The Influence of the Atmospheric Circulation on Diurnal Temperature Range Trends

This study examines the contribution of the atmospheric circulation to variations and trends of the diurnal temperature range (DTR). For the cold seasons (November-March) between 1958/59 and 1994/95, daily surface observations at more than 200 stations in the contiguous United States as well as selected daily fields from the NCEP/NCAR Reanalysis Project are analyzed. Employing simple linear regression, sea level pressure anomaly patterns are identified that are linearly related to daily variations in the DTR. It is found that local positive sea level pressure anomalies, which tend to be associated with fair weather, favor high values of the DTR. The strength of these associations varies geographically due to the effects of nonlinear relationships, the frequency of snow cover, and the complexity of local dynamics. During the 30-year period between 1965/66 and 1994/95, the central and southern United States experienced a decrease in the cold season DTR, while the Northeast and coastal Northwest experienced an increase. Variations in the DTR-related sea level pressure patterns explain significant fractions of the DTR increases in the Northeast and coastal Northwest. In the south-central and southeastern states, an increase in cloudiness accounts for much of the decrease in the DTR, but neither trend is linearly related to changes in sea level pressure. The negative DTR trends in the Great Plains and Southwest are largely unrelated to linear trends in cloudiness and are in the opposite sense as the changes induced by trends in sea level pressure. These findings suggest that the decrease in the DTR over the central and southern United States is attributable either to nonlinear relationships with other climatic variables or to anthropogenic factors such as urbanization and increasing concentrations of greenhouse gases and tropospheric aerosols. Time permitting, some results for other countries and other seasons will also be presented.