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Relation of Dust Emission to ENSO in the Mojave Desert

Determining Climate Thresholds for Wind Erosion in the Mojave Desert: Using ENSO-Related Interannual Variability to Assess Potential Desertification

Gregory S. Okin¹ and Marith C. Reheis²

¹ Department of Geography, University of California, Santa Barbara, California, USA, (okin@geog.ucsb.edu)
² Earth Surface Processes Team, U.S. Geological Survey, Denver, Colorado, USA, (mreheis@usgs.gov)

ABSTRACT

Global circulation models suggest that arid regions throughout the globe are likely to become significantly warmer in the next 50 to 100 years, resulting in increased evapotranspiration. The same models indicate that precipitation will remain nearly constant for these regions over this time period. Thus, the world's mid-latitude deserts are likely to see decreases in the ratio of precipitation to potential evapotranspiration, further dessicating these areas. In arid environments, climatic conditions and geomorphic processes are closely linked. In particular, the dessication of temperate deserts will be accompanied by dramatic increases in aeolian activity.

Strong interannual climate variability may be used to help predict the effect of longer-term climate change on geomorphic processes in arid lands. In particular, interannual climate variability may be used to determine the threshold beyond which wind erosion and dust emission increase dramatically and wind-driven desertification may expand. We compared the strength of the January ENSO (El Niño-Southern Oscillation) anomaly with observed dust events, dust deposition rates, and dune mobility index (DMI=W/(P/PE) where W is the percentage of time that wind is above threshold velocity for sand, P is mean annual precipitation, and PE is potential evapotranspiration) values calculated using hourly climate data from the Mojave Desert. There is a statistically significant negative correlation between ENSO intensity and DMI values for the period from 1973 to 1999 (95% confidence), as well as a high positive correlation coefficient (99% confidence) between DMI values and the number of observed dust events at Daggett, an airport in the Manix Basin, central Mojave Desert. Dust deposition rates also increase with DMI values. In addition, we find that when the DMI is greater than approximately 140, wind erosion increases dramatically and areas of wind-driven desertification (defined by areas of fresh sand) expand. Thus, we predict that longer-term climate changes that increase the frequency of years with DMI > 140 will sharply increase wind erosion of soils and reduce vegetation in the Mojave Desert and throughout the American Southwest and northern Mexico.

Finally, the observed relationship between ENSO state and wind erosion in the Manix Basin underscores the value of using global climate state to predict local geomorphic responses and applied to decision-making for land use. For example, activities that expose soil surfaces to wind erosion could be minimized during especially vulnerable periods accompanying ENSO anomalies, which can now be predicted as much as a year in advance.