Impact of the Desert dust on the summer monsoon system over Southwestern North America
The radiative forcing of dust emitted from the Southwest United States\n(US) deserts and its impact on monsoon circulation and precipitation\nover the North America monsoon (NAM) region are simulated using a\ncoupled meteorology and aerosol/chemistry model (WRF-Chem) for 15 years\n(1995-2009). During the monsoon season, dust has a cooling effect (-0.90\nW m(-2)) at the surface, a warming effect (0.40 W m(-2)) in the\natmosphere, and a negative top-of-the-atmosphere (TOA) forcing (-0.50 W\nm(-2)) over the deserts on 24-h average. Most of the dust emitted from\nthe deserts concentrates below 800 hPa and accumulates over the western\nslope of the Rocky Mountains and Mexican Plateau. The absorption of\nshortwave radiation by dust heats the lower atmosphere by up to 0.5 K\nday(-1) over the western slope of the Mountains. Model sensitivity\nsimulations with and without dust for 15 summers (June-July-August) show\nthat dust heating of the lower atmosphere over the deserts strengthens\nthe low-level southerly moisture fluxes on both sides of the Sierra\nMadre Occidental. It also results in an eastward migration of NAM-driven\nmoisture convergence over the western slope of the Mountains. These\nmonsoonal circulation changes lead to a statistically significant\nincrease of precipitation by up to similar to 40 % over the eastern\nslope of the Mountains (Arizona-New-Mexico-Texas regions). This study\nhighlights the interaction between dust and the NAM system and motivates\nfurther investigation of possible dust feedback on monsoon precipitation\nunder climate change and the mega-drought conditions projected for the\nfuture.