Cool-season regime transition and its impact on precipitation in the Northeast

Past research has pointed to a relationship between cyclogenesis and rearrangement of the large-scale flow, and synoptic conventional wisdom suggests that high-impact weather events are associated with large-scale regime transition. Motivated by these considerations, we investigate cool-season regime changes impacting the northeastern United States from a synoptic climatological perspective.

Daily indices representing the North Atlantic Oscillation (NAO) and the Pacific–North American (PNA) pattern, generated from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis dataset, were used to define large-scale regimes objectively. Upon examining the temporal variability of the indices, large-scale regime transition was defined to be a teleconnection phase change (i.e., a change from negative to positive phase or positive to negative phase) of more than two standard deviations over a seven-day period.

Climatological precipitation anomalies over the Northeast during large-scale regime transitions were calculated from the NCEP Unified Precipitation Dataset. A regime transition was considered to be wet if the Northeast precipitation anomaly for the regime transition period was greater than or equal to +1 standard deviation. Conversely, a regime transition was considered to be dry if the Northeast precipitation anomaly for the period was less than or equal to _1 standard deviation.

Statistical results indicate that large-scale regime transitions from a positive to negative NAO pattern tend to be associated with periods of enhanced Northeast precipitation in the cool season. Results also indicate that large-scale regime transitions from a negative to positive PNA pattern tend to be associated with enhanced precipitation in the Northeast in the cool season.

Relationships between large-scale regime transitions and Northeast precipitation are interpreted using composites of synoptic forcing during wet and dry regime transitions. Synoptic precursors are also identified in order to improve forecast skill of precipitation in the Northeast during large-scale flow reconfigurations.