11:00 AM | *El Nino conditions are starting to appear in the equatorial Pacific Ocean and its specific location raises the prospects for a cold and snowy winter in the eastern US*

The current sea surface temperature anomaly pattern features a large and expanding area of warmer-than-normal conditions across the central-to-western part of the equatorial Pacific Ocean; courtesy NOAA, tropicaltidbits.com

Overview
The Pacific Ocean is the planet’s biggest and its sea surface temperature (SST) pattern has a tremendous influence on all weather and climate around the world.  In particular, El Nino conditions (warmer-than-normal) in the equatorial part of the Pacific Ocean can have a tremendous impact on winter weather in the US depending on its intensity and duration.  In addition to duration and intensity, the specific location of an El Nino in the equatorial Pacific Ocean can be quite crucial to wintertime temperatures and precipitation patterns in the US and current signs point to a weak-to-moderate strength El Nino that is “central-based” and this raises the prospects for a cold and snowy winter in the eastern US.  

El Nino
There have been signs for the past few months that an El Nino would develop in the tropical Pacific Ocean by the latter part of the summer or the early fall and there is now a noticeable region of warmer-than-normal water along and just north of the the equator.  Numerous dynamical and statistical forecast models agree on the formation of a weak-to-moderate El Nino later this summer or early this fall and maintain it through the upcoming winter season. In terms of location, an “eastern-based” El Nino in the equatorial Pacific Ocean features its warmest water close to the west coast of South America whereas a “central-based” El Nino - sometimes referred to as "modoki" - is concentrated in the middle part of the Pacific Ocean.  

The first sign of an "central-based" El Nino may be occurring right now with a noticeable area of warmer-than-normal water situated in the central Pacific.  In addition, some computer forecast models (e.g., NOAA CFSv2, JAMSTEC) suggest the upcoming winter will indeed feature a "central-based" El Nino.  In recent history, strong El Nino’s that were “eastern-based” generally have been associated with warmer-than-normal winters in the eastern US whereas “central-based” weak-to-moderate El Nino’s have been correlated with cold and snowy winters. For instance, two strong and "eastern-based" El Nino’s that resulted in warm winters in much of the eastern US took place during the winters of 1972-1973 and 1997-1998.  On the other hand, two weak-to-moderate El Nino's that were "central-based" and  resulted in cold and snowy winters occurred in 2002-2003 and 2009-2010. 

This graph show forecasts made by dynamical and statistical models for sea surface temperature anomalies in the “Nino 3.4” (central) region of the Pacific Ocean for nine overlapping 3-month periods. The vast majority of the models feature weak-to-moderate El Nino conditions by the upcoming winter season of December/January/February (indicated by arrows). Data source: International Research Institute for Climate and Society/Columbia University

Two reasons why a weak-to-moderate and "central-based" El Nino can increase the chances for a cold and snowy winter in the eastern US are as follows: 1) this type of sea surface temperature pattern tends to provide added moisture and “lift” to the southern branch of the jet stream which, in turn, raises the chances for a storm track across the southern and eastern US - more favorable for snow in the eastern US and 2) higher heights and higher pressure tend to form in this type of pattern over western Canada which usually allows for multiple cold air outbreaks into the eastern US.   The findings in a recent publication support the idea of an increased chance for a cold and snowy winter in the southern and eastern US during a "central-based" El Nino winter season.

Observed anomalies of 500 mb geopotential heights (contours) and surface air temperature anomalies (color shade) in "central-based" El Nino winters (left) resulting in a “di-pole” pattern across the US. Publication source.

Warming in the northern Pacific Ocean
In addition to the "central-based" El Nino sea surface temperature pattern expected in the equatorial part of the Pacific Ocean, there is another important region which too may be signaling that there can be numerous cold air outbreaks this winter season in the eastern US. Specifically, the same computer forecasts models that are predicting a "central-based" El Nino (e.g., NOAA CFSv2, JAMSTEC) both forecast warmer-than-normal water this winter to the south of the Alaskan southern coastline.  This kind of sea surface temperature pattern actually took place on more than one occasion this decade (e.g., 2014-2015, 2013-2014).

The Japan Agency for Marine-Earth Science and Technology (JAMSTEC) sea surface temperature anomaly forecast map for the upcoming winter season of December/January/February features an El Nino focused in the central part of the tropical Pacific Ocean and a large patch of warmer-than-normal water in the Gulf of Alaska (base period for estimation of anomalies is 1983-2006).

If there is a persistent area of warmer-than-normal water tucked in near the southern Alaskan coastline, it can result in consistent high pressure ridging along the Canadian west coast, a condition known to meteorologists as the negative Eastern Pacific Oscillation (-EPO). In turn, this often results in a dip of the upper-level jet stream in the eastern US which allows for the penetration of cold air masses from the northern latitudes into the Mid-Atlantic region during the winter season.  

Final thoughts
While winter is still a few months away, the breakout of warmer-than-normal water in the central Pacific as compared to the eastern equatorial region near the west coast of South America bodes well for a cold and snowy winter in the eastern US.  In addition, the warmer-than-normal water that has formed south of Alaska in the northern Pacific and is predicted by some computer forecast models for the upcoming winter support the notion of numerous cold air outbreaks for the eastern US.  We’ll continue to closely monitor these important factors here at perspectaweather.com to see how they evolve over the next couple of months.  In addition, a few other factors that will be closely monitored here include solar activity, the pressure/temperature pattern across the North Atlantic (e.g., NAO, AO), and the snowpack that forms this fall over cold air source regions such as Siberia and northern Canada. For more information on the "early signs of a cold and snowy winter" click here and...stay tuned…it might be an exciting winter season of 2018-2019.

Meteorologist Paul Dorian
Perspecta, Inc.
perspectaweather.com