Tropospheric warming over the North Indian Ocean caused by the South Asian anthropogenic aerosols: possible implications

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. Our simulations indicate that large amounts of anthropogenic aerosols are transported from South Asia to the North Indian Ocean (the Arabian Sea and North Bay of Bengal). These aerosols are then lifted into the upper troposphere and lower stratosphere (UTLS) by the convection over the Arabian Sea and Bay of Bengal. In the UTLS, they are further transported to the southern hemisphere (30–40° S) and downward into the troposphere by the secondary circulation induced by the aerosol changes. The carbonaceous aerosols are also transported to the Arctic and Antarctic producing local heating (0.002–0.05 K d−1). The presence of anthropogenic aerosols causes negative radiative forcing (RF) at the 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° E–90° E, 8° N–23° N), except over the Indo-Gangetic plain (75° E–83° E, 23° N–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 produced in-atmospheric heating along the aerosol column extending from the boundary layer to the UTLS (0.01 to 0.3 K d−1) and in the stratosphere globally (0.002 to 0.012 K d−1). The heating of the troposphere increases water vapor concentrations, which are then transported from the highly convective region (i.e. the Arabian Sea) to the UTLS (increasing water vapor by 0.02–0.06 ppmv).