Abstract
Past research has indicated that reconfigurations of large-scale flow
regimes can alter regional weather patterns due to shifts in storm tracks and
associated eddy transports of heat, momentum, and vorticity. Conventional
wisdom also suggests that high-impact weather events tend to occur during
large-scale regime transitions. Motivated by these considerations, this research
investigates relationships between large-scale regime transitions and Northeast
precipitation in the cool season (November–April) from a statistical and synoptic
perspective.
In this study, a regime transition is defined as a two-standard-deviation
change centered on zero in the North Atlantic Oscillation (NAO) index or
Pacific/North American (PNA) pattern index over a seven-day period. To identify
regime transitions, a 56-year database (1948–2003) of daily NAO and PNA
indices was generated from the National Centers for Environmental Prediction
(NCEP)–National Center for Atmospheric Research (NCAR) reanalysis dataset.
A daily precipitation anomaly database for the Northeast was derived from the
Unified Precipitation Dataset (UPD) for the same 56-year period.
Key statistical results indicate that transitions from positive to negative
NAO regimes and from negative to positive PNA regimes are associated with
enhanced precipitation in the Northeast. Conversely, transitions from negative to
positive NAO regimes and from positive to negative PNA regimes are associated
with suppressed Northeast precipitation. Results also show that during periods
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surrounding major Northeast precipitation events in the cool season, the NAO
index tends to decrease and the PNA index tends to increase.
To interpret these relationships synoptically, composite analyses were
created of cool-season regime transitions surrounding major precipitation events
in the Northeast. The analyses suggest that synoptic-scale features are
important in these types of large-scale regimes transitions. A positive-tonegative NAO regime transition (a weakening of the North Atlantic jet)
surrounding a major Northeast precipitation event appears to be related to strong
warm air advection in the western North Atlantic downstream of a surface low
associated with the Northeast precipitation event. In the case of a negative-topositive PNA regime transition [an amplification of a trough (ridge) over eastern
(western) North America] surrounding a major Northeast precipitation event, two
synoptic-scale features appear to be important. One feature is persistent warm
air advection that amplifies a ridge over western North America, and the other
feature is a weak cold surge in the Northeast in the wake of the precipitation
event that acts to precondition the atmosphere for a second, stronger cold surge
over eastern North America.
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