Investigation of differences between northern hemisphere winters from 2001-2013 and 1998-2000 leaves little doubt that a large and consequential trend has occurred in the configuration of the polar jet stream which is linked directly to pronounced warming in high northern latitudes. Moreover, the overall trend has remained largely unbroken and appears to have accelerated in the last six winters.

The coupling between Arctic warming and polar jet is oft referred to as arctic amplification. It features a likely, but not yet definitively proven, link to the apparent increase in extreme, high impact winter weather events occurring over varying regions in mid-to-high latitudes in the northern hemisphere.

It’s critical to note that the seasonal averages examined reflect the net effects of considerable variability of weather systems and processes.  As shown below, the single most prominent signal in the means (averages) to emerge from this variability during the 2000s is one closely resembling the negative phase of the North Atlantic Oscillation (hereafter referred to as -NAO). It is well known that periods of -NAO are favorable for major winter storms affecting the mid-high latitudes from eastern North America though western Europe. Preceding and/or accompanying such storms are surges of anomalous cold penetrating further south over eastern U.S. than nominally expected.

It may seem reasonable to believe the odds favor the continuation of this multi-year trend through this winter (and perhaps winters following). However, given limited understanding of factors contributing to arctic amplification, especially as manifest in the predominance of –NAO discussed here, there is no certainty that will be the case.

Note: the positive NAO phase that has dominated through at least the latter half of December is not necessarily a predictor of the overall winter average. (For what it’s worth, Judah Cohen’s oft-cited winter outlook  is non-committal on the predominant sign of the NAO). Models in recent days suggest that a reversal from a positive to negative NAO may occur next week.

Should a –NAO dominate the balance of this winter, the odds do increase for a “big one” (snow lovers’ fantasy come true) affecting the major Mid-Atlantic to northeastern metropolitan regions, possibly, but not necessarily including, the region around Washington, D.C.

Variability in the occurrence and specific configuration of a given -NAO (within a season) and how these factors govern the specifics of nor’easter development are not predictable beyond a week or so. I’m referring here to details pertaining to the frequency, amplitude, timing, location, and regional persistence of major circulation features associated with a –NAO. The latter include mid-to-high latitude blocking patterns which frequently are the dominant influence on the evolution and track of potential Mid-Atlantic coastal storms. (More generally, the same is true in regard to potentially significant winter storms affecting Europe.)

It’s important to keep in mind that that what is being described here is not necessarily a manifestation of climate change (a.k.a. global warming). The 12-year periods examined here are notably short of the 30 years nominally (arbitrarily?) regarded as minimal to warrant being considered climate. This analysis is limited to describing decadal-scale changes based upon evaluation of readily available global analyses – without reliance on model predictions and/or simulations.

Having said this, however, it must be noted that arctic amplification became apparent observationally (see figure 5) without having been predicted by climate models only 10 or so years ago. Nevertheless, it has become the subject of extensive research and media attention in context of being a critical element in the context of global climate change.

Details (technical analysis/discussion)

As shown in the first image (below), Northern Hemisphere winters on average have warmed significantly relative to climatology (average) over much of the arctic compared to lower latitudes since the turn of the century (bottom left panel). Indeed, there is total reversal from a cold (see top left panel) to warm (bottom left panel) arctic (relative to average) and, therefore, to reversal of anomalies in the latitudinal temperature gradient (decreasing temperature from N-S rather than S-N).

Atmospheric dynamics (“thermal wind”) require that winds adjust when such temperature changes occur, moreso where the horizontal temperature gradient is largest. Readily apparent is the polar jet shift southwards (bottom right panel) along with appearance of a pronounced mid-to-high latitudinal belt of weakened westerly winds extending through most of the troposphere from south central Canada through northern Europe and Siberia.

The geographic variability is illustrated by charts of the low-level (850mb, ~ 5,000 ft) temperature (below top) and 250mb (~30,000 ft) zonal wind anomalies (below bottom), the altitude proximate to the core of the polar jet. The zone of  weaker zonal flow in the 2000 winters is regarded as favorable for increasing occurrence of persistent (slow moving) high amplitude (N-S extent) ridges (high pressure) and troughs (low pressure), and blocking patterns. In turn, this is known to be associated with major (possibly extreme) winter storms and highly anomalous meridional (north to south, or south to north) flow bringing cold/warm air intrusions further south/north than nominally expected.

The first and last six 2000 winters overlap with the 12-year means, but the amplitude of the most recent winters is generally more pronounced. Some examples of extreme weather during this period include winters of: 2009-2010 featuring Snowpocalypse and Snowmageddon; 2010–2011 bringing heavy snowfalls and all-time record low temperatures to much of the U.K; 2012-2013 showcasing by the February nor’easter referred to as the Blizzard of 2013 (or, do I dare say, Winter Storm Nemo); 2006-2007 was memorable for the January series of some of the most violent and destructive storms on record affecting much of Europe.

Asymmetries in the temperature and wind fields were found to appear largest in the area extending from central North America, across the Atlantic through most of Europe. The balance of this post, therefore, focuses primarily upon this area, which not incidentally is most relevant to the eastern U.S.

To place the discussion in perhaps a more familiar framework see the 500mb height chart of the winter mean anomalies for the last six seasons (see below). Perhaps not surprising, at least in hindsight on my part, the overall pattern bears an inescapable likeness to the -NAO (see second image below). For snow lovers/haters alike negative -NAO is a significant “heads up” given the generally known connection, for example, increased chances for a significant nor’easter.

Should the trends to -NAO dominant winters persist through this season –and known possible before now –its likely this information would have been incorporated in the recent round of winter season forecasts, maybe even CWG’s. This leaves me with visions of social media lighting up with expressions of euphoric hope/hype from snow lovers and excessive fear/concern of snow haters – all unwarranted with regard to objective reality.

That reality is being cognizant of significant interannual and sub-seasonal variability. Included is the oft-overlooked fact that there is no single manifestation of -NAO 500mb height fields. Some appear more and some less favorable for a nor’easter depending upon the specifics of the location, spatial orientation, and amplitude of the basic features. The Snowpocalypse and Snowmaggedon winter of 2009/2010 was in a class of its own in terms of a favorable pattern for snowstorms. On weekly time scales, periods of -NAO recur in seemingly random fashion with differing variations of the theme.  In principle, this is much like there being no single manifestation of El Niño and its link to specific weather events.

In my humble but knowledgeable opinion, there is and may never be modeling systems and strategies able to provide reliably useful forecasts of winter season means generally, and specifically in differentiating between positive and negative NAO dominant winters, let alone the array of possible expressions of -NAO when it occurs. At best, consistently reliable forecasts of probabilities of week two means seem achievable.

Finally, for at least those readers less familiar with such things (and have actually made it this far), it’s essential to note that favorable -NAO is only one of a very large number of ingredients that must that come together at the right place and time to result in extreme weather. Perhaps most relevant is an El Niño winter, which is absent this winter. Otherwise, -NAO tilts the odds in that direction but all else is mostly a function of random natural variability unpredictable beyond a few days to perhaps two weeks or so.

Bottom Line: Arctic warming, regardless of causative mechanism (warm season diminishing sea ice, radiative processes, fall to winter Siberian snow, etc.) must be coupled with the corresponding wind field (thermal wind relationship), which in turn plays an important role in weather events. In this investigation of before and after turn of the century winters, this warming signal is manifest in the form of an unmistakable –NAO signal. More specifically, the before and after winters show a shift in the polar jet southward with a zone of weaker zonal winds to the north at mid-to-high latitudes. This is recognized as conducive to development of high amplitude, slow moving, features (e.g., blocking) favorable for significant to extreme weather events.

Further references