OHweather 2012-2013 Winter Forecast.
First off…my previous two winter forecasts…10-11 with images in tact:
09-10…images not in tact:
Just in case anyone was curious. I did not make an outlook last year.
Above is a current look at the sea surface temperature anomalies across the Atlantic and Pacific Oceans. An El Nino began to develop over the summer, but appears to have lost some steam with a cold tongue of water near the equator over the eastern Pacific. There is a warm gyre of water over the northwest Pacific stretching to the east around a cold pool of water near the Aleutian Islands, with cooler water near the west coast of the US/Canada.
As mentioned above, the eastern Nino regions have cooled notably of late. The western most Nino region has held steady, however we are not truly in an El Nino at this time.
Beneath the surface, previously warm waters have been replaced by near normal or even slightly below normal water temperatures during the month of September and into October. This does not bode well for an attempt at developing an El Nino, as any upwelling will bring cooler subsurface waters to the ocean surface and further weaken any attempt to develop an El Nino.
Although the 90-day average SOI is weakly negative, September and thus far October have featured predominantly positive SOI’s. This contributes to stronger trades over the equatorial Pacific, which enhances upwelling of cooler water to the ocean surface, and as mentioned above, these sub-surface waters are near normal and will thwart any developing El Nino if upwelled.
The most recent ENSO forecast models plume shows some potential additional warming in ENSO region 3.4 through the fall, but nothing significant. Given the waters in the Nino regions have cooled further since these models were initialized, it appears unlikely that we will see a moderate or strong El Nino in the DJF timeframe. The best bet is for a warm but neutral ENSO this winter, with some chance at a weak Nino, although those chances don’t appear to be too high at this time. The warmest waters will be in the western Nino regions.
The Pacific Decadal Oscillation (or PDO) is a cycle of sea surface temperature anomaly placement over the entire Pacific Ocean that generally maintains itself for two to three decades. The current sea surface temperature anomaly map posted above more closely matches the right hand picture, which is the cold phase of the PDO, although the cold pool that has recently developed near the Aleutians gives it a bit of a neutral look.
However, we are currently in a multi-decadal cold phase of the PDO and the overall current look remains cold. So, for my expected Pacific pattern, I am using analogs that consisted of neutral warm ENSO conditions and cold PDO conditions…
DJF SST Composite of my Pacific Analogs:
DJF 500mb height anomalies with above SST based analogs:
The pattern shown is interesting but in general matches –PDO climo…with a negative PNA and resulting troughiness over the western/central US. It should be noted that there are only limited signs of an enhanced sub-tropical jet in the above composite image, with the only signal being a negative anomaly off the SW coast of the US. This points to a more active polar jet than normal with a slightly perkier subtropical jet in the analog years. Precipitation anomalies in these years did show slightly above normal precip across the southern/eastern US these years in all three winter months. It should be noted that for the above image, I was ONLY considering Pacific sea surface temperatures, and it is only one variable being looked at for this composite map. When looking at the month by month breakdown, the troughing was focused in the western and central US in December and January, and farther east in February.
Potential for High Latitude Blocking
As is generally well known, high latitude blocking forces the jet stream to buckle more frequently and become amplified, which allows for stormier conditions and more extreme temperature anomalies. High latitude blocking is nearly as important to what occurs during a winter as what happens out in the Pacific Ocean.
Quasi Biennial Oscillation (QBO):
The QBO essentially measures the wind anomaly in the stratosphere over the equator. This affects us because a positive or westerly QBO tends to be more hostile towards blocking, while a negative or easterly QBO tends to be more favorable for sudden stratospheric warming events and high latitude blocking during the winter months.
The correlation for a +QBO in the winter months (1949-2011) shows that in general, when the QBO is in its positive, or westerly phase, that lower than normal heights tend to persist over the high latitudes, meaning the high-latitude blocking is generally less likely during a +/westerly QBO than a -/easterly QBO.
As can be seen two images up, the zonal wind anomalies at 30mb and 50mb were positive going into last winter but have since slackened and become negative. The March-September 2012 period featured a good amount of high latitude blocking, possibly as a result:
Given the zonal wind anomalies continue to be strongly easterly, with down-welling evident as the 200mb zonal wind anomalies begin to decrease, along with the tropical 500mb temperatures increasing, it is a fairly good bet that winter 2012-13 will feature a –QBO…possibly strongly…which may favor high latitude blocking.
The NAO has been prevalently negative since May 2012. In fact, every month has featured an average NAO of -.59 or lower since May. Since 1950, there have only been 6 instances since 1950 in which at least 4 of the 5 months from May-September have featured an NAO of -.50 or lower, per this data available on the CPC website:
Of those 6 instances, 4 of the 6 following meteorological winters featured an NAO that averaged less than “0”:
1957-58: .12/-.54/-1.06/avg -.49
1968-69: -1.4/-.83/-1.55/avg -1.26
1993-94: 1.56/1.04/0.46/avg +1.02
1998-99: 0.87/0.77/0.29/avg +0.64
2008-09: -0.28/-.01/0.06/avg -.08
2010-11: -1.85/-0.88/0.7/avg -.68
The 500mb height anomalies over the NAO region the following winters, on average, looked like this:
So, although the sample size isn’t extremely large, years that featured an overwhelmingly negative NAO from May-September were twice as likely to see a negative NAO DJF period than a positive NAO DJF period. So, this slightly favors high latitude blocking this winter. In two of the four negative NAO years in this set of “analogs,” the NAO trended from strongly negative early towards more of a neutral NAO. In the other two years, as can be seen above, the trend was the other way around.
Atlantic Sea Surface Temperatures:
Since 1950, there have been 14 winters which have seen an average NAO of less than -0.50. The Atlantic sea surface temperatures in the July-September period leading up to those winters looked like this:
The water temperature this July to September looked like this:
There are very few correlations in the Atlantic Ocean between JAS periods leading up to –NAO winters and the 2012 JAS period…although there are some similarities in the NE Pacific. This year, much of the northern Atlantic Ocean saw normal to above normal sea surface temperatures, with well above normal water temperatures surrounding and south of Greenland. In the JAS periods leading up to –NAO winters, the water across the north Atlantic was typically cooler than normal, with another tongue of cooler than normal waters along the equator. This variable does not support a –NAO winter this year.
When examining years in which the JAS Atlantic SSTs somewhat matched this year’s Atlantic JAS SSTs (whose composite JAS SSTs looked like this):
The following winter NAO looked something like this:
This is a –NAO look although the sample size is somewhat small, and the composite anomalies are not extremely strong (20m on the high end). The composite Pacific SSTs in these winters did feature a cold PDO, however the composite also shows a neutral-cool ENSO. Thus, this variable, when all is combined, does not strongly favor a predominant negative or positive NAO winter. The above Atlantic SST analogs featured the strongest blocking in January, with west based blocking in February, for what it’s worth.
In the same years used in the Atlantic Sea Surface Temperature section above for –NAO years here is what the precipitation rate looked like in the JAS period leading up to the above used –NAO years. Note the large area of above normal precip along the equator just east of the dateline:
This year’s map does not quite look like the above map. There was above normal precip east of the dateline along the equator, but not as much as in the JAS periods leading up to previous strongly –NAO winters. There was also above normal precip west of the dateline, which did not occur in the previously strong –NAO winters. There was an area of below normal precip stretching from the Central American coastline east towards Hawaii, which also occurred in the JAS periods leading up to –NAO winters. Overall, this variable does not conclusively point to a negative or positive NAO winter this winter.
When delving deeper, and looking at previous JAS periods that had similar tropical precipitation anomalies (that look like this):
The following winter high latitude blocking looked like this:
Which is a solid west based –NAO, overall. These winters, when averaged out, featured a neutral to maybe cool PDO in the Pacific and a neutral but on average slightly warm ENSO. These analogs may have some merit this winter, and support at least some high latitude blocking. These analogs also had warm water temps around Greenland in the JAS period, which lends some credence to this idea. In these “analog” years, the composite NAO was negative all three winter months, but trended from near neutral to strongly negative from December to February.
There is some suspicion that decreased (or at least a change in) sunspot activity may correlate to an increase in winter time sudden stratospheric warming events/high latitude blocking. Although my current breadth of knowledge doesn’t include how this forcing mechanism may work, I’m going to explore the issue a little bit right now.
We are nearing the peak of solar cycle 24. This cycle, thus far, has been extremely tame compared to cycle 23 which peaked near 2000 with sunspot numbers between 150 and 175. This cycle peaked at near 100 for a brief period of time in late 2011 before diminishing again. Based upon trends since the beginning of 2012, it is reasonable to expect a peak of somewhere around 75 at some point in 2013, and it seems to be a safe bet that winter 2012-2013 will feature sunspot numbers, at least monthly, of between 50 and 75.
While looking through sunspot data since 1949, I found 5 years that featured sunspot numbers general hovering between 50-75 in the months leading up to winter that remained in that area through winter. The composite 500mb height anomalies over the high latitudes looked like this:
Sunspot date found here:
While not a strong anomaly, the AO and NAO appear to be slightly positive, on average, during winters with similar sunspot activity to what I am expecting this winter. This is a small sample size and not a strong signal, which again makes it hard to make a definitive call based on this variable to this winters NAO. It should also be noted that in the solar forcing “analogs,” the NAO trended from positive on average in December to neutral in January to negative in February.
All in all, through some mixed signals, there are more signs of a negative NAO winter overall than a positive NAO one; however the signals aren’t extremely strong. This may be a winter that sees alternating periods of blocking and no blocking, with blocking probably being somewhat more likely after December.
July-September Pattern Analogs:
Above is a look at the JAS 2012 pattern across North America. Some key features to point out are strong Greenland blocking leading to a trough over the eastern US, along with lower than normal heights between Alaska and Siberia leading to ridging over western North America.
When looking through reanalysis data from 1948 onward, it was evidently rare to have ridging over western Alaska during summer with a –NAO over the North Atlantic. There are only 4 years where the JAS composite 500mb height anomaly maps were satisfyingly close to this year’s prevailing 500mb pattern for the JAS period…here is the mean 500mb height anomaly in these “analogs”:
Not a perfect match by any stretch…however, it seems unusual to see good high-latitude blocking over Greenland and a strong low 500mb height anomaly near Alaska…resulting in a trough in the eastern US…in the JAS period. The winters following these summers played out as follows:
In general, the analogs, based strictly on the proceeding JAS period indicate that high latitude blocking is more likely in January than February in the roll forward analog years…there is also some ridging off the west coast of the US in each month of the analog years. However, the sample size is extremely small, and the Pacific in general this year does not match the JAS “analogs”…here was the JAS Pacific SSTs those years:
Thus, these analogs will not be given much weight in this forecast.
Putting it all together:
In general, I have found that:
-Based on my methods used, a variable NAO appears to be likely this winter, with periods of negative and positive NAOs likely. When these periods occur is in question, although I’d favor a more negative NAO overall as the winter progresses.
-Based on my Pacific analogs, a dominant polar jet appears more possible than a dominant subtropical jet. However, a warm-neutral ENSO supports a more lively sub-tropical jet. This may result in quite a gradient setting up somewhere over the central US with a fair amount of precip near this gradient. This also means it may be necessary to throw standard “Nino climo” out the window.
-The general trend seems to be a trough in the western US due to ridging over the central Pacific and possibly into Alaska, with the pattern possibly shifting eastward through the winter. This is also supported by the NAO analogs showing a more negative NAO as the winter goes on in general, although not in every case. With a warm but only weakly so ENSO, the cold PDO may result in this winter behaving more like a La Nina than an El Nino, or potentially as some sort of hybrid of the two.
Based on this, here are my monthly maps:
December (temps then precip):
January (temps then precip)…
February (temps then precip):
“For fun” snowfall forecast (overall):