Control of Storminess over the Pacific and North America by Circulation Regimes: The role of large-scale dynamics in weather extremes

Circulation regimes are identified from a cluster analysis applied to 5-day means of the combined anomalies of 500 hPa geopotential height and the 250 hPa zonal wind over the extended Pacific-North America region obtained from ERA-Interim reanalyses. Five regimes are identified: the Arctic Low (AL), Pacific wavetrain (WT), Alaskan Ridge (AR) and Pacific Trough (PT) and Arctic High (AH). Regime dependent anomalies of the frequency of atmospheric rivers and measures of the storm tracks (850 hPa covariance between high pass filtered meridional wind and temperature and 300 hPa variance of high pass filtered meridional wind) are shown to be significant. The circulation regimes provide statistically significant modulation of observed precipitation (obtained from the Multi-Source Weighted-Ensemble data set). We examine regime-specific anomalies, normalized anomalies and the occurrence of extreme values of precipitation. We identify regions in which the ratio of frequency of occurrence of extremes (5th and 95th percentile) to the climatological frequency in each regime is significantly high. Precipitation is decreased over the Pacific Northwest in the AL regime, but is increased over the central and Eastern US, with a large increase in the probability of extreme precipitation, enhanced atmospheric river occurrence and stronger storm tracks. In the WT regime, Pacific atmospheric rivers are shifted towards Alaska, accompanied by a decrease in storm track magnitude over the Eastern North Pacific. The AR regime is characterized by an increase in moisture flux into the Southwest, increases in extreme precipitation and atmospheric rivers and storm track related heat transport. Increases in precipitation, and atmospheric rivers along the Southwest coast are noted in the PT regime, along with increased precipitation over Texas and Florida. The Northeastern region experiences increases in extremely dry conditions. Significant drought conditions occur over the Midwest US. Significant increases in precipitation over a broad region of the Northwest occur during the AH regime.