highlattop

The Importance of High Latitude Blocking in The Upcoming Winter

In our winter forecast, we discussed certain signals that supported high-latitude blocking developing for the middle and late periods of this upcoming winter. High-latitude blocking often comes with negative phases of the North Atlantic Oscillation (NAO), the Artic Oscillation (AO), and/or the Eastern Pacific Oscillation (EPO). High-latitude blocking is one most important factors in any winter forecast, as it typically supports colder temperatures and larger snowstorms for the Northeast US. This is an even more important factor in during a strong El Nino winter, as the Pacific or Subtropical jet is generally more active and stronger.

Research from Al Marinaro (@wxmidwest ) brought to light a strong correlation between the sea-level pressures in North Pacific and the NAO modality, during +ENSO (El Nino) winters. North Pacific sea-level pressures less than 1013 mb had a -NAO value on average for the December, January, February, and March period (DJFM). Sea-level pressures above 1013 mb during the month of October had either a positive NAO value or very close to netrual in the following winter. An official pressure for North Pacific reading has not been released for October yet. However, NCEP/NCAR reanalysis tells us that sea-level pressures were on average around 1010 mb over the region 65N-30 & 160E-140W in the North Pacific. That would be well-below the 1013mb threshold for a -NAO DJFM.

 

NP SLP NAO Indicator & ObservationOne of the first topics of discussion, among many meteorologists and hobbyist this Autumn, was the fact that this correlation did not work very well last year at all.  Despite the sea-level pressure below 1013mb in North Pacific in October 2014, we still saw very positive +NAO on DJFM average. However, last year winter’s El Nino started very weak.  The +ENSO forcing and influence was not great — and Al Marinaro notes that the correlation works best during at least moderate ENSO years.  Solar and geomagnetic activity was on the higher side last year as well, and this combination reduced wave activity flux (WAF) or Rossby waves. All in all, this helped to support a stronger stratospheric vortex.

This years El Nino is much stronger, and solar radio flux is has declined from last year. This will help increase Wave Activity Flux that will eventually lead more weakening of the stratospheric polar vortex and thus more high-latitude blocking during the mid to late late winter period. In addition to the North Pacific SLP, Al Mariano also found another correlation between 700mb zonal winds in the north pacific and a DJFM NAO phase, during +ENSO winters.  When October 700mb zonal winds in the North Pacific are greater than 8.5 m/s, the ensuing winter typically features a positive NAO modality. The 700mb zonal wind speed for this October was 3.47 m/s. This is well below 8.5 m/s threshold for a -NAO.

 

700mb Zonal Winds

While the above research and interpretation is quite extensive, we also researched the NOAA climate data for any substantial correlations. This past July we saw a AO at -1.356 and NAO at -3.18. These are both near record low levels for July. Within our winter forecast, we discussed the correlaktion between the negative Arctic Oscillation modality in July — and a negative modality in the following winter. When one takes a 500mb anomaly for the winters following the top 10 lowest AO values in July, the results are impressive. There is a very pronounced high latitude blocking signal over Greenland and the Davis Straight westward to Alaska and the Bering Straight. A deep upper level low is positioned farther southwest in the Northeast Pacific, and anomalous troughing is present over the Eastern US. Despite these strong numbers, we did err on the side of caution with using these correlations — due to the variation among analog years and the surrounding conditions.

July -AO to DJF 500mb anomalyThe Quasi-Biennial Oscillation (QBO) will be in a westerly or positive phase during this winter.  Past moderate or strong El Nino winter analogs with the +QBO of the also support more high-latitude blocking by February. The 500mb composite for each winter month, for all those years, indicates a large upper-level low or deep trough over the Gulf of Alaska in December supporting higher heights over much of United States. This is supported on many long term forecast models already with above normal heights and temperatures in November and December. This upper level low/trough begins to retrograde in January and subtropical jet becomes more active.  By February, the upper-level low/trough is positioned south Aleutians supporting more ridging along the West Coast,  and a large upper level trough is present over the Eastern United States with high-latitude blocking over Greenland.

DJF Mod-Strong w +QBO 500mb anomaly

Below are the positive QBO values for each of the previous years from October to March. Most of these years had the +QBO peak in the October or November  before the following winter, and then a slow decline in the positive values through March. The +QBO in the 1982-83 winter continued to rise until peaking in April 1983.

+QBO Values During Mod-Strong Nino Events

All those winters, except 1987-88, had colder and snowier conditions by February and slightly above normal snowfall for New York City metro. Below are snowfall totals from some notable major winter storms for NYC Metro Area from some of those years. These storms also came with during periods of high-latitude blocking. (Source: the Northeast Snowstorms Volume II: The Cases)

February 14-17, 1958 – La Guardia Airport,NY 10.1″

March 18-21, 1958 –  Central Park,NY 11.7″

January 11-14 1964 – Central Park,NY 12.5″

February 10-12, 1983 –  La Guardia Airport,NY 22.0″

In addition, as mentioned in a previous premium post–Monitoring the Stratosphere in November–the QBO value at 30mb for October is +13.36. This was the highest positive value at this level for October going to back to records since 1948.  Typically, the positive QBO phase promotes a deeper, more profound polar vortex in the troposphere and stratosphere, while the negative QBO phase supports a high-latitude blocking with a weaker, more mobile polar vortex. The QBO influence here is more minimal with values between +10.00 and -10.00. The highest values on record with the positive QBO phase aren’t higher than about 15.00. The graph below also indicates this +QBO phase is likely near peak. So while we still expect the QBO to average above +10.00 for the entire winter, a slow decline may begin by February or March.

The QBO phase shown in a zonal wind anomaly (m/s) at 30 hPa oscillating between westerly  (around 90°) and easterly (around 270°) for intervals back to March 1961.  This QBO began it's westerly phase in July 2015 and is entering the 45° to 90°  quadrant where the wind anomalies have reached their peak in this phase (image courtesy of Sam Lillo @splillo

The QBO phase shown in a zonal wind anomaly (m/s) at 30 hPa oscillating between westerly (around 90°) and easterly (around 270°) for intervals back to March 1961. This QBO began it’s westerly phase in July 2015 and is entering the 45° to 90° quadrant where the wind anomalies have reached their peak in this phase (image courtesy of Sam Lillo @splillo

 

However, the QBO is not the only factor with high-latitude blocking. Another factor is solar activity, which can switch around the typical influences supported from the each QBO phase. Periods of higher solar activity have been linked to a stronger polar vortex and thus less high-latitude blocking during the winter. High-latitude blocking and a weaker polar vortex has been more prevalent during winters with lower solar activity. In fact, a new research article came out this week, with evidence of a connection between the solar maximum period and the North Atlantic cold pool and three-year lag behind with the maximum -NAO modality. All of this being said, we currently sit amid a weaker solar cycle with a double peak of sunspot activity in late 2011 and early 2014. So this may not correlate to a maximum -NAO modality until 2016-17 winter season.

Back during the 2009-10 winter — which many remember for its high latitude blocking, cold and snow — we were at the solar minimum and in a moderately strong -QBO phase. We saw high-latitude blocking for much of that winter, and it even supported a colder and snowier December. Last winter, we saw a spike in sunspots, solar radio flux, geomagnetic storms–thus a stronger polar vortex and less high-latitude blocking.

This year looks very much like a blend, with no wildly anomalous trend in either direction. A middle road is likely, as we sit between the maximum and minimum of solar cycle 24. At this time, we are seeing another small uptick sunspots and minor-moderate geomagnetic storms. So we expect to see very little in the way of high-latitude blocking developing through December. This amount of solar activity  is predicted to decline early next year which will bring us closer levels observed during 2012-13 and 2013-14. Those winters also featured a trend towards negative AO/NAO/EPO modality in the mid-late winter months.

History of sunspots and solar radio flux and projections near the minimum

Sunspots and Solar Radio Flux predicted to closer to levels reached between during 2012-13 and 2013-14 winters

 

Lastly, our forecasters continue to monitor an anomalous surface high pressure or ridge that has been over Eastern Europe in October. There is a correlation to strength of the high over this area and the strength of polar vortex, at this time of year. Depending on the strength and position of this high, it could halt the intensification of polar vortex that typically occurs in early in the season. This could allow for in -NAO blocking pattern to develop in January. This research on this topic has been noted recently by Anthony Masellio @antmasiello.  

Eastern Europe High

To summarize, our confidence continues to be high that high latitude blocking will develop during the middle to latter part of this winter. The variables discussed above were the basis for that conclusion in our winter forecast, and they continue to remain generally on track as we move forward through the month of November. As noted in our winter forecast, we expect December to be a warmer than average month with less than average snow — and a positive AO/NAO and EPO leading to a non-wintry pattern.

Changes are likely to begin in Mid January, with wintry weather increasing as the month goes on. Finally, February will be the coldest and more wintry month with a negative modality of the AO/NAO and EPO, and the potential for a few bigger snowfall events and high latitude blocking. We expect this to continue into March.

Nevertheless, there are also other major factors — such as the MJO, PDO, and ENSO peak, which will continue to evolve and change the atmospheric evolution. These will have impacts on the forecast moving forward — and we’ll be discussing them in our next technical post installment in the upcoming week. Happy Sunday!

The above post was a collaborated, written and edited by Miguel Pierre and John Homenuk.

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