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Record Setting AO and SOI Combo Work with Other Factors to Create a Wild Winter

By Joe D'Aleo
Monday, March 1, 2010

It has been a winter for many to remember (or forget). There are lots of reasons why. The sun is just coming out of a very long slumber for one. Lets look at some of the other factors likely at play.

In April 2009, we talked about Mt Redoubt’s eruption (and later Russia’s Sarychev) and the effect it might have on high latitude blocking and cooling summer and winter. Starting on March 22, a series of major eruptions have taken place from Mt. Redoubt in Alaska. The biggest exceeded 65,000 feet in height. More than a dozen eruptions as high as 60,000 have followed.

Climatologists may disagree on how much the recent global warming is natural or manmade but there is general agreement that volcanism constitutes a wildcard in climate, producing significant global scale cooling for at least a few years following a major eruption. However, there are some interesting seasonal and regional variations of the effects.

Oman et al (2005) and others have shown that though major volcanic eruptions seem to have their greatest cooling effect in the summer months, the location of the volcano determines whether the winters are colder or warmer over large parts of North America and Eurasia. According to their modeling, tropical region volcanoes like El Chichon and Pinatubo actually produce a warming in winter due to a tendency for a more positive North Atlantic Oscillation (NAO) and Arctic Oscillation (AO).


The North Atlantic Oscillation (NAO) Index first found by Walker in the 1920s, is the north south flip flop of pressures in the eastern and central North Atlantic. The difference of normalized MSLP anomalies between Lisbon, Portugal and Stykkisholmur, Iceland has become the widest used NAO index and extends back in time to 1864 (Hurrell, 1995), and to 1821 if Reykjavik is used instead of Stykkisholmur and Gibraltar instead of Lisbon (Jones et al., 1997).

Arctic Oscillation (also known as the Northern Annular Mode (NAM) Index) in defined as t
he amplitude of the pattern defined by the leading empirical orthogonal function of winter monthly mean NH MSLP anomalies poleward of 20ºN (Thompson and Wallace, 1998, 2000). The NAM /Arctic Oscillation (AO) is closely related to the NAO.

In the positive phase of these large scale pressure oscillations, low pressure and cold air is trapped in high latitudes and the resulting more westerly jet stream winds drives milder maritime air into the continents.

The summer of 2009 had the most negative AO since 1950. Which explains the cold summer (especially July).

Summers with a very negative AO above have a cold anomaly centered in the nation’s midsection.

Last summer saw that pattern.

The AO has stayed very negative this winter. In fact again the most negative of any winter since 2009/10. It has averaged almost 3.5 standard deviations negative. In both December and February, it has reached more then 5 STD.


Negative AO/NAO winters are cold and often snowy in the US and Europe.


This year we have set all-time seasonal snow records in the Mid-Atlantic, soon in Texas and parts of other states. Records for snowfall duration are being set in many areas, more typical of the northern tier.

The worst freeze hit the Florida citrus since 1989 and records for duration of cold were wet in many places in Florida and the southeast. January ties as 5th coldest January in Key West and January 7 through 13 ties coldest week where records have been kept since 1873. Many died in Eastern Europe and western Russia from the extreme cold. In China, the coldest weather since 1971 damaged crops and in Mongolia heavy snows and the extreme cold killed up to 20 million livestock according to Sky News.

England has turned back the clock to the early 1800s with cold and snow. In parts of Scotland it was the coldest December and January and for the second year in a row, England got significant snow. This is part due to the similarity with regards to the ultra-long solar minimum to that of the late 1700s and early 1800s, the so-called Dalton Minimum. We have covered that in earlier Intellicast stories like here.

The strongest blocking years have a warm polar stratosphere and mid troposphere.  Certainly that has been the case this year.

The most negative AO years when composited show the widespread cold.

This winter has been cold especially in the southeast through February 16th.

And certainly it has been cold also all across Asia into Europe.



It also has been an El Nino year, the case in about half the top ten most negative AO years. All measures of ENSO have had it at least a moderate event. The Southern Oscillation Index (SOI) has peaked this month, the others in December. ENSO has different effects depending on other factors like the Quasi-biennial Oscillation (QBO) and the state of the Pacific Decadal Oscillation (now in its longer term cold mode again like the 1950s through much of the 1970s).




Sir Gilbert Walker was generally recognized as the first to find large scale oscillations in atmospheric variables. As early as 1908, while on a mission to try and explain why the Indian monsoon sometimes failed, he assembled global surface data and did a thorough correlation analysis.


On purely statistical grounds through careful interpretation, Walker was able to identify three pressure oscillations: a flip flop on a big scale between the Pacific Ocean and the Indian Ocean which he called the Southern Oscillation (SO); a second oscillation, on a much smaller scale, between the Azores and Iceland, which he named the North Atlantic Oscillation; and a third, between the areas of high and low pressure in the North Pacific, which Walker called the North Pacific Oscillation.


Walker further asserted that the SO is the predominant oscillation, and had a tendency to persist for at least one to two seasons. He went so far in 1924 as to suggest the SOI had global weather impacts and might be useful in predicting the world’s weather. He was ridiculed by the scientific community at the time for these statements. Not until four decades later was the Southern Oscillation recognized as a coupled atmosphere pressure and ocean temperature phenomena (Bjerknes 1969) and more than two decades further before it was shown to have statistically significant global impacts and could be used to predict global weather/climate at times many seasons in advance. Walker was clearly a man ahead of his time. 




The Southern Oscillation Index (SOI) is the oldest measure of the large-scale fluctuations in air pressure occurring between the western and eastern tropical Pacific (i.e., the state of the Southern Oscillation) during El Niño and La Niña episodes (Walker et al. 1932). Traditionally, this index has been calculated based on the differences in air pressure anomaly between Tahiti and Darwin, Australia. In general, smoothed time series of the SOI correspond very well with changes in ocean temperatures across the eastern tropical Pacific. The negative phase of the SOI represents below-normal air pressure at Tahiti and above-normal air pressure at Darwin. Prolonged periods of negative SOI values coincide with abnormally warm ocean waters across the eastern tropical Pacific typical of El Niño episodes. Prolonged periods of positive SOI values coincide with abnormally cold ocean waters across the eastern tropical Pacific typical of La Niña episodes.

As an atmospheric observation-based measure, SOI is subject not only to underlying ocean temperature anomalies in the Pacific but also the intraseasonal oscillations like the Madden-Julian Oscillation (MJO). The SOI often shows month-to-month swings even if the ocean temperatures remain steady due to these atmospheric waves. This is especially true in weaker El Nino or La Nina events as well as La Nadas (neutral ENSO) conditions.  Indeed, even the changes week-to-week can be significant. For that reason, other measures are often preferred.


The Southern Oscillation Index has dropped on a daily basis to an amazing 8 STD negative in early February. For the first 19 days, it has been second most negative February since 1870.

This fits the combination of a negative AO El Nino very well (bottom right).

Easterly QBOs tend also to have more widespread snow in Eurasia and eastern and central North America (top right in graphic below).

So as is almost always the case the seasonal patterns are the result of the combination of natural global oscillations called teleconnections. In this case some combination of the AO/NAO, ENSO. QBO and solar and perhaps the lingering effects of high latitude volcanism.

Note: This unfortunately is the last of the series of Dr. Dewpoint reincarnations. This time around, the lead time for publication was such that the topics could be not as timely (I had the tools to publish myself the first time) and the updates were always midnight Mondays. The author will be providing a new blog with frequent posts in real time covering weather events and medium and long range forecasts. Learn more by writing me at