Summary: What are the leading influences on North America winter 2018-19 climate? Typical of climate patterns of recent years ENSO has a diminished role despite presence of an El Nino likely to gain strength. The lead modes of climate influence are the warmer than normal Pacific Ocean in the tropics and northern latitudes near the Dateline and warmer than normal Norwegian Sea. The anomalous warmth in the central tropical Pacific Ocean will continue to cause convection releasing heat pole ward that amplifies a western North American high pressure ridge (and downstream upper trough). The pattern described has already lead to record eastern North America snow cover. In-between the North American and northern Europe/northwest Russia snow cover the open ice-free waters (north of Europe) warm the atmosphere and cause semi-permanent high latitude/high pressure causing the persistent 2018 polar vortex in the polar region to split and shift southward. Not quite a “polar vortex” winter but strong enough to cause the northern portion of an El Nino-produced South/East U.S. storm track to be mostly snow causing cold air to hang over the Northeast.
Discussion: There are two leading modes of climate variability identified as primary contributors to the North America winter 2018-19: 1.) Central tropical Pacific Ocean warmth (Fig. 1) and 2.) Open water of the wintertime Norwegian Sea (Fig. 2). Other factors such as El Nino, transient Madden Julian oscillation and stratospheric warming events can over-rule these 2 leading factors at times.
The central tropical Pacific warmth will create an environment that allows persistent convection occasionally enhanced by the Madden Julian oscillation (MJO). The heat release pole ward of this convection warms the atmosphere and causes the mean location of an upper level high pressure ridge to reside over western North America. Beneath the upper ridge temperatures are warmer than normal and precipitation limited.
Downstream the upper ridge is a semi-permanent presence of the polar vortex in northeast/east North America causing expansive snow cover and generation of cold air masses. The 30-day 500 MB anomaly analysis reveals this pattern has already evolved (Fig. 3).
An El Nino winter usually produces an active southern U.S. storm track. Given the big chill residing over eastern Canada expect the eastern portion of the El Nino storm track to feature more snow than normal in the Northeast Corridor this winter season.
Once again the constricted polar ice cap leaves the North Atlantic Ocean to the north of Europe ice free. The relative warm ocean water surrounded by snow covered land mass to the west (Canada) and east (northern Europe/northwest Russia) will favor the generation of high latitude/high pressure blocking patterns which can energize middle latitude storm tracks causing more snow and cold.
The circumstances described leading to increased risk of high latitude/high pressure blocking (i.e. negative North Atlantic oscillation) is a large change from the persistent climate pattern of 2018 in which the polar vortex was near home base (far northern latitudes) and record strength.
Snow cover is the key to a cold winter. Currently, snow cover is already historically expansive across the North-Central to Northeast U.S. remarkable before Thanksgiving (Fig. 4). The tropical Pacific is a leading contributor to this event. Removing the snow cover as meteorological winter arrives becomes very difficult due to the lowering less effective sun angle. If the snow cover holds across the northeast quarter of the U.S. the winter could be extremely cold in the Northeast and very uncharacteristic of an El Nino winter.
There are always caveats in climate forecasts reliant on many variables beyond just El Nino southern oscillation (ENSO). If the warming biased toward the central tropical Pacific Ocean plus warming of the Pacific in the far northern latitudes shifts east a milder El Nino climate would emerge.
One other factor requiring mention in regard to the winter outlook (or any climate outlook) is the arrival of solar minima. Unproven is the direct link of solar intensity at any one given time on earth’s climate. However, statements on cumulative effects should be addressed. The solar cycle which averages 11 years in length from maxima-to-minima and back to maxima has changed quite dramatically the past 15-20 years.
After an intense peak intensity of the solar maxima in 1999-2000 the descent into solar minima forecast for 2005-2006 lasted much longer (4 years) at extremely low intensity (Fig. 5). The maxima that followed in 2012-2014 was choppy in intensity and about half the strength of the 1999-2000 solar maxima. More quickly than expected the following solar minima has already developed and solar intensity is lower than forecast. Another lengthy solar minima is likely.
Accumulatively, solar energy into the earth’s atmosphere has been below the historical average for about 15 years. Precisely what that means to weather is not well understood but recognizing the presence of relative inactive sun requires monitoring. The last period of an inactive sun was 1800-1830 (“Dalton Minimum”) when the earth’s atmosphere cooled.
Forecast models are in general agreement on weak-to-moderate El Nino for winter 2018-19 (and well into 2019). The latest NCEP CFS V2 indicates the weak-to-moderate intensity (Fig. 6). Combining operational Nino index (ONI) and multivariate ENSO index (MEI) to identify the best ENSO analog years due to their weak-to-moderate NINO34 SSTA signatures and weak MEI characteristics identifying the weaker coupling between the atmosphere and the El Nino oceanic warming 2004-05 and 2014-15 are chosen.
To project the average upper air pattern across the polar region an optimum climate normal (OCN) is used. OCN is the average climate over a 10-year (sometimes 15-year) period compared to the standard 30-year climatology due to a persistent change in climate most notable in recent years. The 500 MB anomaly OCN indicates presence of strong high latitude/high pressure blocking but not in the exact right position (Fig. 7) to enable negative NAO indices yet the sensible weather pattern has more often than not been representative of –NAO. The regime identified is likely due to the constricted polar ice cap influence on climate accelerating during the past 10-15 years.
The North America forecast is based on the ENSO analog years 2004-05 and 2014-15 plus the 10-year OCN to project reasonably reliable expectations for winter 2018-19 plus spring and summer 2019.
Fig. 1: A leading contributor to global climate is the placement of convection in the equatorial region. Convection has a tendency to appear most consistent where the ocean surface is warmest.
Fig. 2: The constricted polar ice cap of the past 20+ years has left the Norwegian Sea free of ice and increased the ability of a relative warm ice free ocean to warm the atmosphere above leading to increased risk of high latitude/high pressure blocking patterns which force the polar vortex out of the polar region to energize middle latitude snow storms and attendant cold.
Fig. 3-4: The influence of tropical Pacific Ocean warming near the Dateline has contributed to semi-persistent high pressure ridging across western North America causing a downstream polar vortex over Quebec as indicated in the 30-day 500 MB anomaly analysis (left). The upper air pattern has caused above normal snow cover from the North-Central to Northeast U.S. (right).
Fig. 5: The 11-year solar cycle has changed characteristics significantly in this century with an unusually lengthy solar minima in 2007-2010 and possibly the arrival of another lengthy and intense solar minima beginning this year separated by a weak and choppy solar maxima 2012-2014.
Fig. 6: The 11-year solar cycle has changed characteristics significantly in this century with an unusually lengthy solar minima in 2007-2010 and possibly
Fig. 7: Optimum climate normal is a measure of the average condition over a 10-year period (versus standard 30-year period).