Griteater’s Winter Outlook (18-19)
“It is our view that doing the science and going through the steps is as important as coming up with the right answer. A lot of people don't understand this, as the emphasis in our culture is on getting the right answer. For example, if I forecast the winter pattern based upon the number of socks in my dresser drawer and the forecast turns out to be correct, it doesn't mean my technique was scientifically based and/or it can be used in the future.”
Dave Tolleris (wxrisk.com) – November 2016
ENSO Related Parameters
SSTs – it looks like we are going to have a borderline Weak – Moderate Nino. Given our low ONI starting point this fall, I think it will be difficult to reach Moderate Nino using a definition of requiring at least 3 consecutive Nino 3.4 ONI tri-monthlies of 1.0 or higher.
Equatorial SOI for Aug-Oct was -0.2. Here is how that compares with all years since 1949 - https://twitter.com/griteater/status/1059539528803848192
Global AAM for Sep-Oct was right around 0. Here is how that compares with other El Ninos since 1950 - https://twitter.com/griteater/status/1059536786706305024
Upper Level VP Anomalies averaged over Jul-Oct show subsidence over the Indian Ocean and Western Atlantic, with uplift over much of the Pacific and over the Eastern Atlantic / Africa.
OLR anomalies averaged over Jul-Oct show below normal convection extending from Africa thru the Indian Ocean. Otherwise, no significant anomalies are noted.
Equatorial Upper Ocean Heat Content east of the dateline is above to well above normal.
Equatorial Sea Level Anomalies are above normal across the full Pacific basin
MEI – the Oct and Nov releases of the MEI have it on the borderline of neutral / weak Nino.
El Nino Base (SSTs) – with the October data now in, here is an updated graph showing the progression of this year’s Nino SST base (thick black line) compared to other Ninos since 1950. Looking at the 24 prior Ninos on the chart, 22 of the 24 either remained in the relative portion of its “swim lane” or moved west from Fall to Winter. The only 2 years that made a noteworthy move east were 82-83 (mod east > strongly east) and 86-87 (mod west > weak west). We are solidly in the west-based camp this year, and I would expect that to continue thru the winter.
The previous graph is based on the E Pattern vs. C Pattern of El Nino SSTs from this paper from Takahashi et al. Here is an image of the E vs. C Patterns. In my view, this image captures the best method for visualizing the SST patterns of El Ninos at the opposite ends of the spectrum.
Global SSTs – outside of the equatorial Pacific, noted SST anomalies are: 1) cool anomalies in the eastern Indian Ocean and Maritime Continent, 2) mostly warm North Pacific anomalies, 3) cool/warm dipole in the North Atlantic, and 4) warm Arctic.
PDO – the PDO isn’t in a strongly positive or strongly negative phase at the moment.
QBO – I have been maintaining updates to the Berlin QBO chart since updates were dropped on the site for some reason back in March. With the latest update now in for October, we see that a flip from -QBO to +QBO has now occurred at 30mb. The +QBO is descending down through the stratosphere at a steady rate. I now expect the +QBO to reach 40mb by December, and reach 50mb by Jan-Feb.
Solar Parameters – if we use a simple 1 to 5 scale of 1-Very Low, 2-Low, 3-Moderate, 4-High, and 5-Very High to describe the current solar forcing, here’s how I would rate a few of the key parameters:
Solar Flux / Sunspots: 1-Very Low
Solar Geomagnetic Activity / Solar Wind: 2-Low to 3-Moderate due to periodic increased geomagnetic activity from the release of enhanced solar wind from rotating coronal holes.
In the big picture, solar forcing is relatively low, but it’s not super low like the winter of 2009-2010, and we likely won’t reach this solar cycle’s absolute minimum, with the possible combination of very low solar flux / sunspots and very low geomagnetic activity, until sometime over the next 2 years.
Pacific / North American Pattern
El Nino Patterns
As a part of putting this outlook together, I first binned each of the +ENSO winters (El Nino and Positive Neutral) since the 1870’s based on the Pacific / North American pattern for Dec-Mar. I used Sep-Nov to Jan-Mar ONI from Eric Webb’s Ensemble ONI for ENSO determination.
Not all +ENSO winters matched a specific pattern, but here is the breakdown of the 7 common +ENSO patterns that I noted:
If we remove the high end strong and super Ninos on the high end, and remove the years that are barely +ENSO on the low end (via SSTs), we get the following results:
Here’s a view of each pattern in order of most common to least common in the previous chart, using composites of the cases from 1895 to 2014 (these are the dates I can retrieve for composites of both 500mb and detailed U.S. surface temperatures):
Note: in the last image, 1987 didn’t actually have an East Aleutian Low, but its pattern of zonal flow, ridge in southern Canada, and U.S. temperature distribution of warm north / cool south fits the mold here. I left out the super ninos in these composites which would have all been in this group.
The next step is to try and determine the base pattern for the upcoming winter from those previous images.
On statistics alone, we see that the first 2 patterns have occurred more than any of the others during El Ninos (Aleutian Low / Western North America Ridge & North Pacific Ridge / Eastern U.S. Trough).
El Nino Strength and SST Base
Outside of East Based Super Ninos, the data suggests that the Nino SST base doesn’t necessarily give us strong clues regarding the specifics to expect with the winter pattern. The Nino base does however provide a general framework for the expected U.S. temperature pattern.
Ninos that were East Based, Trending East into East Based, or Basinwide – in 75% of cases (6 of 8), the result was warm anomalies over most of the U.S. or warm in the East. Exceptions: 1976-1977, 1969-1970.
Ninos that were West Based, Trending More West Based, or Trending into West Based – in 75% of the cases (12 of 16), the result was cool anomalies over most of the U.S., cool in the East, or cool in the SE / South. Exceptions: 2004-2005, 1994-1995, 1991-1992, 1953-1954.
Velocity Potential (VP) Anomaly Pattern
I reviewed the summer to fall progression of the VP anomaly pattern for each Nino since 1979 (there are limitations with the VP data prior to ~1979). I viewed the maps in 3-month increments (Jun-Aug, Jul-Sep, Aug-Oct). Along with SSTs, the advantage of using VP data is that it tends to give clues ahead of time about what we may be able to expect with the upcoming winter pattern.
There were 2 types of patterns that I saw while reviewing the VP data, which I will refer to as 2-cell and 4-cell. Here is a summary:
Here are Jul-Oct VP anomaly / subsequent winter pattern composites for each VP Anomaly Pattern Type (I removed the Super Nino years in the 2-Cell composite):
This year’s VP Anomaly pattern is in the 4-Cell camp, and with enhanced subsidence in the Indian Ocean.
Repeating El Nino Patterns
Anthony Masiello (twitter: @antmasiello) recently made the astute observation that “a lot of weaker Nino winters are like chameleons; they take on the characteristics of the interannual (but usually intradecadal) climate period.” - https://twitter.com/antmasiello/status/1046792232248991744
With that in mind, I went down perhaps a slightly different path and noted that regardless of strength, El Nino patterns do tend to be similar within their time period. Here is a list of El Ninos that occurred in a time period close to each other and exhibited similar winter patterns.
This concept suggests that the upcoming winter may very well share some pattern characteristics with the recent Weak El Nino of 2014-2015.
The High Latitudes
North Atlantic Oscillation (NAO)
We are currently in the midst of a remarkable stretch of +NAO readings. The NAO has been at least +1.00 or higher in 7 straight months, and in 9 of the last 10 months.
In addition, the NAO has been negative in only 3 of the last 20 winter months (Dec-Mar).
This is a strong +NAO background environment.
Greenland Blocking Index (GBI)
I’m a sucker for a good teleconnection index. One index that I’m surprised I personally haven’t seen any mention of in the weather/climate community is the Greenland Blocking Index, or GBI (though it may be in the Research Portal, ha). The GBI is quite similar to the NAO, but what I like about the GBI is that its key anomaly center calculation at 500mb is located squarely in the most influential blocking region of the NAO domain, Greenland and the Davis Strait. Another way to view the GBI is to say that it is an index of the west-based NAO.
In contrast to the specific domain focus of the GBI, there are various calculations for the NAO. Some of the calculations are based on a difference in normalized SLP between Portugal and Iceland, or the Azores and Iceland (Station-Based), while others are based on SLP or 500mb anomalies across a wider longitudinal region of the North Atlantic (Principal Component-Based).
Here is a link to the paper describing the GBI from Edward Hanna et al. - https://rmets.onlinelibrary.wiley.com/doi/epdf/10.1002/joc.4673
And here is a link to the GBI timeseries with monthly data - https://www.esrl.noaa.gov/psd/gcos_wgsp/Timeseries/GBI_UL/
With the GBI, the sign is reversed from the NAO, so in simplified terms, +GBI means Greenland Blocking or above normal heights over the GBI domain, while -GBI means a lack of Greenland Blocking or below normal heights over the GBI domain.
Here are composites for some of the strongest Greenland Blocking winters on record alongside some of the weakest Greenland Blocking winters on record (since 1950).
Below are those same composite years for the September, October, and November prior, with my comments on what I see as the key differences.
For September, note the difference in and around Greenland. The -GBI has a clear signal for negative height anomalies.
For October, note the difference in far North Central Russia and into the Arctic along 90E. There are positive anomalies in this region on the +GBI composite, and negative anomalies on the -GBI composite.
For November, note the difference across the Arctic. There are large scale positive anomalies on the +GBI composite and large scale negative anomalies on the -GBI composite.
In essence, in the +GBI progression, above normal heights build over the Arctic in September, become stretched out across far northern Eurasia in October, then congeal over the polar cap in November. In the -GBI progression, below normal heights build over Greenland in September, become stretched out across northern Russia in October, then congeal over the polar cap in November.
As for this fall, the September map (below) is clearly in the -GBI camp.
There isn’t a clear signal on the October map (not shown). For November, ensemble forecasts as we progress through the month hint at a 500mb pattern that is more closely tied with the -GBI camp, with the stratospheric and tropospheric polar vortices becoming moreso centered over the polar cap.
In totality, I’d say the fall pattern at 500mb is more suggestive of a -GBI winter (i.e. lower amounts of Greenland blocking compared to normal).
North Atlantic SST Dipole Pattern
On the Sep-Oct averaged SST map, there is a dipole pattern in the North Atlantic with cold anomalies stretched out over the far North Atlantic just SE of Greenland and warm anomalies extending from off the Northeast U.S. coast over to the Azores.
The last 13 times this type of dipole pattern was present in the fall, 10 of the subsequent winters went on to have a -GBI winter (i.e. low amounts of Greenland blocking). In addition, 10 out of the 13 Decembers of those same years experienced a -GBI. The 13 winters matching this North Atlantic SST dipole pattern are: 17-18, 15-16, 13-14, 11-12, 94-95, 92-93, 90-91, 89-90, 88-89, 84-85, 83-84, 82-83, and 75-76.
Interestingly enough, the correlation fails miserably when looking at years prior to 1975. From 1900 to 1974, I found on reanalysis charts 15 cases of said North Atlantic SST dipole pattern. In those 15 cases, only 5 led to a -GBI winter.
I’m going to lean on the more recent climatology here and say that this year’s North Atlantic SST pattern favors a -GBI in both December and averaged over the full winter.
QBO, Solar, & The Stratospheric Polar Vortex (SPV)
Based on the current QBO progression of early winter -QBO in the 40-50mb layer with a mid-late winter switch to +QBO, the best QBO matches for the upcoming winter in the key 30mb to 50mb layer are:
4 of those 6 winters saw a sudden stratospheric warming (SSW) in early winter (exceptions: 77-78 & the Pinatubo contaminated winter of 94-95). Here are a few more details about each of those occurrences:
1954-1955 – Canadian Warming in Dec / SSW in Jan (per the Berlin site data)
1984-1985 – SSW in early Jan (per Amy Butler’s SSW table)
1998-1999 – SSW in mid-Dec
2001-2002 – SSW in Late Dec / Early Jan
There is debate about the mechanism for how the QBO modulates the Stratospheric Polar Vortex (SPV), but this paper from Garfinkel et al. states that a -QBO in the lower stratosphere at 50mb leads to a weakened SPV, regardless of downward propagation of the QBO.
Finally, in 3 of the 6 years listed above (54-55, 84-85, and 01-02), the 500mb pattern in Dec consisted of below normal heights in Alaska and Greenland, with above normal heights over northwest and/or north-central Eurasia. This pattern configuration is similar to those hinted at on current extended ensemble model runs. Ridging in northwest and/or north central Eurasia is a precursor pattern for disturbing the SPV.
Given all of the above, in my view, the chances of an SSW or significant SPV weakening in early winter (Dec – early Jan) are higher than the climatological mean (note: if an SSW were to occur this winter, it would likely lead to a cold period in the Eastern U.S. given that the background ENSO state favors an Eastern U.S. trough).
Other Forecast Thoughts
Trends Over Recent Winters
Over the past 4 winters (15-16 Super Nino Excluded), we have seen a rather consistent pattern of Alaska & Western North America Ridging / Hudson Bay Low Vortex / +NAO / and higher than normal heights across the southern tier. I would expect this winter to fit in with the overall theme here, but with adjustments.
My biggest takeaway from the Nov release of the Euro Seasonal is that it lessened the amount of Greenland blocking compared to the Oct release, particularly in Dec and Jan. It also moved the core of the Eastern U.S. negative anomaly center from the Southern Mississippi Valley to off the Northeast coast.
Northern Stream / Southern Stream
I have a largely west to east zonal flow regime forecasted for Dec.
For the Jan-Mar time period, I would roughly project the following with respect to Northern Stream / Southern Stream flow:
1) Northern Stream dominant flow 50% of the time
2) Split Flow pattern 25% of the time (Jan would be the target month)
3) Other non-related pattern 25% of the time (pattern variability)
Cold Air Outbreaks
I would project the number of cold air outbreaks east of the Rockies to be slightly higher than the winter normal, especially in Jan & Feb.
Report Card From Last Year’s Outlook
My best call from last year’s outlook was when I stated that I “favor the development of a “North” North Pacific Ridge that is variable, but biased to the northwest in the mean winter pattern.”
My worst call from last year’s outlook was my projection of a Western U.S. Trough / Eastern U.S. Ridge for December.
Finally, here are the grades for my Snowfall and Dec-Mar Temperature maps: