Abstract
Kerala conside red the land of monsoons, located on the southwest coast of India, received contrasting rainfall during the northeast monsoon (NEM) seasons of 2019 (excess) and 2020 (deficit). The reasons for the contrasting NEM rainfall during these two consecutive years were studied by analysing the atmospheric circulation pattern and its interaction with warm ocean waters by using wind data from the stratosphere-troposphere (ST) radar and reanalysis products. The study reveals that weak and variable westerly winds over the southern Indian Peninsula coupled with the location of the Inter-tropical Convergence Zone (ITCZ) north of the peninsula prevented the timely onset of NEM over its core regions during 2019. However, 2019 was an anomalous year with an extreme positive Indian Ocean Dipole (IOD) with unusual cyclonic activity over the Arabian Sea (AS) during the NEM season. These anomalous conditions favoured large amounts of rainfall, especially over the northern parts of Kerala during the latter half of October and compensated for the rainfall deficiency during the early half of October. In 2020, the subdued cyclogenesis over the AS combined with near-normal sea surface temperatures (SST) resulted in reduced convective activity over Kerala during the initial part of the NEM season. After the withdrawal of the southwest monsoon (SWM), the formation of two cyclones in the Bay of Bengal (BoB) and the frequent passage of easterly troughs from the BoB across the southern peninsula brought in anomalous rainfall over the east coast and some parts of Kerala, but the central and northern parts of Kerala largely remained rainfall deficient.
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Acknowledgements
We would like to express our sincere thanks to the Ministry of Earth Sciences (MoES), Government of India, for supporting the ST Radar Facility at ACARR, CUSAT. We sincerely acknowledge and thank the Science and Engineering Research Board (SERB), Department of Science and Technology (DST), Government of India for providing the grant and for their assistance in the design and conception of the ST-radar facility at CUSAT. All figures were prepared using free software ferret and R. IMD daily gridded rainfall data was downloaded from the website http://www.imdpune.gov.in. ECMWF ERA5 reanalysis data was accessed from https://cds.climate.copernicus.eu. NOAA Optimum Interpolation (OI) Sea Surface Temperature (SST) V2 was downloaded from https://psl.noaa.gov/data/gridded/data.noaa.oisst.v2.html.
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Funding was provided by the Ministry of Earth Sciences (MoES), Government of India, to support the ST Radar facility at ACARR, CUSAT, Kochi.
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All authors contributed to the study conception and design. Material preparation, data collection, and analysis were performed by Syam Sankar, Achanya Unni V, Abhiram Nirmal C S, Vijaykumar P, Rejoy Rebello, and Rakesh V. The first draft of the manuscript was written by Syam Sankar. K Mohanakumar and Abhilash S validated, supervised and edited the manuscript. All authors read and approved the final manuscript.
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Fig. S1
Vertical velocity (Pa/s) at 850 hPa averaged during a) 1st October and 16th October (FHO) 2019 b) 1st October and 16th October (FHO) 2020 c) 17th October and 31st October (SHO) 2019 and d) 17th October and 31st October (SHO) 2020 (PNG 521 kb)
Fig. S2
Monthly Hovmuller plots of divergence (105 sec-1) (a,b) and vertical velocity (Pa/s) (c,d) averaged over the Indian longitudes (75oE-85oE) and averaged from 1000 hPa to 850 hPa pressure levels, from August to December during 2019 and 2020. (PNG 325 kb)
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Sankar, S., Achanya Unni, V., Abhilash, S. et al. Contrasting features of northeast monsoon during 2019 and 2020 in response to the delayed withdrawal of southwest monsoon. Theor Appl Climatol 153, 19–34 (2023). https://doi.org/10.1007/s00704-023-04467-4
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DOI: https://doi.org/10.1007/s00704-023-04467-4