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South Asian Haze Forcing: Remote Impacts with Implications to ENSO and AO

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  • 1 Center for Clouds, Chemistry and Climate, Scripps Institution of Oceanography, La Jolla, California
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Abstract

Aerosols are regionally concentrated and are subject to large temporal variations, even on interannual timescales. In this study, the focus is on the observed large interannual variability of the South Asian (SA) haze, estimating the corresponding variations in its radiative forcing, and using a general circulation model to study their impacts on global climate variability. The SA haze is a widespread haze, covering most of South Asia and the northern Indian Ocean during December–April. 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, two numerical studies were conducted with two extreme locations of the forcing: 1) extended haze forcing (EHF) and 2) shrunk haze forcing (SHF). The former has the forcing extending to 10°S, while the latter is confined to regions north of the equator.

Each of the two haze forcing simulations was implemented into a 3D global climate model (NCAR CCM3) with a prescribed SST seasonal cycle to estimate the sensitivity of the model climate to the aerosol forcing area. In both simulations, the haze forcing was prescribed only during the dry season between November and April. Over India where the forcing is centered, the simulated climate changes are very similar between EHF and SHF. In remote regions, however, the responses differ remarkably. Focusing on the remote effects of the haze, it is shown that some of the recent observed boreal-wintertime changes of the southwest Asian monsoon, El Niño–Southern Oscillation (ENSO), and the Arctic Oscillation (AO) could be explained by the SA haze forcing and its fluctuation.

First, both simulations reveal the wintertime drought over southwest Asia, with the EHF generating far more severe drought. Second, the EHF experiment simulates a poleward shift of the Northern Hemisphere (NH) zonal-mean zonal momentum during the winter season, while the SHF effect rather moves the NH extratropical zonal momentum only slightly equatorward. Thus, the interannual fluctuations in the extension of the haze forcing area can explain the recently documented increased variability of the AO.

Third, the EHF significantly suppresses the convection in the western equatorial Pacific during the boreal wintertime, and the SHF leads to much less suppression. Since the western Pacific convection suppression would weaken the trade winds over the Pacific and induce warm anomalies in the eastern basin, it is proposed that the SA haze may be partially responsible for the observed El Niño–like warming during the recent decades. When the convection suppression in the EHF experiment is imposed in the Cane–Zebiak Pacific ocean–atmosphere model, the coupled model actually simulates a warm bias similar to the observed El Niño trends of the recent decades. These findings have to be verified with a fully coupled ocean–atmosphere climate model.

Abstract

Aerosols are regionally concentrated and are subject to large temporal variations, even on interannual timescales. In this study, the focus is on the observed large interannual variability of the South Asian (SA) haze, estimating the corresponding variations in its radiative forcing, and using a general circulation model to study their impacts on global climate variability. The SA haze is a widespread haze, covering most of South Asia and the northern Indian Ocean during December–April. 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, two numerical studies were conducted with two extreme locations of the forcing: 1) extended haze forcing (EHF) and 2) shrunk haze forcing (SHF). The former has the forcing extending to 10°S, while the latter is confined to regions north of the equator.

Each of the two haze forcing simulations was implemented into a 3D global climate model (NCAR CCM3) with a prescribed SST seasonal cycle to estimate the sensitivity of the model climate to the aerosol forcing area. In both simulations, the haze forcing was prescribed only during the dry season between November and April. Over India where the forcing is centered, the simulated climate changes are very similar between EHF and SHF. In remote regions, however, the responses differ remarkably. Focusing on the remote effects of the haze, it is shown that some of the recent observed boreal-wintertime changes of the southwest Asian monsoon, El Niño–Southern Oscillation (ENSO), and the Arctic Oscillation (AO) could be explained by the SA haze forcing and its fluctuation.

First, both simulations reveal the wintertime drought over southwest Asia, with the EHF generating far more severe drought. Second, the EHF experiment simulates a poleward shift of the Northern Hemisphere (NH) zonal-mean zonal momentum during the winter season, while the SHF effect rather moves the NH extratropical zonal momentum only slightly equatorward. Thus, the interannual fluctuations in the extension of the haze forcing area can explain the recently documented increased variability of the AO.

Third, the EHF significantly suppresses the convection in the western equatorial Pacific during the boreal wintertime, and the SHF leads to much less suppression. Since the western Pacific convection suppression would weaken the trade winds over the Pacific and induce warm anomalies in the eastern basin, it is proposed that the SA haze may be partially responsible for the observed El Niño–like warming during the recent decades. When the convection suppression in the EHF experiment is imposed in the Cane–Zebiak Pacific ocean–atmosphere model, the coupled model actually simulates a warm bias similar to the observed El Niño trends of the recent decades. These findings have to be verified with a fully coupled ocean–atmosphere climate model.

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