Has northern Indian Ocean cloud cover changed due to increasing anthropogenic aerosol?

The recent Indian Ocean Experiment (INDOEX) observed high aerosol concentrations with a sizeable soot fraction over the northern Indian Ocean. This aerosol mix substantially absorbs solar radiation, and recent modeling studies have proposed that the resulting atmospheric heating reduces daytime cloud cover. The present study tests this hypothesis by investigating whether low-level cloud cover has decreased over the northern Indian Ocean between 1952 and 1996, a time period when south Asian anthropogenic emissions have greatly increased. The observed slight increase in cloud cover indicates that other processes must compensate soot solar heating. A similar increase in cloud cover observed over the relatively clean southern Indian Ocean suggests the increase over the northern Indian Ocean does not have a special regional anthropogenic aerosol origin.