Wednesday, August 3, 2016

A Factor in Winter: The Pacific Decadal Oscillation

Good morning everyone. As I mentioned, I am going to start to highlight factors that can drive our winter pattern over the next month. Of course we cannot make any predictions yet, but it is a good time to at least take a look at how different variables impact the season.

One of those variables is the Pacific Decadal Oscillation. In simple terms, the PDO is a sea surface temperature pattern in the north pacific. It has two phases positive and negative. Both phases tend to effect the overall pattern in different ways.

Looking above you see two different phases of the PDO. On the left is the positive phase and on the right is the negative phase. Notice how in the positive phase we have a horseshoe of warmer than normal water off coast of North America extending down towards equator. Also notice the cold pool of water off shore. The opposite of this is on the right image.


The next image shows our current sea surface temperate anomalies.  It may not be as clear cut as my example image, but we are currently in a positive PDO.

In fact, if we take a look at model projections for winter you can see how there is evidence this continues..


So what does this mean?

In most cases you want a +PDO for a good winter season. This pattern supports a trough south of the Aleutian islands of Alaska which in turn supports a ridge in the west and trough in the east. The next image below shows all the winters that have had a +PDO and how the jet stream set up as an example...

Cooler colors are low pressure or dips in the jet stream and warmer colors are higher pressure or a rising jet stream. Pretty strong signal for colder and stormier than normal conditions along the east. Of course, this is not only factor that matters and is simply only a blend of many years some of which did not look like this. 

One of those other major factors will be once again the El Nino Southern Oscillation (ENSO). This past winter we had a killer strong El Nino which dominated our weather pattern. This year that has shifted and we are more towards an La Nina state. However, La Ninas are not as favored during +PDO's (there has been research done on this connection) which lines us up for a very weak La Nina or no La Nina at all.

I will comment more on this at another time along with other factors that will drive the winter forecast. 

Thanks for reading. 

4 comments:

  1. What happened to those big snowstorms that came out of Southwest, redeveloped off the coast of Florida, slowly crawled up the coast and gave huge areas of the Northeast/ Mid Atlantic, and the South big snows? I don't mean just the coastal areas. I mean big snows simultaneously for coastal/interiors regions from Virginia to New England. Binghamton/ Elmira, NY; good chucks of Ohio and PA have not seen 25" snows for years.

    ReplyDelete
    Replies
    1. Those are good old classic "Miller A" storms. For that to happen you need a big ridge out west and a very very cold air mass out ahead of the developing storm to support snow from coast to inland. Blocking high pressure over Greenland also aids in holding in this cold air. We will see one of those again. The big storm last winter was close to that as it originated from a big piece of energy that came across the country and completely closed off, but the high pressure was too strong so it couldn't crrep enough north. The blizzard of 93 was the best example of an epic storm that developed in gulf then worked it way up the whole coast. Mixing issues and rain near coast tho.

      Delete
  2. Now both accuweather and the farmer's almanac are calling for a brutally cold and snowy winter across much of the country.

    ReplyDelete
    Replies
    1. Yeah I saw that. There are factors that could align to produce a great winter. HOWEVER, I can equally see the other case as of now- in particular a winter where we have inland running storms.

      I am waiting for updates on the La Nina, Pacific Decadial Oscillation along with other ocean patterns over next 2 months. We will also have Eurasia snow cover to analyze.

      I will say this, if you want a good winter you want to bet against a La Nina and bet against any cold water anomalies popping up along the pacific coast.

      Delete