This presentation investigates relationships between large-scale regime transitions and cool-season (November–April) Northeast precipitation from statistical and synoptic perspectives. In this study, regime transitions are identified as periods when the North Atlantic Oscillation (NAO) index or Pacific–North American (PNA) pattern index undergoes a two-standard deviation change centered on zero over seven or fewer days. To identify regime transition periods, 56-year (1948–2003) time series of daily NAO and PNA indices were generated from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis dataset. A daily precipitation anomaly time series for the Northeast derived from NCEP’s Unified Precipitation Dataset for the same 56-year period was used to calculate precipitation anomalies during these periods.
Statistical results indicate that two types of large-scale regime transitions are conducive to above-normal cool-season precipitation in the Northeast: (i) positive-to-negative NAO transition periods and (ii) negative-to-positive PNA transition periods. Statistical results also show that the NAO index tends to decrease and the PNA index tends to increase in conjunction with major cool-season Northeast precipitation events. Composite analyses of positive-to-negative NAO regime transitions bracketing major Northeast precipitation events reveal that extratropical cyclones associated with these precipitation events contribute to the evolution of a negative NAO regime by transporting warm air to high latitudes and creating a high-over-low (Rex-type) block.
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