Effects of the South Asian Absorbing Haze on the Northeast Monsoon and Surface–Air Heat Exchange

Abstract

The effects of the south Asian haze on the regional climate are assessed using the National Center for Atmospheric Research Community Climate Model version 3 (CCM3) at the T42/L18 resolution. This haze, as documented during the Indian Ocean Experiment (INDOEX) campaign (1995–2000), consists mainly of anthropogenic aerosols, and spans over most of south Asia and the north Indian Ocean. It reduces the net solar flux at the surface by as much as 20–40 W m⁻² on a monthly mean basis and heats the lowest 3-km atmosphere by as much as 0.4–0.8 K day⁻¹, which enhances the solar heating of this layer by 50%–100%. This widespread haze layer is a seasonal phenomenon limited to the dry period between November and May. The imposed haze radiative forcing leads to several large and statistically significant climate changes during the dry monsoon season, which include cooling of the land surface, and warming of the atmosphere. These temperature change features lead to the stabilization of the boundary layer that results in a reduction of evaporation and sensible heat flux from the land. The dynamical response to the aerosol forcing is surprisingly large. The aerosol forcing weakens the north–south temperature gradient in the lower level, which results in an enhancement of the area mean low-level convergence and a northward shift of the ITCZ. The increase in low-level convergence leads to increased convective rainfall and latent heat release, which in turn leads to a further increase in low-level convergence. This positive feedback between the low-level convergence and deep convective heating increases the average precipitation over the haze area by as much as 20%. The ocean surface undergoes a suppression of evaporation. Because of this decreased evaporation accompanied by the increase in the haze-area precipitation, the precipitation over the rest of the Tropics decreases, with a large fraction of this decrease concentrated over the Indonesian and the western Pacific warm pool region. The prescribed dry monsoon haze effect affects the summertime wet monsoon too, but a detailed analysis has to await the availability of year-round aerosol data. The major inference from this study is that the effects of absorbing aerosols on the regional climate can be quite large. The simulated surface temperature response was very sensitive to the ratio (R) of the surface aerosol forcing to the atmospheric forcing. The R itself varies from −1.5 in clear skies to about −0.5 in overcast skies over ocean, and available experimental data are not sufficient to constrain its value more narrowly.