant left rainfall asymmetry during its passage. Analysis from mid-tropospheric (600 hPa) equivalent potential temperature (K) reveals the presence and meander of dryline along the western Bay of Bengal (BoB) (< 325 K). The upper ocean response during life history of Amphan is analysed from Argo floats within vicinity of cyclone track. The key finding in this study is that mechanical mixing and intense precipitation are responsible for the changes in mixed layer depth and barrier layer along the Amphan track. These results indicate that the presence of dryline in the middle troposphere is crucial for the recurvature of Amphan track during premonsoon season. This study highlights that large-scale environmental and ocean–atmosphere interactions for the rapid intensification of cyclone in the North Indian Ocean.

Aerosol dipole pattern over India: consequences on rainfall and relation with wind circulations

Vissa Naresh Krishna, Tyagi Bhishma

Acta Geophysica

2021

Vol. 69, no. 6

2475--2482

The aerosol concentrations are changing in rapid phase in the recent decades over Indian subcontinent. A general conclusion that is postulated from various studies indicated high aerosol loading over the Indian subcontinent. The aerosol concentrations are found to alter cloud-precipitation chemistry globally. Analysing the trends of aerosol concentrations over the Indian subcontinent is crucial to understand in depth the aerosol–precipitation relationship over these regions. In the present study, the long-term trend (2000–2019) of aerosols, precipitation and winds has been analysed over the Indian subcontinent. The work aims to identify the zones of significant increasing/decreasing Aerosol Optical Depth concentrations and alter the rainfall patterns over these regions. Mann–Kendall test has been employed for trend analysis. An aerosol dipole pattern is observed along the eastern-western boundaries of the monsoon trough over the Indian subcontinent (India, Bangladesh and Pakistan), which shows an inverse relationship with the rainfall patterns. Over the north-western region of the Indian subcontinent, the reduced pollution and enhanced precipitation may further alter the land-sea thermal gradient, resulting in the weakening of monsoonal circulation. Enhanced pollution over eastern India is adjusting the precipitation distribution along with the weakening of low-level jet and moisture transport. The present study provides an evidence of aerosol–precipitation relation over the Indian subcontinent using long-term datasets that has profound application in better understanding the weather systems.