Integrated regional enstrophy and its correlation with regime transitions in the Northern Hemisphere
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Integrated regional enstrophy (IRE) is the enstrophy integrated over a portion of the globe at a specific level in the atmosphere. Previous work has shown this quantity is correlated to the positive Lyapunov Exponent for hemispheric flow, and as such is a measure of flow stability or predictability. In this study, IRE is calculated at 500-hPa over an area that encompasses 0� to 70� N in the Northern Hemisphere. The data sets used were the 500-hPa initial and forecast fields for the Global Ensemble Forecasting System (GEFS) (on a 1� x 1� latitude-longitude grid) provided by the National Oceanic and Atmospheric Administration (NOAA) Weather Prediction Center (WPC) and the National Centers for Environmental Prediction / NOAA reanalysis (on a 2.5� x 2.5� latitude-longitude grid) archived in Boulder, CO. The GEFS forecast fields were provided every 24-h out to 240-h. By examining these forecasts over a year, we have found that significant changes in IRE values are a good predictor of flow regime transition, and several cases were found. We also found that the IRE forecasts identified these regime transitions reliably out to four days, and the skill decreased significantly after this time. Additionally, a threshold for changes in IRE was found for the cases studied here and this threshold varied by season.