Exploring dynamics of Indian summer monsoon rainfall trends asymmetry: An event in early twentieth century warming and mid-twentieth century cooling episode

Abstract

The shifting pattern of Indian summer monsoon rainfall (ISMR) holds paramount importance on a global scale, demanding urgent attention due to its profound impact on freshwater availability, food production, and the economy of India, which is home to ~18% of the global population. The notable decline and subsequent recovery of ISMR during recent global warming and subsequent hiatus (from the 1970s onwards) have been associated with diminished and amplified land-sea thermal contrast, resulting from accelerated anthropogenic interventions. A similar global warming trend followed by a hiatus is also present in history called Early Twentieth Century Warming (ETCW) and Mid-Twentieth Century Cooling (MTCC), respectively. Examining the spatial variability of ISMR trends during this period and comprehending the dynamics behind such variability is crucial for understanding the current and future climate. To achieve this objective, the analysis focuses on district-wise summer monsoon rainfall patterns in four rainfall homogeneous regions across India (South Peninsula, Central, Northwest, Northeast), employing the established method of Chakra et al. 2023. Notable increasing rainfall trends are observed in the South Peninsula and Northwest India, contrasting with an opposite trend in Northeast India, covering more than one-third of the regions between the 1930s and 1960s. Central India also experiences an increasing trend during this period but over a limited area. This spatial heterogeneity in rainfall trends contributes to an east-west rainfall trend asymmetry in India, prompting further exploration for understanding causal processes. An increase in wind strength is noted in the Arabian Sea and the southern Bay of Bengal, while a decrease is observed in the Northern Bay of Bengal. The wind updraft rate varies across the Indian region, with a higher rate over the Arabian Sea and southern Bay of Bengal, and a lower rate in northern India and the Tibetan Plateau. Analysis of surface temperature trends reveals a higher warming rate in the northern tropical Indian Ocean compared to the southern tropical Indian Ocean. This difference resulted in a meridional thermal contrast conducive to a stronger Hadley circulation, bringing increased rainfall to the western part of the Indian region. Concurrently, the cooling trend over northern India and the Tibetan plateau led to weak updraft and increased downdraft, ultimately reducing wind strength over the northern Bay of Bengal, and creating unfavorable conditions for rainfall over the Northeast region. Hence, the east-west asymmetry in the trend of ISMR is linked to the latitudinal disparity in temperature trends during the periods of Early Twentieth Century Warming and Mid-Twentieth Century Cooling.