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NAO/AO/QBO/Solar

The NAO has been consistently negative since the spring of 2008, with only a handful of interruptions on the positive side. What has been causing this plummet in NAO values over the past few years? There are multiple mechanisms at work, and I believe that the Sun is/has ultimately been a major driver of the NAO.

Solar eruptions are linked to the North Atlantic Oscillation. Dr Theodor Landscheidt, a successful solar forecaster, showed how variations in solar output, solar wind, geomagnetic activity, galactic cosmic rays, among other periodicities, impact the modality of the NAO.
Geomagnetic activity indices measure the affect of solar eruptions on Earth. Note the figure below; a strong correlation was found between the geomagnetic aa index and the NAO index over the past half century. Early in the period, particularly prior to 1955, geomagnetic activity had a significant impact on the stratosphere but not the troposphere. However, since then, its impact extended down into the troposphere. Studies have speculated that as the Sun has become more active, with increasing irradiance/output by the mid 1900s, its impact on the troposphere strengthened. For the period 1970-2000, the correlation coefficient for the geomagnetic aa and the NAO was 0.93, an outstanding correlation.

Galactic cosmic rays (GCR) have been shown to have a strong inverse correlation to geomagnetic activity and thus the NAO. The concept in a nutshell is as follows: in times of high geomagnetic activity, the solar wind is stronger, which prevents a significant amount of galactic cosmic rays from reaching the Earth’s surface. When geomagnetic activity is lower in conjunction with a weaker solar wind, more GCR can penetrate through the atmosphere. Since GCR are an ionizing radiation, they act as a sort of nuclei for cloud formation, particularly low level clouds. Thus, surface cloudiness increases and global temperatures generally cool when GCR are high. There is likely a connection between increases/decreases in cloud cover over the Atlantic (via variations in GCR) and the NAO.

Below is the comparison of NAO and GCR values since 1950. Note the strong negative correlation.

When comparing the solar sunspot cycles to geomagnetic activity, notice there is a pretty significant lag between the maxima/minima of each. In terms of correlation with NAO values, geomagnetic activity and GCR have higher correlations than the sunspot curve itself (although the latter is important as well).

If we take a look at the following geomagnetic Ap index graph, its strong correlation to the NAO becomes apparent (geomagnetic Ap index is correlated almost perfectly with the aa index). The NAO relative minima periods of the mid 50s, the 60s-early 70s, late 70s, mid 80s, mid/late 90s, and right now correlate to the lower Ap values. The NAO relative maxima periods of the early 60s, mid 70s, early and late 80s, early-mid 90s, and the mid 2000s correlate to the higher Ap values. The NAO and Ap index curves are very well correlated over the past 40-50 years.

In summation, since 1950, we have seen an intensifying correlation between geomagnetic activity and the NAO index, possibly due to a strengthening stratospheric vortex post 1970 (Thompson, et al 2000), With that being said, what about the QBO and other NAO forcing mechanisms? The solar factors, namely geomagnetic activity and GCR provide a fairly strong argument that NAO values will remain on the negative side for this upcoming winter. In addition, the Sun is probably main driver of low NAO values since 2008, as shown above.
In terms of the QBO – it is known that the westerly phase tends to promote a stronger polar vortex and the easterly phase weakens the vortex (much like the effect of geomagnetic fluctuations on the polar vortex). However, which is more important in terms of NAO modality – QBO or geomagnetic activity?

Let’s take a look at some select years in the past few decades, and examine the states of both the QBO and geomagnetic ap index, and their corresponding impact on the NAO phase.

1972-73
1973-74
1974-75
1979-80
1983-84
1988-89
1989-90
1991-92
1993-94
2000-01

All of the above years featured a -QBO (easterly), yet all winters (DJF) had a +NAO average. If we look at the geomagnetic ap index for those winters, they all featured either high or rising values.

1977-78
1978-79
2008-09

All of these years featured a +QBO (westerly), yet the winters had a –NAO average. The geomagnetic ap index was either low or decreasing during those seasons.

Thus, in 13/40 years since 1970 the QBO and NAO were inversely correlated and did not follow the expected signal (+QBO/+NAO, -QBO/-NAO). So 67.5% of years followed the expected QBO/NAO combination. However, the geomagnetic ap index-NAO correlation since 1970 was quite a bit higher.

The QBO and geomagnetic ap index appear to have a relationship, which is not surprising; solar activity impacts stratospheric circulations/patterns. With that being said, geomagnetic activity appears to be more important than the QBO in terms of connection to NAO modality. In times of lower geomagnetic activity and –QBO, the –NAO potential is likely enhanced significantly. An example of this is last winter, 2009-10, a blockbuster snow winter over the Mid atlantic due to an active southern stream and sustained blocking.

The question is – what will happen next winter? We have a +QBO, moderate to strong La Nina, and record low geomagnetic activity. There’s no doubt in my mind regarding the solar effect on the NAO over the past couple years. In fact, the NAO has remained negative this autumn even with an intense Nina and +QBO. In 1955, we had a strong nina and +QBO until January 1956. The latter part of the winter turned slightly negative QBO. Geomagnetic activity dropped significantly in the mid 50s, which IMO was a major forcing in the significant –NAO of the 1955-56 winter.

Another NAO forcing mechanism is the Atlantic multidecadal oscillation (AMO). If we compare the NAO and AMO curves over the past several decades, there’s an apparent connection between the upswing and downswing of the AMO, and NAO values. Notice how the NAO averaged positive after the AMO minima of 20s and the 70s, while the NAO averaged negative after the AMO maxima of the mid/late 40s, and the mid 2000s. The NAO has been trending more negative over recent years, and this fits the overall AMO cycle as well. The NAO has its own decadal cycle as shown in the first image, and the curve is trending down since 2000, but the unprecedented solar factor is definitely aiding in the sustained negative NAO since 2008.

Therefore, the NAO factors for the upcoming winter are as follows: mod-strong La Nina, +QBO, declining AMO decadal cycle, declining NAO decadal cycle, record low geomagnetic activity, etc. Based upon the discussion above, I believe the solar influence is probably the most influential of all factors. However, the powerful Nina will be a factor fighting against the development of sustained blocking this winter, which will likely keep the NAO from being anywhere near as low as it was last winter, (when the El Nino and –QBO enhanced it).

With that said, I believe it is probable that a weakly negative NAO will persist for the winter season regardless of the powerful Nina. Since the AO has a strong correlation with the NAO, I anticipate a near-neutral AO, with spells of negative and positive (although not too positive).

PDO/PNA/EPO/ENSO

The Pacific signaling should easier to determine compared to the Atlantic this year. We currently have a powerful, basin wide La Nina event, with the latest region 3.4 reading at -1.5c, which is considered borderline strong.

The Pacific Decadal Oscillation (PDO) is clearly negative, with the September value coming in a -1.61 per the JISAO data. La Nina’s with strongly negative PDO’s tended to be more common in the 1950-1975 period, when the PDO decadal phase was negative. Over the past several years, the PDO downswing has returned. The onset was noticeable in the moderate El Nino of 2006-07, as the PDO was very low for a warm ENSO event. However, the PDO has been trending down since the late 90s, gradually.

Looking at the SSTA profile, the Pacific set-p is one that favors MJO waves remaining in the early octants (2-6), as very cold water pushing into ENSO region 4 could aid in halting their progression eastward across the tropical Pacific. The Gulf of Alaska is about near normal on its northern periphery and down the Pac NW coast, but colder than normal further SW. This Pacific regime generally supports a negative PNA/positive EPO pattern, with a vortex in the Gulf of Alaska, enhancing the jet into the NW US.

Forecast models – the latest CFS has ENSO region 3.4 within strong territory should the middle part of the winter, prior to weakening by Feb/March (typical of Nina climo). The ECMWF is in solid consensus with the CFS idea; region 3.4 numbers hovering in the -1.5c to -2.0c range for the better part of the winter, before becoming moderate late winter.

Therefore, I believe it is possible that this La Nina event will peak strong in the trimonthlies, meaning we’ll see three months with region 3.4 SSTA at -1.5c or lower. It’s not out of the question a couple weekly readings fall as low as -1.8c to -2.0c. So no doubt we have a powerful La Nina on our hands, whether it peaks at -1.5c or -2.0c, it’s going to be a major play in United States weather (as well as globally).

With regards to the Pacific North American index (PNA), 9/11 moderate to strong La Nina’s featured a negative PNA average for the winter (DJF). 1999-2000 and 2007-08 were the only two years that had a positive PNA. Over the past 6 months, the PNA has largely been positive. In fact, it’s somewhat unusual for this to be happening. 7/11 moderate to strong La Nina’s featured a –PNA in the August-October period. The years that didn’t were 1954, 1998, 1999, and 2007. But 1954 and 1998 shifted to a –PNA by the winter time. Based upon the intensity of this La Nina event and the declining PDO, I believe the PNA will average negative for the winter, although not strongly so, as I’m expecting some fierce resistance on the Atlantic side. The EPO should be opposite the PNA sign, with the likelihood of troughiness in Gulf of Alaska pretty high.

Putting it all together

An interesting pattern may unfold this winter, with the potential high for the NAO averaging on the negative side for DJF, and the AO not as positive as it normally would with the present ENSO regime. However, we have a strong La Nina event in place, and that will undoubtedly play a significant role in the temperature/precipitation distribution across the United States. It’s going to be a bit difficult to analog this winter, or compare it to previous years, as there aren’t many similarities. We have an unprecedented solar set-up with record low geomagnetic activity; we have a –QBO shifting slightly positive in the means, a strong La Nina, and a relatively favorable Atlantic SSTA pattern. Big time conflicts between the Pacific and Atlantic signal fighting for dominance may be the name of the game this winter. The Pacific trying to torch much of the nation, which will surely happen at times, and the NAO overwhelming parts of the US with colder than normal temps and snowfall. 1954-55, 1955-56 are probably the best overall analogs for 2010-2011 IMO. Both occurred in a relative geomagnetic minimum with negative NAO values, both were at a similar place PDO/AMO wise, and both were mod-strong La Nina events (1955-56 was very strong like this one may be). 1973-74 would be up near the top if the solar/NAO fit with this year, but it doesn’t. 1970-71 is pretty good but the La Nina this year will be quite a bit stronger, thus warmer than that winter.

December should be the coldest month relative to normal in the Northeast, with colder anomalies continuing to hold on in January with a favorable NAO pattern. February should be the warm month with the Nina induced SE-ridge going wild, pushing above normal temps further west. Overall, DJF temp departures probably won’t be as warm as most strong La Nina’s, with the more favorable Atlantic regime expected. As for precipitation and storm tracks – there’s likely to be a mixture this winter. We’ll see a good amount of lakes-cutters and inland runners producing Ohio Valley and Northern New England snow, but in times of –NAO, we could see northern stream Miller B clippers dive southward and produce snow in the I-95 corridor. I anticipate December 15th-January 15th to be the snowiest relative to normal for most of the Eastern I-95 corridor, Richmond to Boston. The biggest snows for interior/NNE may be in February with warmer temps but also plentiful precipitation.

Forecast for NYC

Temps
December: -1 to -2
January: 0 to -1
February: +1 to +2
Overall: 0 to -1
Snowfall: 25-32” (near normal)

Bonus cities snowfall:
Philly: 15-22”
Washington DC: 8-15”
Boston: 50-57”

Verification:

Here are my last couple paragraphs from my winter outlook issued in October (I’ve bolded the calls within it).

“An interesting pattern may unfold this winter, with the potential high for the NAO averaging on the negative side for DJF, and the AO not as positive as it normally would with the present ENSO regime. However, we have a strong La Nina event in place, and that will undoubtedly play a significant role in the temperature/precipitation distribution across the United States. It’s going to be a bit difficult to analog this winter, or compare it to previous years, as there aren’t many similarities. We have an unprecedented solar set-up with record low geomagnetic activity; we have a –QBO shifting slightly positive in the means, a strong La Nina, and a relatively favorable Atlantic SSTA pattern. Big time conflicts between the Pacific and Atlantic signal fighting for dominance may be the name of the game this winter. The Pacific trying to torch much of the nation, which will surely happen at times, and the NAO overwhelming parts of the US with colder than normal temps and snowfall. 1954-55, 1955-56 are probably the best overall analogs for 2010-2011 IMO. Both occurred in a relative geomagnetic minimum with negative NAO values, both were at a similar place PDO/AMO wise, and both were mod-strong La Nina events (1955-56 was very strong like this one may be). 1973-74 would be up near the top if the solar/NAO fit with this year, but it doesn’t. 1970-71 is pretty good but the La Nina this year will be quite a bit stronger, thus warmer than that winter.

December should be the coldest month relative to normal in the Northeast, with colder anomalies continuing to hold on in January with a favorable NAO pattern. February should be the warm month with the Nina induced SE-ridge going wild, pushing above normal temps further west. Overall, DJF temp departures probably won’t be as warm as most strong La Nina’s, with the more favorable Atlantic regime expected. As for precipitation and storm tracks – there’s likely to be a mixture this winter. We’ll see a good amount of lakes-cutters and inland runners producing Ohio Valley and Northern New England snow, but in times of –NAO, we could see northern stream Miller B clippers dive southward and produce snow in the I-95 corridor. I anticipate December 15th-January 15th to be the snowiest relative to normal for most of the Eastern I-95 corridor, Richmond to Boston. The biggest snows for interior/NNE may be in February with warmer temps but also plentiful precipitation.”

Temp departure forecast for DJF in NYC versus actual:

Temps
December: -1 to -2 / Actual: -4.5 Grade: C
January: 0 to -1 / Actual: -2.4 Grade: B
February: +1 to +2 / Actual: +1.4 Grade: A

Overall: 0 to -1 / Actual: -1.8 Grade: B

Snowfall: 25-32”
Updated call in mid Jan: 50-57″
Grade: C+

Overall grade for winter 2010-11: B

My assessment of this winter’s forecast — I thought both the temp departure values and trend throughout the winter (coldest in December, still cold Jan, warm Feb) worked out very well. I was happy to get the period of heaviest snowfall correct, Dec 15-Jan 15, in conjunction w/ the best greenland blocking. However I’m disappointed I didn’t see the historic snow totals coming for the NYC area. Was hard to make a call that far above normal given other factors, and I wasn’t sure whether the -NAO/AO pattern would be a very snowy one. Either way, snowfall is the most difficult aspect to predict long range, so it is not weighted as much as the temperature calls, which were good overall. What I’m most happy about is my NAO/geomag connection theory working out well, as NAO values were low most of the winter until February.

Biggest regrets — not going colder in the SE US in December, and not going snowier in NYC. I had the idea correct for a poor winter in the mid atlantic and a great one for Boston, but was not bullish enough in our region (NYC/NJ/PHL).

Bonus cities snowfall:
Philly: 15-22”
Washington DC: 8-15”
Boston: 50-57”


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