Impact of Springtime NAO on Weather Conditions and Snow Melting in the Southwestern US

Abstract:

It is reported in this study 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 to surface air temperature and snow melting over the broad southwestern U.S. (SWUS) region in the early warm season. Monthly time-scale correlation and composite analyses using three different multidecadal temperature datasets reveal that NAO-related upstream circulation positively affects not only the means, but also the extremes, of the daily maximum and minimum temperatures in the SWUS. This NAO effect is primarily linked with the positioning of upper-tropospheric anticyclones over the western United States that are associated with development of the positive NAO phase. This link has been strengthened in the last 30-yr period (1980–2009), compared to the previous 30-yr period (1950–79).

We further examine the impact of NAO on snow melting using snow equivalent water (SWE) of automated snow telemetry (SNOTEL) over SWUS. The significant negative correlations widespread in SWUS between the NAO average and zero-SWE date during snow melting periods, indicate that frequent positive (negative) NAO phases enhance (delay) snow melting inter-annually. This linkage is found to be stronger at the low-elevation sites below 3200 m. The underlying mechanism for this linkage is that positive (negative) NAO phases tend to bring not only warmer (colder) but also drier (wetter) weather conditions to the SWUS region in spring. These results emphasize the role of NAO on springtime weather conditions, snowpack characteristics, and related water resource over the SWUS region.