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.