Thursday, January 12, 2012

What is Driving Our Not-so-Cold Winter?

We've grown accustomed to cold and snowy starts to winter.  The last two have featured rounds of cold and snow with steady snow cover. This winter hasn't behaved like her predecessors.  Why the change?  wasn't La Nina supposed to drive this winter's cold?  Not so fast.

The answers lie within several regions of the northern hemisphere and the indices that describe their behavior. Here are the regions and their parent indices in no particular order.  I'll explain what each index measures and describe the impact each index has on our weather here in northern Ohio.


We'll describe each teleconnect and show how each plays a role in our weather patterns across the Great Lakes and northern Ohio. It gets complicated so I've tried to keep the technical explanations to a minimum.


The Arctic Oscillation (AO) is a measure of the opposing pressure patterns of the mid-latitudes (across Canada and US) and the polar north.

When the index is POSITIVE, high pressure over the US and Canada drive storm tracks farther north keeping temperatures above normal across the eastern US (see left side of graphic above). When it is NEGATIVE (see right), the pressure tends to be lower across the US and Canada with higher pressure over the polar region. This causes storm tracks to come from the northwest ushering in colder than normal air.

The North Atlantic Oscillation (NAO), the AO's cousin, describes the pressure changes between low pressure over Iceland and the semi-permanent high over the Azores (eastern Atlantic).

Both of these teleconnections play direct role in how much and to what degree the cold air/storm track and intensity becomes in winter across the eastern US and Ohio.  The lower or more negative the AO and NAO levels, the more frequent and deeper the cold air.

The problem is that these pressure variations are incredibly random and very difficult to predict much further than a week ahead of time. This is why we refer to these as "wild cards".

I plotted the average AO and NAO levels for the Novembers and Decembers since 1991.

The last 3 winters had AO/NAO levels well below zero. See how the NAO and AO run hand in hand. When the AO goes down, the NAO follows. Notice below how many days (graph below) over the last two early winters the AO/NAO were well below zero

The pressures over the arctic were alot higher, the pressure/storm tracks across the Great Lakes were much lower, the cold air much colder. The shades of blue show temperatures below normal across Ohio and the  eastern US.

The pattern resulted in snows like  this...

This year, we count only 9 days with AO levels below zero! Not many at all.

Yet this year, the levels were very high. The highest levels in decades! What did this do? It kept the polar jet stream strong and consistant which keeps the cold air locked up north.  The pressure tendencies in the north Atlantic were such that it steered the storm tracks well north of Ohio.  All of this kept unseasonable temperatures for November and December. 
I plotted the average November and December combined temperatures for each year since 1991 on the AO/NAO plot. Notice that the warmer periods match up well with the higher levels of the
AO/NAO; colder periods for the lower AO/NAO levels.

The Pacific Ocean has also played a pivotal role in our pattern this year. TWO OTHER INDICES were very good guides on this milder pattern. The first is the PACIFIC-NORTH AMERICAN OSCILLATION. Its the general pressure pattern over this area shaded in red.

The higher the pressure pattern, the stronger the ridge over the western US and Canada, the stronger the trough/cold air over the US. This, unlike the other indices, means that lower than average pressure was centered over the western US/Canada and higher than average pressure was over the eastern US.

This winter, the PNA has been mainly negative.

The ocean temperatures in the tropical Pacific have been below average since last fall. This below normal ocean sea surface temperature pattern is a classic LA NINA PATTERN.

This change in ocean temperatures buckels the jet stream across North America which alters the storm tracks so that they come from the northwest. Last winter, we had more than 9 cold front-type snows from Canada. This year, ONLY TWO!

How does this LA NINA compare to others that share similar characteristics (time of peak/length of duration)? Use like to use the MEI Index because it factors in PRESSURE DIFFERENCE, WINDS, AIR TEMPERATURE, CLOUD COVER and SEA SURFACE TEMPERATURES in the tropical Pacific Ocean. Here is a graph that plots this LA NINA event to the others that compare favorably. Notice that it is similar to the three year LA NINA back in the early/mid 1970s.

How can we summarize what has happened so far?

  • While the LA NINA pattern was present last winter allowing for clipper snows from Canada, the ARCTIC/NORTH ATLANTIC PATTERN continued to drive the cold air south reinforcing the La Nina storm track.  

  • This year, the LA NINA is present yet the ARCTIC/NORTH ATLANTIC CONNECTION is noticably absent.  So instead of reinforcing the colder air/northwesterly storm tracks, it shifted the storm track more south featuring above normal rain (7+ inches for Cleveland in November and December).The ridge over Ohio and the east coast pushed snow producing CLIPPER SYSTEMS north and the temperatures higher.   
  • The cooler than normal Pacific water temps off of the coast of California and Alaska kept the cold and storminess further west (NEGATIVE PNA SHOWN EARLIER) while further enhancing theridge of dry/milder weather across the Great Lakes

How about snow totals comparing this year through January 12th to the last two winter? MUCH LOWER

How will these TELECONNECTIONS change in the weeks ahead?  More on that later after this rare round of snow moves through. :)