Sunday, July 7, 2013

Pacific Decadal Oscillation Turns Positive

The Pacific Decadal Oscillation (PDO), a major driver of weather patterns, especially in the winter, has turned positive recently.

The PDO has two phases: positive and negative. In the positive phase, above normal sea surface temperatures preside over the waters immediately offshore the coastline from Alaska down to the Pacific Northwest. In the open northern Pacific waters, a body of below normal sea surface temperatures are observed. It can be thought of as warm water along the coastline surrounding the colder waters in the open northern Pacific. In the negative phase, it's exactly the opposite. Cooler waters are dispatched along the coastline from Anchorage to Seattle, and warmer than normal waters cover much of the northern Pacific.

Until recently, we have been in a negative PDO phase. This meant that precipitation trends during the winter were very dry along most of the Southern US, including the Gulf Coast states. This dry trend continued into the Mid-Atlantic, before very slight above normal precipitation anomalies were favored in the Great Lakes region. The negative PDO tends to favor a very wet West Coast. In the temperature department, a negative phased PDO meant nearly everyone in the nation experienced increased chances above normal temperatures (outside of the Northern Plains, which was favored to experience below normal temperatures).

Now that we are entering into a positive PDO that looks like it won't be going away anytime in the next several weeks, let's see what the positive PDO composites for temperature and precipitation look like.

The top image shows precipitation trends in positive PDO years, with the bottom image depicting temperature trends in positive PDO years. For precipitation, the Pacific Northwest joins the southern Ohio Valley in being favored for a drier winter. It is worth noting that those in the Pacific Northwest have much higher chances for drier weather, as the dry anomalies in the southern Ohio Valley are very slight. The Gulf Coast experiences a wetter than normal winter, as does a portion of northern California. Oddly enough, the precipitation composite is very similar to that of the El Nino winter.
In temperatures, much of the southern half of the nation is favored for below normal temperatures in a positive PDO winter. This cool trend stretches from New England into skiing country in Nevada, Utah and Wyoming. The Northern Plains and Upper Midwest are faced with warmer than normal conditions in a positive PDO.

Remember, the Pacific Decadal Oscillation is just one of many factors that go into forecasting in the long range- although this composite may say one precipitation trend is likely, another factor may say the opposite precipitation trend is probable. Don't put all your eggs in one basket; even if you like what the positive PDO typically brings to the US, don't base your whole winter's expectations solely on these two images above.



Cameron Jourdan Fry said...

IF we were to judge the upcoming winter solely on QBO and about heads spinning this fall when winter forecasts start to bust out. Thankfully there are more clues and it's still too early to make any calls. This is a good sign for cold/snowy winter lovers like myself. But this map is almost a 180 from the QBO map posted last week ;)

Luna said...

okay this is just a light and funny comment... You had caps on for 'aNDREW' :P

Anyway, wahoo cold and wet!

Anonymous said...

I just want a nice warm, dry, winter here in Iowa! Everyone else that loves snow & cold can have it!
I say good reddens to that cold white stuff & let it keep on passing me by here in my neck of the woods! Bring on the sun & warm temps!