Abstract
In this study, we present observational evidence, based on satellite aerosol measurements and MERRA reanalysis data for the period 1979–2011, indicating that absorbing aerosols can have strong influence on seasonal-to-interannual variability of the Indian summer monsoon rainfall, including amplification of ENSO effects. We find a significant correlation between ENSO (El Nino Southern Oscillation) and aerosol loading in April–May, with La Nina (El Nino) conditions favoring increased (decreased) aerosol accumulation over northern India, with maximum aerosol optical depth over the Arabian Sea and Northwestern India, indicative of strong concentration of dust aerosols transported from West Asia and Middle East deserts. Composite analyses based on a normalized aerosol index (NAI) show that high concentration of aerosol over northern India in April–May is associated with increased moisture transport, enhanced dynamically induced warming of the upper troposphere over the Tibetan Plateau, and enhanced rainfall over northern India and the Himalayan foothills during May–June, followed by a subsequent suppressed monsoon rainfall over all India, consistent with the elevated heat pump (EHP) hypothesis (Lau et al. in Clim Dyn 26:855–864, 2006. doi:10.1007/s00382-006-0114-z). Further analyses from sub-sampling of ENSO years, with normal (<1-σ), and abnormal (>1-σ) NAI over northern India respectively show that the EHP may lead to an amplification of the Indian summer monsoon response to ENSO forcing, particularly with respect to the increased rainfall over the Himalayan foothills, and the warming of the upper troposphere over the Tibetan Plateau. Our results suggest that absorbing aerosol, particular desert dusts can strongly modulate ENSO influence, and possibly play important roles as a feedback agent in climate change in Asian monsoon regions.
Highlights
Atmospheric aerosols, clouds and precipitation are key building blocks of the energy and water cycles of the earth’s climate system (Haywood and Boucher 2000; Ramanathan et al 2001; Bellouin et al 2005; Wang 2013)
Results are obtained based on high-minus-low composite analysis carried out separately for (a) pure” ENSO (PENSO), i.e., ENSO years with normal normalized aerosol index (NAI) over northern India, (b) Elevated Heat Pump (EHP), i.e., years with NAI exceeding one standard-deviation, and (c) COM, i.e., combined effects of La Nina (El Nino) years and high aerosol optical depth (AOD) years
During EHP years, the basic response is similar to PENSO, except that the magnitude of the anomalous rainfall and rising motion is stronger over the Himalaya foothills and the Bay of Bengal in May–June
Summary
Atmospheric aerosols, clouds and precipitation are key building blocks of the energy and water cycles of the earth’s climate system (Haywood and Boucher 2000; Ramanathan et al 2001; Bellouin et al 2005; Wang 2013). Ganguly et al (2012) investigated the time-scale dependent responses of the South Asian monsoon to anthropogenic aerosol forcing using the Community Atmosphere Model (CAM5) with a fully predictive aerosol life cycle They demonstrated that the slow response associated with sea surface temperature (SST) changes is generally stronger than the fast atmospheric response. On intraseasonal time scales, Manoj et al (2011) found that atmospheric heating by absorbing aerosols over central India during long breaks can create a large meridional temperature gradient at low levels between aerosol-rich central India and the pristine equatorial Indian Ocean They showed that active spells are produced due to the pronounced moisture convergence and deep convection in central India, suggesting. Temperature, water vapor and winds are from MERRA (Modern Era Retrospective-Analysis for Research and Applications) reanalysis data (Rienecker et al 2011)
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