On the relationship between the North Atlantic Oscillation and early warm season temperatures in the southwestern US

Abstract:

This work reports that the North Atlantic Oscillation (NAO), which has been known to directly affect winter weather conditions in Western Europe and the eastern United States, is also linked with the surface air temperature over the southwestern United States (SWUS) in the early warm season. In this study, monthly time scale correlation and composite analyses were performed using three independent multi-decadal temperature datasets (i.e., NCEP/NCAR reanalysis (60 years), CRU (60 years), and NARR (30 years)) between 1950 and 2009. Results reveal that NAO-related upstream circulation not only positively affects the means, but also the extremes of the daily maximum and minimum temperatures in SWUS. This effect is primarily linked with the positioning of upper-tropospheric anticyclones over western US that is associated with the development of the positive NAO phase, through changes in low-tropospheric wind directions as well as suppression of precipitation and enhanced shortwave radiation at surface. This effect is observed in SWUS only during the March-June period because the monthly migration of anticyclones over the western US follows the migration of the NAO center over the subtropical Atlantic Ocean. This linkage of SWUS temperature with NAO has been strengthened in the last 30-year period (1980-2009) compared to the earlier 30-year period (1950-1979). Our results imply different spatial NAO patterns with different timescales from months to several decades.