Abstract
The Indian subcontinent is one of the hotspots of deadly heat stress.
Several attribution studies have shown increasing trends in heatwaves
and its linkage with dry spells over South Asia. However, very few
studies have investigated concurrent or successive (lagged d-day)
occurrence of humid heat stress (high temperature compounded by
humidity) and precipitation extremes within a short time window. Using
gauge-based observation records of the last five decades, we have
analyzed the concurrence of extreme wet bulb temperature,
Tw and peak rain events in 9 urban locations of India,
distributed over climatologically heterogeneous regions. We find a
larger fraction of the population is exposed to a significant increase
(more than 1% and up to 2.5%/decade) in mean and extreme
Tw (around 1%/decade) in several sites than solely
accounting dry-bulb temperature trends. This prompted us to analyze the
compound hazard associated with storm events preceded by extreme
Tw (assessed through ≥ 95 – 98.5th
percentile exceedances and annual maxima series) up to a week of
occurrence of the event. Considering synchronicity between two drivers
(extreme Tw and peak rain), we demonstrate cities
located across the western half of the country showed positive
dependence, whereas those located over the eastern half show negative
dependence. While negative correlation suggests the concurrence of dry
and hot episodes, the positive correlation suggests robust amplification
in precipitation extremes. This is confirmed by the large upper tail
distributions of peak rain events during the core monsoon season (June
– September) to locations showing positive dependence. Based on extreme
Tw-precipitation sensitivity, we propose compound heat
stress – rain-induced flood hazard model for densely populated areas.
Understanding drivers of peak runoff responses would benefit risk
management, insurance, and flash flood forecast, devising flood
resilience under climate change.