The North Atlantic Oscillation: Impact on Snowfall Conditions in the Northeastern United States

The North Atlantic Oscillation (NAO) is one of the main components of atmospheric circulation variability within the middle and high latitudes of the Northern Hemisphere, particularly impacting winter weather patterns in northeastern United States. Previous research has indicated greater snowfall totals and higher frequency of snowfall days in the Northeast during a NAO negative phase due to repeated polar outbreaks; yet, the NAO positive phase has also been associated with extreme snowfall events in this region. This study examines the relationship between the NAO and winter (November – March) snowfall totals in northeastern U.S. between 1961 and 2010. Two case studies of recent winter events with differing NAO phases were evaluated to provide insight on how both NAO phases can produce significant snowfall in portions of the Northeast.

The analysis revealed an inverse relationship between NAO phase and seasonal snowfall, with positive (negative) NAO index years associated with lower (higher) average snowfall and snowfall days. Significantly greater snowfall during the NAO negative phase was mainly located along the East Coast as well as the interior southern half of the study region. A composite analysis of various tropospheric variables (e.g., 500-hPa heights) showed NAO negative years produced greater snowfall due to more extreme weather conditions affecting the Northeast, such as below normal sea level pressure, a deepened mid-tropospheric trough and weaker upper-level westerlies that permitted more frequent polar outbreaks. The intrusion of cold polar air into the interior U.S. generates more extreme temperature gradients and produces snowfall farther south than the NAO positive phase. In addition, the eastward displacement of the storms in the NAO negative phase along with the available moisture from the Atlantic Ocean creates more snowfall along the East Coast. These results correspond to the spatial distribution of snowfall that occurred during the February 2010 winter storms that produced significant snowfall throughout most of the Northeast, particularly around the mid-Atlantic.

On the other hand, stations indicating no relationship between NAO and snowfall were mainly located in western and central New York, northern Vermont, and Maine. These regions of the Northeast correspond to those that receive snowfall in either NAO phase, as exhibited in the case studies, due to the effects of nearby lakes and higher terrain related to the storm tracks through these areas. Winter storms track over this northern region (of the Northeast) regardless of NAO phase whereas snowfall in the mid-Atlantic states is more dependent on a strong southerly track (i.e. Cape Hatteras/Nor'easter low) associated with the NAO negative phase. Since western New York and northwestern Pennsylvania border Lake Ontario and Lake Erie, the stations in these regions commonly receive lake-effect snow as a result of great fetch due to the west-east orientation of both lakes. Stations in central and northern New York receive lake-effect snow as additional lift is generated when the moist air reaches the Allegheny Plateau and the Adirondack Mountains.