A Simple Definition of Flow Regimes and its Relationship to High Impact Weather Events

Historically, maxima in the variance of the mass field have been one of the metrics used to identify storm tracks. Consequently, a simple definition of flow regimes is developed here using the variance of the 500 hPa height field as well as the variability in precipitation over different time scales (7, 15, and 30 days) to identify active versus quiescent periods in the entrance and exit regions of the climatological storm tracks. Furthermore, a metric is developed to assess flow stagnation which here is defined as a 24 h period of sustained geostrophic easterlies at 500 hPa calculated over 10 degrees longitude bands. This flow stagnation is then combined with a measure of air mass quality through an assessment of the layer-averaged equivalent potential temperature anomalies to highlight regions that are susceptible to high impact precipitation events. Preliminary results suggest that low variance (decreased storm track activity) regimes in the eastern North Pacific are associated with a combination of an equatorially displaced and westward retracted jet in the North Pacific basin as well a preference for the positive phase of the Pacific North American pattern. Flow stagnation in this regime occurs preferentially in the Gulf of Alaska and extends southwestward along the West Coast of North America with a separate maximum situated in the southwestern United States in association with anticyclonic wave breaking. These patterns are conducive to atmospheric river events in the Pacific Northwest and Alaska as well as a propensity for cut-off low formation in the Southwestern United States. In the western North Atlantic, low variance regimes are associated with an equatorially displaced and weakened jet (consistent with the negative phase of the North Atlantic Oscillation) with abnormally warm conditions over central Canada. Flow stagnation and increased precipitation in this regime occurs preferentially in Ontario and Quebec with a strong decrease in anticyclonic wave breaking and reduced wind shear suggested over the portions of the subtropical Atlantic.