Study finds a widespread three-fold rise in compound dry and hot summer monsoon extremes during the past decades over India

Figure1: Spatial distribution of change in frequency per decade in the occurrence of the Ccmpound Dry and Hot Extremes (Top panel) and Compound Wet and Cold Extremes (Bottom panel) for the recent period (1977-2019) relative to the base period (1951-1976) during Indian summer monsoon months across homogenous regions of India.

We imagine droughts, floods, heatwaves, and cold waves if we hear the word natural extremes or disaster. Indeed, they are not rare confirms by the recently released IPCC sixth climate assessment report. However, the report emphasizes that most extreme events are interconnected and concurrence is defined as a compound event. How about two extremes occurring together? Maybe simultaneously or one leads to another at the same place or two different geographically separated locations (Russian heatwave and Pakistan Flooding). Imagine a situation if India faces the worst drought and severe heatwaves together. The frequency and spatial extent of warm extremes, such as droughts and heatwaves are increasing unprecedented due to increased global mean temperatures, land surface feedback, ocean-atmospheric coupling, land-use change, cloud cover, and aerosol feedback on regional climates. These global changes have a significant adverse impact on the economy, eco-hydrological systems, agriculture, and population.

Simultaneous occurrence of two or more natural extremes impacts society greater than their univariate counterparts. Therefore, assessment of compound extremes is essential. In particular, compound extremes associated with precipitation and temperature are more disastrous in hydroclimatology.

In our recently published article in Nature Scientiifc Reports, we disentangled the frequency and spatial extent changes of compound dry and hot extreme (CDHE) and compound wet and cold extreme (CWCE) across India at a regional scale using an empirical approach.

Our findings revealed that the frequency of CDHE (CWCE) has increased by 1-3 events per decade (decreased by 1-2 events per decade) for the recent period (1977-2019) relative to the base period (1951-1976). This increasing (decreasing) pattern of CDHE (CWCE) is high across North-central India, Western India, North-eastern India and South-eastern coastlines. The increasing frequency of CDHE is mainly attributed to the Indian Ocean SST variability post-1976/77 climate shift. Several studies concluded that the influence of equatorial Pacific Ocean SST has decreased, and eastern equatorial Indian Ocean SST has increased after the climate shift leading to an early withdrawal of ISM. As a result, the length of ISM has shrunk, causing an increased (decreased) frequency of CDHE (CWCE). Therefore, considering the joint conditions of hydro-climatic variables and multivariate modelling is essential to advance informed understanding of compound extremes climate change adaptation. Moreover, identified regions with an increased likelihood of compound dry and hot extremes are critical for risk management strategies.

In brief, the study is of great societal relevance to the agrarian economy of the densely populated, resource-stressed Indian subcontinent and can help design the appropriate strategies from the viewpoint of compound extremes. These findings are described in the article entitled Disentangling increasing compound extremes at regional scale during Indian Summer Monsoon, recently published in the Scientific Reports. Ravi Kumar Guntu conducted this work from the Department of Hydrology, Indian Institute of Technology Roorkee, under the supervision of Dr. Ankit Agarwal