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

Abstract

Atmospheric concentrations of South Asian anthropogenic aerosols and their transport play a key role in the regional hydrological cycle. Here, we use the ECHAM6-HAMMOZ chemistry–climate model to show the structure and implications of the transport pathways of these aerosols during spring (March–May). Our simulations indicate that large amounts of anthropogenic aerosols are transported from South Asia to the northern Indian Ocean and western Pacific. These aerosols are then lifted into the upper troposphere and lower stratosphere (UTLS) by the ascending branch of the Hadley circulation, where they enter the westerly jet. They are further transported to the Southern Hemisphere (∼15–30∘ S) and downward (320–340 K) via westerly ducts over the tropical Atlantic (5∘ S–5∘ N, 10–40∘ W) and Pacific (5∘ S–5∘ N, 95–140∘ E). The carbonaceous aerosols are also transported to the Arctic, leading to local heating (0.08–0.3 K per month, an increase by 10 %–60 %). The presence of anthropogenic aerosols causes a negative radiative forcing (RF) at the top of the atmosphere (TOA) (−0.90 ± 0.089 W m−2) and surface (−5.87 ± 0.31 W m−2) and atmospheric warming (+4.96 ± 0.24 W m−2) over South Asia (60–90∘ E, 8–23∘ N), except over the Indo-Gangetic Plain (75–83∘ E, 23–30∘ N), where RF at the TOA is positive (+1.27 ± 0.16 W m−2) due to large concentrations of absorbing aerosols. The carbonaceous aerosols lead to in-atmospheric heating along the aerosol column extending from the boundary layer to the upper troposphere (0.1 to 0.4 K per month, increase by 4 %–60 %) and in the lower stratosphere at 40–90∘ N (0.02 to 0.3 K per month, increase by 10 %–60 %). The increase in tropospheric heating due to aerosols results in an increase in water vapor concentrations, which are then transported from the northern Indian Ocean–western Pacific to the UTLS over 45–45∘ N (increasing water vapor by 1 %–10 %).