Interaction of Dust Radiative Forcing and the Asian Summer Monsoon

Wind-blown dust is ubiquitous over the Arabian Sea during the Asian summer monsoon. Absorption of solar radiation within the layer acts as an elevated heat source, which can induce ascent and precipitation. Whether the monsoon circulation is strengthened by aerosols depends upon the response of the Indian Ocean. If surface radiative forcing by aerosols is compensated over the Arabian Sea by a reduction in evaporation, then atmospheric latent heating will decrease and oppose radiative heating within the aerosol layer. In contrast, if the surface forcing is balanced by a reduction in ocean heat export, then evaporation need not decrease and radiative heating within the aerosol layer can strengthen the monsoon without opposition. Consideration of the ocean response resolves an apparent contradiction among previous numerical simulations whereby the monsoon either strengthens or weakens in response to aerosol radiative forcing. When sea surface temperature (SST) is prescribed, a surface energy constraint is absent, and aerosol radiative forcing at the surface is implicitly balanced by a reduction in ocean heat export, allowing a strengthened monsoon. Conversely, the monsoon weakens in most models where SST is calculated and evaporation rather than ocean heat export decreases to balance the reduction of incident sunlight beneath the aerosol layer. To represent the full range of feedbacks between the monsoon, the Indian Ocean, and dust radiative forcing, the Principal Investigators (PIs) will perform experiments with an atmospheric general circulation model (AGCM) coupled to fully interactive models of the ocean circulation and dust aerosol cycle. Unlike other absorbing aerosols, dust concentration is a sensitive function of the climate through the surface wind speed. A previous calculation found that the dust load was increased by strengthened monsoonal winds driven by dust radiative forcing. The simulated relationship will be compared to observations of key variables such as dust amount, wind speed, evaporation, and precipitation over the Arabian Sea, on both daily and seasonal time scales. The daily response to dust is of interest because dust outbreaks are episodic, in contrast to the slow seasonal variations of absorbing aerosols created by pollution. The research will join relatively few previous studies about the interaction of the monsoon and aerosol radiative forcing, and help to understand contrasting results about the effect of aerosols upon the monsoon strength (that result from the effect of prescribed versus calculated SST, for example). Studies considering the particular effect of dust aerosols have so far not considered the effect of ocean heat transport. Calculation of the climate response to dust is necessary to distinguish the global and regional response to anthropogenic forcing by greenhouse gases and other aerosols. Results from the study will be used to improve a publicly available AGCM, including a model of the dust cycle, and will be incorporated into teaching and presentations to general audiences by the PI. By the end of the project, a doctoral student will complete her dissertation under the supervision of the PI.