In order to quantify an objective definition of a weather regime, teleconnection indices such as the North Atlantic Oscillation (NAO), the Pacific/North American (PNA) pattern, the East Pacific Oscillation (EPO), and the West Pacific Oscillation (WPO) were used to characterize preferred modes of atmospheric circulation. Using daily-averaged 500 hPa geopotential height data from the National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis dataset, daily values from January 1948 to December 2001 were produced for each index, according to the procedure defined by the Climate Diagnostics Center (CDC). Upon examining the variability of each time series, a major weather regime change was preliminarily defined to be a greater than two standard deviation change in an index over a seven day period.
As an intermediate step in determining correlation between Northeast precipitation events and regime transitions, a normalized height anomaly index was developed to identify anomalously low 1000 hPa geopotential heights in the Northeast and vicinity. Daily values of this index are being used in conjunction with daily values of change for each teleconnection index in order to determine whether a correlation exists between surface height anomalies in the Northeast and regime transitions. Future work will determine whether more major precipitation events can be expected during regime changes as compared to climatology. Composite analyses will be constructed so that characteristic signatures of significant large-scale regime changes may be identified. These composites and results from case studies will be used to determine whether a causal relationship exists between a regime change and a major precipitation event.