Abstract
Droughts have become more severe and recurrent over the Indian sub-continent during the second half of the twentieth century, leading to more severe hydro-climatic and socio-economic impacts over one of the most densely populated parts of the world. So far, droughts have mostly been connected to circulation changes concomitant with the abnormal warming over the Pacific Ocean, prevalently known as “El Niño”. Here, exploiting observational data sets and a series of dedicated sensitivity experiments, we show that the severity of droughts during El Niño is amplified (17%) by changes in aerosols. The model experiments simulate the transport of boundary layer aerosols from South Asian countries to higher altitudes (12–18 km) where they form the Asian Tropopause Aerosol Layer (ATAL) (~ 60–120°E, 20–40°N). During El Niño, the anomalous overturning circulation from the East Asian region further enriches the thickness of aerosol layers in the ATAL over the northern part of South Asia. The anomalous aerosol loading in the ATAL reduces insolation over the monsoon region, thereby exacerbating the severity of drought by further weakening the monsoon circulation. Future increases in industrial emissions from both East and South Asia will lead to a wider and thicker elevated aerosol layer in the upper troposphere, potentially amplifying the severity of droughts.
Highlights
We have considered only strong El Niño years 1957, 1965, 1967, 1972, 1977, 1982, 1991, 1994, 1997, 2002, 2004, 2009 and 2015 when the amplitude of the SST anomalies in either eastern or central tropical Pacific exceed one standard deviation
We considered monsoon season of the co-occurring El Niño year since lead-lag www.nature.com/scientificreports correlations between all Indian summer monsoon rainfall and Niño-3 index exhibits a maximum negative correlation for zero lag year[33,68]
It should be noted that aerosols in the troposphere have an important influence on precipitation by modifying cloud microphysics over shorter timescales. previous study[10] has shown that, over the timescale of a week, the rainfall over central India (16.5–26.5°N, 74.5–86.6°E) is positively correlated with the concentration of natural aerosols such as desert dust and sea salt over the Arabian Sea
Summary
Aerosols interact in many ways with the monsoon - the enhancement or suppression of the Indian summer monsoon rainfall depends on its duration and scale (including remote influences), along with its distinct direct and indirect radiative effects[8,9,10] In addition to this conventional understanding, the presence of the recently discovered aerosol layer in the upper troposphere and lower stratosphere (UTLS) known as “the Asian Tropopause Aerosol Layer (ATAL)” covering South Asia (defined here as the region spanning 0–100°E, 20–45°N) during the monsoon season[11] may have a role in controlling the monsoon precipitation. Indian emissions have an important contribution to the ATAL due to the vicinity of the centre of Asian monsoon convective updraft[20] This region shows a higher growth in sulfate aerosol compared to China[21] leading to a pronounced contribution to the ATAL composition. Using satellite observations and model simulations, we explore for the first time the contribution of anomalously high aerosol loadings over the UTLS region during El Niño in worsening drought conditions over the Indian subcontinent
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