Effects of local and remote black carbon aerosols on summer monsoon precipitation over India

K S Krishnamohan,Angshuman Modak,Govindasamy Bala

doi:10.1088/2515-7620/ac18d1

Abstract

In this study, we perform idealized climate model simulations to assess the relative impacts of an increase in local black carbon (BC) aerosols (located over the Indian region) and the remote BC aerosols (located outside the Indian region) on the summer monsoon precipitation over India. We decompose the precipitation changes into fast adjustments triggered by the introduction of the forcing agent and slow response that is associated with the global mean temperature change. We find that a 60-fold increase in the ‘present-day’ global distribution of BC aerosols leads to an increase in precipitation over India, which is mainly contributed by an increase in remote BC aerosols. When remote BC aerosols are increased, the fast adjustments contribute to an increase in precipitation in association with the warming of the northern hemisphere land and the related northward Intertropical Convergence Zone (ITCZ) shift. For an increase in local aerosols too, by enhancing the upper tropospheric temperature meridional gradient in the Indian region, the fast adjustments contribute to an increase in precipitation over India. The slow response contributions in both cases are related to the regional patterns of SST change and the resulting changes to meridional temperature gradient in the Indian region and zonal circulation changes. The net precipitation change over India is an increase (decrease) for an increase in remote (local) BC aerosols. As the interpretation of our results relies on ITCZ shift related to planetary energetics, differing land-ocean response and meridional temperature gradients in the Indian region, the results from our study are likely to be robust across climate models.

Highlights

Black carbon (BC) aerosols absorb solar radiation in contrast to sulfate aerosols which primarily scatters solar radiation
60-fold increase in the ‘present-day’ global distribution of BC aerosols leads to an increase in precipitation over India, which is mainly contributed by an increase in remote BC aerosols
It is shown in Modak and Bala (2019) that the BC aerosol forcing leads to relatively a larger warming of the northern hemisphere compared to the southern hemisphere, and the resulting change in interhemispheric temperature difference leads to a northward shift in the location of ITCZ and an enhancement of precipitation in northern hemisphere tropics

Summary

Introduction

Black carbon (BC) aerosols absorb solar radiation in contrast to sulfate aerosols which primarily scatters solar radiation. Lau et al (2006) find that the absorbing aerosols along the slopes of the Tibetan plateau could heat the air in these elevated levels during the pre-monsoon months and strengthen the monsoon circulation This in turn increases the precipitation over northern India (the so called ‘elevated heat pump’ mechanism - Lau et al 2006, Lau and Kim 2006). Lau et al (2006) find that precipitation could decline over central India because of the BC induced surface cooling Other studies such as Meehl et al (2008) and Kovilakam and Mahajan (2016) find that the increase in the meridional tropospheric temperature gradient due to absorption of solar radiation by BC aerosols could enhance the monsoon circulation and precipitation over India

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