83rd Annual

Tuesday, 11 February 2003
Interannual variations in the south Asian haze forcing: Implications to ENSO
Chul Eddy Chung, SIO/Univ. of California, La Jolla, CA; and V. Ramanathan
Aerosols are regionally concentrated and are subject to large temporal variations, even on interannual time scales. The fundamental reasons for this large variability are the short life times of aerosols and the important role of transport in regulating their regional concentrations. In this study, we focus on the observed large interannual variability of the South Asian haze, estimate the corresponding variations in its radiative forcing, and use a general circulation model to study the impact on global climate variability. The South Asian haze is wide spread, covering most of the North Indian ocean including the Arabian sea and the Bay of Bengal. The southernmost extent of the haze varies year to year from about 10° S to about 5° N. In order to understand the impact of this interannual variation in the haze forcing, we conducted two numerical experiments with two extreme locations of the forcing: 1) extended haze forcing and 2) shrunk haze forcing. The former has the forcing applied northward of 10°S, while the latter is confined to regions north of the equator. These two cases represent two extreme phases of the satellite retrieved AODs (Aerosol Optical Depths) over the Indian Ocean in the recent years.

Each of the two numerical experiments was implemented into the NCAR/CCM3 with the climatological (but seasonally varying) SST to estimate the climate sensitivity to the area of the aerosol forcing. Over India where the forcing is centered, the simulated climate changes are very similar between the two experiments. In remote regions however, the responses differ substantially. First, both experiments simulate the wintertime drought over southwest Asia, with the extended forcing simulating far more severe drought.

Second, the extended forcing significantly suppresses convection in the western equatorial Pacific during the boreal wintertime, and the shrunk forcing leads to much less suppression. Since the western Pacific convection suppression would relax the trade winds over the Pacific and induce warm anomalies in the eastern basin, we propose that the Indo-Asian haze is partially responsible for the observed El Niņo like warming in the recent decades. When the convection suppression in the extended forcing experiment is imposed in the Cane-Zebiak Pacific ocean/atmosphere model, the coupled model simulates a bias towards the warm phase at the magnitude of the observed El Niņo like warming. The shrunk forcing also led the Cane-Zebiak model to simulate a bias towards the warm phase but at half the magnitude.

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