The merits and limitations of using ENSO phase as a winter forecasting tool in the northeastern United States

A 64-year climatological record for the cold season in Syracuse, New York was analyzed for temperature, liquid equivalent precipitation, and snowfall with the intent of determining the value of El Niño Southern Oscillation (ENSO) phase as a seasonal forecasting tool in the northeast United States. Statistical evidence suggests that El Niño winters are characterized by a distinct trend in which warmer temperatures and below normal snowfall occurs during the first month of winter followed by colder temperatures and above normal snowfall for the second month of winter. The analysis of a dimensionless snowfall to liquid equivalent precipitation ratio reveals a corresponding trend of low ratios during the first month of winter and high ratios during the second month of El Niño winters. While characteristics of La Niña winters differed little from ENSO neutral winters during the first month of winter, the second 30 day period yielded anomalously low snow/precipitation ratios, suggesting the more frequent occurrence of warmer temperatures accompanying precipitation events consistent with a westward displaced storm track. Major snow events, defined as two-day totals exceeding a foot, were more than five times more likely to occur for El Niño winters compared to neutral winters. Conversely, major snow events were rarely observed during La Niña years, most especially during the second month of winter. It is suggested that the greater frequency of heavy snowfalls during El Niño winters is related to a more pronounced and disturbed sub-tropical jet stream, which ultimately provides for more phasing opportunities with trough-like features in the polar jet. This combination is known to be associated with the development of major east coast winter storms.

While the La Niña winter of 2008-2009 lived up to statistical expectations, the first month of the 2009-2010 El Niño winter did not follow suit, defying the expectations of observing a warm period during the first month of winter. Upon reanalysis, it becomes clear that the North Atlantic Oscillation (NAO) was decisively locked into its negative phase from early December through mid-January. An extended period of negative NAO phase is known to yield a synoptic scale trough over the Eastern United States allowing for reinforcing surges of cold polar air to infiltrate the east coast. The prolonged period of negative NAO phase appeared to be responsible for an unprecedented situation in the climate record, in which colder than normal temperatures were observed for the first 30 days of a significant El Niño winter.

The sum total of climatologically derived evidence suggests that the leading modes of atmospheric variability in the Pacific and Atlantic basins, represented respectively by ENSO and NAO, collectively influence cold season weather patterns in the northeast United States. However, their teleconnections operate independent of one another, having different mechanics and timescales. While ENSO phases tend to last for several months, NAO phases tend to vary on weekly or even daily timescales. While ENSO phase is most strongly tied to the enhancing/suppressing of vorticity in the sub-tropical jet stream, the NAO is most strongly related to the troughing/ridging character of the polar jet stream in the Northeast United States. Despite their independent nature, major east coast winter storms often rely on southern stream vorticity, a known byproduct of El Niño, as well as the presence and position of cold air supplied by troughs associated with the negative phase of the NAO. Therefore, by throttling the degree and frequency of cold air and associated trough-like features in the polar jet during a given winter, phase locking of NAO can alter the expectation of a seasonal forecast strictly based on ENSO phase.