Synoptic Characteristics of Surge-Producing Extratropical Cyclones along the Northeast Coast of the United States.

Extratropical cyclones (ETCs) are responsible for most of the large storm-surge events in the northeastern United States. This study uses the ECMWF atmospheric reanalysis of the twentieth century (ERA-20C) and NOAA tide gauge data to examine the local, regional, and large-scale atmospheric circulation accompanying the 100 largest ETC-driven surge events at three locations along the northeastern coast of the United States: Sewells Point (Norfolk), Virginia; the Battery (New York City), New York; and Boston, Massachusetts. Results from a k-means cluster analysis indicate that the largest surges are generated when slowly propagating ETCs encounter a strong anticyclone, which produces a tighter pressure gradient and longer duration of onshore winds. The strength of the anticyclone is evident in the middle and upper troposphere where there are positive 500-hPa geopotential height anomalies overlying the surface anticyclone for the majority of clusters and nearly all of the five biggest surge events. Multiple clusters feature a slower-than-average storm and a strong anticyclone, indicating that various circulation scenarios can produce a large storm surge. This favorable environment for large surge events is influenced by well-known modes of climate variability including El Niño, the Arctic Oscillation (AO), the North Atlantic Oscillation (NAO), and the Pacific-North American (PNA) pattern. ETCs are more likely to produce a large surge during El Niño conditions, which have been shown to enhance the East Coast storm track. At Boston and the Battery, maximum surge occurs preferentially during the positive phase of PNA and the negative phases of AO/NAO. [ABSTRACT FROM AUTHOR]