Anthropogenic emissions from South Asia reverses the aerosol indirect effect over the northern Indian Ocean

Abstract

Atmospheric aerosols play an important role in the formation of warm clouds by acting as efficient cloud condensation nuclei (CCN) and their interactions are believed to cool the Earth-Atmosphere system (‘first indirect effect or Twomey effect’) in a highly uncertain manner compared to the other forcing agents. Here we demonstrate using long-term (2003–2016) satellite observations (NASA’s A-train satellite constellations) over the northern Indian Ocean, that enhanced aerosol loading (due to anthropogenic emissions) can reverse the first indirect effect significantly. In contrast to Twomey effect, a statistically significant increase in cloud effective radius (CER, µm) is observed with respect to an increase in aerosol loading for clouds having low liquid water path (LWP < 75 g m−2) and drier cloud tops. Probable physical mechanisms for this effect are the intense competition for available water vapour due to higher concentrations of anthropogenic aerosols and entrainment of dry air on cloud tops. For such clouds, cloud water content showed a negative response to cloud droplet number concentrations and the estimated intrinsic radiative effect suggest a warming at the Top of the Atmosphere. Although uncertainties exist in quantifying aerosol-cloud interactions (ACI) using satellite observations, present study indicates the physical existence of anti-Twomey effect over the northern Indian Ocean during south Asian outflow.

Tropospheric warming over the northern Indian Ocean caused by South Asian anthropogenic aerosols: possible impact on the upper troposphere and lower stratosphere

“…The transported aerosols enhanced the AOD by 0.18 %-0.8 % (30 %-80 %) over the northern Bay of Bengal and by 0.02 %-0.12 % (20 %-60 %) over the Arabian Sea. This is consistent with previous studies where 50 %-60 % enhancements in the AOD over the tropical Indian Ocean due to anthropogenic aerosols have been reported (Satheesh et al, 2000;Jose et al, 2020). Chemical analysis of aerosols observed over the southeastern coastal Arabian Sea also shows the dominance of anthropogenic aerosols having sources over the Indian region (73 %) (Aswini et al, 2020).…”

“…Chemical analysis of aerosols observed over the southeastern coastal Arabian Sea also shows the dominance of anthropogenic aerosols having sources over the Indian region (73 %) (Aswini et al, 2020). Analysis of MODIS satellite observations (2003-2017) likewise shows that anthropogenic sources contributed ∼ 60 %-70 % to the aerosol loading over the east coast and west coast of India (Jose et al, 2020).…”

Increased aerosols can reverse Twomey effect in water clouds through radiative pathway

“…Additionally, it has been suggested that the excessive presence of hygroscopic aerosols and/or giant CCN can suppress super saturation and hinder the activation of smaller particles into cloud droplets, leading the positive relationship 20 – 22 . Some studies further showed that intense competition of water vapor due to increased aerosols can lead the evaporation of small cloud droplets, and thereby the positive relationship 14 , 15 . These studies provide a clue that the positive relationship can result from pathways different than the pathway of direct aerosol-cloud interaction, but these pathways are yet to be understood in detail.…”

Balloon-borne aerosol–cloud interaction studies (BACIS): field campaigns to understand and quantify aerosol effects on clouds

“…For example, the invigoration effect is proposed for convective clouds (Rosenfeld et al, 2014a) under the influence of updraughts. The first indirect effect (Twomey effect) and the second indirect effect (Albrecht effect) for liquid clouds are influenced by mixing (Costantino and Bréon, 2010), turbulence, and entrainment (Jose et al, 2020;Schmidt et al, 2015;Small et al, 2009). Although the first indirect effect is reasonably well understood, observational limitations pose a serious challenge in understanding and/or evaluating other hypotheses.…”