Dust Amplification of ENSO Related Rainfall Anomalies over Australia in the NOAA/GFDL Coupled Model 3

The Lake Eyre basin in southern Australia is the most intense source of dust on the continent, with highly variable emission rates that depend on the water level, or lack of water, in the lake.  Summertime precipitation in the region is strongly influenced by ENSO, and major drought events in Australia are often connected to El Niño conditions.  Here, we use a suite of multi-century simulations from the NOAA/GFDL coupled model CM3 to demonstrate that El Niño related rainfall anomalies in Australia are amplified by dust from Lake Eyre, which drives a positive feedback between the two.

            We multiply dust emission in each simulation by a scalar ranging from 0 to 5 in order to produce a wide range of dust loads over Australia.  We then analyze the regression coefficient between the Nino3.4 index and DJF precipitation over the continent for each simulation.  We find that while El Niño conditions reduce rainfall over Australia in all simulations, the strength of the relationship increases as dust is increased.  When there is zero dust emission from Australia, DJF rainfall is reduced by 29% over the Lake Eyre region per unit of the Nino3.4 index, while in the 5x dust emission simulation the reduction has grown to 54%.  We further find that the rainfall reduction is caused by the radiative effects of dust.  By scattering incoming solar radiation, dust decreases the net radiation to the surface by as much as 40 W/m², which in turn reduces evaporation.  With less evaporation comes reduced humidity and CAPE, increased CIN, and less deep convection.

Suppression of rainfall by dust also gives rise to a positive feedback in the model.  In the updated version of CM3, dust emission responds to both soil moisture and vegetation cover.  Reduced rainfall creates drier soils and less vegetation, both of which enhance dust emission above its previous level.  This excess dust in turn helps to continue suppressing rainfall.  Properly simulating this relationship may be important to accurately simulating drought in Australia.