Compound Hot-Dry Events in Urban India: Variability and Drivers

The precipitation deficit-temperature feedback can severely impact multiple sectors, such as reduction in crop yield to critical infrastructure failures, especially in low latitude areas (< 30°N). Typically, a heatwave event coincides with a significant decline in surface wind speed due to atmospheric blocking and is often compounded by persistent precipitation-deficit leading to meteorological droughts. Anomalous warm-and-dry air, which comes in torrents, results in an abrupt increase in air temperature that strengthens the local land-atmosphere feedback via soil desiccation. Based on daily meteorological observations covering the 1970-2018 period, first, I show a spatial coherence in the timing of unprecedented hot-dry events over major urban and peri-urban locations of the Indian sub-continent (8°4'N and 37°6'N). Surface wind data confirms a significant decline in low wind speed over most of the locations, especially over the eastern coastal plains of the country. Further, the compound occurrence of extreme temperature and low wind speed act as a preconditioning driver for sequential short (or long)-duration precipitation deficits across most of the sites. A copula-based joint distribution framework incorporating the compounding effect of high temperature, low wind speed, and precipitation deficit reveals a T-year severe hot-dry event tends to become more frequent. Finally, I show a median 6-fold amplifications in compound hot-dry frequency than that of the expected annual number of 50-year temperature extreme. The inferred amplifications are more pronounced in low-lying urban-coastal areas than in the interior locations, where decadal changes in (significant) increase in extreme temperature at several locations are contrasted by a concurrent decrease in surface wind speed.