The role of quasi-biennial variability in the spring 2010 circulation anomalies over the Western United States

The circulation of the tropical stratosphere is dominated by a ~28-month cycle of zonal winds, however, similar ‘quasi-biennial' (~2-year) modes have also been widely identified in surface climate and tropospheric dynamics. In recent years and decades, several studies have identified a significant ~26-month QB mode in winter and annual hydroclimate variables and proxies over the central Western United States, including instrumental precipitation records, river flows in California, and precipitation-sensitive tree-ring records dating back 500-1000 years. Analysis of monthly 500 hPa geopotential heights from the NW US has revealed a strongly periodic, nonseasonal ~26-month cycle that appears to produce the hydroclimate anomalies when extreme phases are aligned with the winter storm season. In the spring of 2010, the geopotential height oscillation reached a minimum in March (based on an objective sinusoidal projection from data covering 1949 to 2007. This projected minimum coincided with unusual spring circulation anomalies in the NW US, including persistently negative 500 hPa height anomalies, cool temperatures over the Pacific NW, and a prolonged winter storm season that extended into June. These anomalies, whose sign and duration is consistent with QB activity over the past 20-60 years, is tentatively attributed to the QB mode over the Western US and the broader Northern Hemisphere. Further projection and testing of the QB oscillation may lend insight to its origins, and perhaps predictive capability in high northern latitudes.