It's No Longer You Grandfather's Winter: The Relationship Between Changes in the Nocturnal Polar Vortex, Terrestrial Boundary Conditions, and Snowfall Climatology in the Urban Corridor of the Northeastern U.S.

Beginning in 1997 the average winter snowfall for Boston's Logan Airport (BOS) has increased from 42 inches / yr ( 107 cm) to 52 inches / year ( 133 cm). Additionally, the frequency of synoptic scale cyclones producing >20 inches ( 51 cm) per event has increased from 1:50 yr to 1:6 yr during this same period. In the Mid-Atlantic urban corridor, although seasonal snowfall has not reflected a significant increase, this region has been impacted by extreme single snowfall events resulting in widespread >20" accumulations e.g. the so-called "snowmeggedon" events of 2003, 2009, and 2016. Moreover, significant snowfall events in this region have also been associated with anomalously cold temperatures.

Such changes in snowfall climatology may be attributed, in part, to wave flux activity ( WAFz) resulting in "sudden stratospheric warming events" ( SSW) leading to thermal forcing of the stratospheric polar vortex. eventually propagating downward impacting the tropospheric polar vortex. The outcome of this phenomenon are 1)coupling between the stratospheric and tropospheric polar vortices 2) circulation asymmetries commensurate with a more amplified mid-latitude circulation 3) bifurcation of the polar vortex into sub-vortices that migrate into the mid-latitudes historically associated with extreme winter weather events in the northeastern U.S.

In addition, changes in the Pacific Decadal Oscillation ( PDO) and SST in the North Pacific basin e.g. warm anomalies in the vicinity of the Gulf of Alaska ( the so-called "warm blob") and its associated thermal forcing has contributed toward the formation of a persistent mid-tropospheric ridge across western North America coupled with a downstream trough over eastern North America. For example the record snowfall winter of 2014-15 in Boston ( 11- inches, 279 cm) was the outcome of this western ridge - eastern trough Rossby wave pattern that persisted from late January into early March. This pattern also featured extreme cold air outbreaks, and frequent ( and a few case explosive) cyclogenesis along the Mid-Atlantic coast, or the intensification of migratory cyclones e.g. "Alberta Clippers" as they propagated off the coast. Cyclonic activity was further amplified by above avg SST across the wester North Atlantic basin in the vicinity of the northeast coast.

This presentation will feature an overview of these changes in climatology drawing upon the most recent research in this area.