Forcing Of Dust Aerosols Research Articles

Redistribution of Indian summer monsoon by dust aerosol forcing

Previous studies based on models and observations show that dust aerosols have an impact on the timing and distribution of the Indian summer monsoon but the transition of various Indian summer monsoon‐associated atmospheric features which unfolds in the process is still not clear. Through regional climate simulations, this study identifies the different phases of the change in the mean distribution of monsoon related characteristics in response to local dust forcing. The deposition of dust along the slopes of the Tibetan Plateau results in a heated atmospheric column (elevated heat pump) over there starting from May. This drags the monsoonal flow towards the foothills of the Himalayas before the conventional monsoon onset over India, which tends to decrease the rainfall over central India and increase it along the foothills of the Himalayas during May and June. July is the period of transition during which the reversal of various atmospheric conditions takes place. The elevated heat pump completely vanishes with the wet deposition of dust, which reduces the vertical wind over the Tibetan Plateau. The removal of dust increases the temperature over the Indo‐Gangetic plain, which efficiently drags the moisture carrying southwesterly towards central India. Meanwhile, stronger horizontal divergence of moisture towards the Bay of Bengal reduces the rainfall over central India and these atmospheric conditions further intensify during August. These results provide strong evidence of the role of dust aerosols in shaping the Indian summer monsoon distribution over India.

Intensification of North American Megadroughts through Surface and Dust Aerosol Forcing*

Abstract Tree-ring-based reconstructions of the Palmer drought severity index (PDSI) indicate that, during the Medieval Climate Anomaly (MCA), the central plains of North America experienced recurrent periods of drought spanning decades or longer. These megadroughts had exceptional persistence compared to more recent events, but the causes remain uncertain. The authors conducted a suite of general circulation model experiments to test the impact of sea surface temperature (SST) and land surface forcing on the MCA megadroughts over the central plains. The land surface forcing is represented as a set of dune mobilization boundary conditions, derived from available geomorphological evidence and modeled as increased bare soil area and a dust aerosol source (32°–44°N, 105°–95°W). In the experiments, cold tropical Pacific SST forcing suppresses precipitation over the central plains but cannot reproduce the overall drying or persistence seen in the PDSI reconstruction. Droughts in the scenario with dust aerosols, however, are amplified and have significantly longer persistence than in other model experiments, more closely matching the reconstructed PDSI. This additional drying occurs because the dust increases the shortwave planetary albedo, reducing energy inputs to the surface and boundary layer. The energy deficit increases atmospheric stability, inhibiting convection and reducing cloud cover and precipitation over the central plains. Results from this study provide the first model-based evidence that dust aerosol forcing and land surface changes could have contributed to the intensity and persistence of the central plains megadroughts, although uncertainties remain in the formulation of the boundary conditions and the future importance of these feedbacks.