Impact of Low-Level Anthropogenic Absorbing Aerosol on Monsoon Precipitation and Air-Sea Heat Exchange

We assess the effect of the recently discovered South-Asian haze on the monsoonal circulation over the Tropical Indian ocean and the S. Asian continent. The significance of these man-made aerosols has been estimated by the INDian Ocean EXperiment (INDOEX) field campaign. The aerosol layer, largely confined between 1000 mb and 700 mb layer, contains as much as 10% (of fine particle mass) absorbing black carbon and covers a large region extending over most of N. Indian ocean, S. Asian and SE Asian continent. The absorbing aerosol heats the low-level atmosphere by as much as 1.0K/day on a monthly/diurnal mean basis, while both absorbing and scattering aerosol reduces the net solar flux at the surface by as much as 25 to 50 W/m^2. The haze layer is a seasonal phenomenon limited to the period between October to May.

NCAR CCM3 was employed to simulate the regional climate change due to the low-level diabatic heating increase and the net surface radiative flux reduction. The first study of the impact of this haze on the monsoon has been reported by Kiehl, Ramanathan and Hack (AGU, Fall 2000 meeting). In this study we explore the sensitivity of the monsoon to the efficieny of absorption. Two numerical experiments were conducted: 1) One in which the atmospheric absorption nearly balances the reduction in surface solar radiation, i.e, the ratio (R) of surface forcing (a negative qunatity) to atmospheric forcing(a positive quantity) being -1.0; this case simulates the observed cloudy sky aerosol forcing; 2) The second case considers a ratio R=-1.5 which simulates the aerosol forcing in clear skies. The model integration was made with CCM's atmospheric and land surface components, and with prescribed seasonal cycle of SST. 42 years of output was used for experiments, and 84 model years for control run.

The dominant effect of the absorbing haze layer includes the following: warming of the lower atmosphere by about 1 to 2 K; surface cooling which varies from -0.5 to -1 K for R=-1.5 to negligible values for R=-1; large perturbation to the ITCZ and the precipitation which includes the following: the region connecting Sri Lanka and Malaysia has increase of 2~3 mm/day in precipitation; the rainfall gets weakened by comparable amplitude over Indonesia; the ITCZ over the western Indian ocean is pushed southward. In May, north-migrating heavily-precipitating area becomes widened meridionally. The aerosol effect on the net heat flux into the ocean is negative (meaning flux reduction) in both experiments, in spite of offseting by the simulated reduction in latent heat flux to atmosphere. The major inference from our study is that effects of anthropogenic aerosols on regional climate can be quite large.