Arabian Sea tropical cyclones intensified by emissions of black carbon and other aerosols

 Amato T. Evan, James P. Kossin,Chul ‘Eddy’ Chung & V. Ramanathan

Nature 479,94–97 (03 November 2011) doi:10.1038/nature10552

Abstract

  Throughout the year, average sea surface temperatures in the Arabian Sea are warm enough to support the development of tropical cyclones, but the atmospheric monsoon circulation and associated strong vertical wind shear limits cyclone development and intensification, only permitting a pre-monsoon and post-monsoon period for cyclogenesis. Thus a recent increase in the intensity of tropical cyclones over the northern Indian Ocean is thought to be related to the weakening of the climatological vertical wind shear. At the same time, anthropogenic emissions of aerosols have increased sixfold since the 1930s, leading to a weakening of the southwesterly lower-level and easterly upper-level winds that define the monsoonal circulation over the Arabian Sea. In principle, this aerosol-driven circulation modification could affect tropical cyclone intensity over the Arabian Sea, but so far no such linkage has been shown. Here we report an increase in the intensity of pre-monsoon Arabian Sea tropical cyclones during the period 1979–2010, and show that this change in storm strength is a consequence of a simultaneous upward trend in anthropogenic black carbon and sulphate emissions. We use a combination of observational, reanalysis and model data to demonstrate that the anomalous circulation, which is radiatively forced by these anthropogenic aerosols, reduces the basin-wide vertical wind shear, creating an environment more favourable for tropical cyclone intensification. Because most Arabian Sea tropical cyclones make landfall, our results suggest an additional impact on human health from regional air pollution.

Points

 *   The South Asian atmospheric brown cloud (ABC) is a 3-km-thick layer of pollution over the northern Indian Ocean (NIO) and Indian subcontinent that results from human emission of aerosols such as black and organic carbon and sulfates. When over water the dominant surface-radiative effect of the ABC is a decrease in solar insolation, which is enhanced significantly by black and organic carbon aerosols. There is a northward gradient of fine-mode aerosol optical depth over the Arabian Sea (Fig. 1a) and therefore a southward gradient in aerosol surface forcing (that is, negative surface forcing is greater in magnitude to the north). Positive forcing associated with greenhouse gases is offset in the NIO by the sixfold increase in emissions since 1930, resulting in a southward gradient in sea surface temperature (SST) trends (Fig. 1b) and a decrease in the climatological summertime northward SST gradient. The atmospheric response to this anomalous SST gradient is a weakening of the southerly cross-equatorial surface flow and upper-level tropical easterly jet, which define the Indian monsoon circulation, through anomalously high sea-level pressure to the north and low sea-level pressure to the south.

Fig 1.  a, Genesis points (circles) and tracks (solid lines) of pre-monsoon tropical cyclones during the period 1979–2010. Storms with an lifetime maximum intensity (LMI) of more than 50 m s⁻¹ are indicated with a filled circle at the genesis point and thick track lines. Shaded contours represent annual long-term mean fine-mode aerosol optical depth (AOD) from the MODIS Terra and Aqua instruments averaged over 2003–2009. b, The 50-year change in observed SST, averaged over 55°–75° E. The SST change is defined as the average of the monthly linear trend from 1955–2004, multiplied by 50.