Local and regional climate change in the Mojave Desert, USA
In this study, the role of urbanization and atmospheric circulation on the climate of the Mojave Desert was examined using a variety of methods and datasets to uncover patterns of change at the local to regional level over the last sixty years. The three basic research questions that informed the dissertation were; (1) what is the effect of urbanization on local to regional climate; (2) what are the regional trends in Mojave Desert climate; and, (3) can trends in local to regional climate be explained by changes in atmospheric circulation?
Urban effects were widespread with mean and minimum temperature trends 2 to 3 times greater than the regional average. In general, minimum temperatures increased by 2.5--5°C at urban stations, while rural areas warmed by less than 1.5°C, if at all. In the largest city, Las Vegas, a daytime urban cool island linked to increased air pollution and atmospheric moisture was observed. In the smaller Mojave cities, such as St. George, UT, daytime heat islands formed as maximum temperature increased by 1--3°C. Regional urban effects were observed in a day-of-week (DOW) cycle in precipitation downwind of Las Vegas that matched DOW air pollution (PM10) variability. Between 1994 and 2003, 32% more precipitation occurred during the Wednesday to Friday (weekday) period than the Saturday to Monday (weekend) period with average precipitation intensity 13% greater during the weekday period.
The Mojave Desert is experiencing summer-like conditions earlier in the year due to a 2--3°C increase in the average spring temperature, while temperatures are also increasing during the summer. Thus, a longer and hotter summer season is occurring throughout the Mojave.
The frequency of warm atmospheric circulation patterns increased at the expense of colder ones leading to temperature increases in the Mojave Desert. For example, 65% of the regional trend in winter minimum temperature is due to a change in circulation patterns alone. Overall, the data suggest that the combined effect of urbanization and atmospheric circulation changes have produced hotter conditions in the Mojave Desert. There is less evidence suggesting that an increase in global greenhouse gases has caused the regional trends.