Wednesday, December 14, 2011

Checking in on the PDO

Last week we checked on the AO Index to see how it has been behaving of late, and there hasn't been much change regarding that in the last 7 days.

Today, let's check in on the North Pacific. The main index we look at here is the PDO (Pacific Decadal Oscillation). I'm sure you've heard of this and generally know what it means. However, if you do not, fear not, as here are a couple links to give you a quick primer.

Note that I normally loathe Wikipedia as an official "source," but I know that experts in the field generally take good care of Wiki articles on atmospheric principles and concepts. So for the most part, Wiki articles on meteorology are trustworthy.

Now, let's take a look at how we've been doing lately.

The PDO clearly appears locked in a negative (cold) phase. The chart at the right shows you how the PDO has behaved since 2005. There have been suspicions that we've entered into a longer term negative phase of the PDO (starting back in 1998), where we can go long stretches registering colder than normal water in the Pacific. And I think if there were still questions after 1998, they've been mostly answered at this point.

November's PDO value (via the NCDC) registered in at a hefty -2.87. We briefly dipped under -2.5 in 1998 and have had several dips into the -2s, but the last time* we registered a value under -2.75 was March of 1956!

*Author's Note. There is another set of PDO values run by the University of Washington. There are some differences in how these two values are calculated and some questions as to which set of data is more relevant to weather/climate linkage. Regardless, the November PDO value from UWash was -2.33, and the last time they registered anything under -2.25 was back in December of 1961. So either set tells you that it has been AT LEAST 40 years since we've seen the North Pacific this cold!

On the left is a correlation map from NOAA's Earth System Research Laboratory (this week's link to bookmark). It shows how surface temperatures in the US from December to February correlate to the PDO alone. Notice that, typically, the PDO correlates strongly positive in the Pacific Northwest and Northern Rockies and strongly negative on the Gulf Coast (as much as -0.7!). A positive correlation means that as the PDO goes up or goes down, surface temperatures will generally follow it (warmer when the PDO is positive and cooler when it's negative).

Alternatively, a negative correlation means surface temperatures will do the opposite depending on the PDO: Warmer when it's negative and cooler when it's positive.

So the map generally makes sense that it would be colder in the Northwest and warmer on the Gulf Coast in a negative PDO situation. Locally, the correlation is around -0.5...which isn't exactly strong, but it's not bad. So typically, in a negative PDO situation, you would expect milder winter temps, which is indeed what we've had so far this year. Of course, as we've learned in the last few years, the PDO alone is not the sole driver of weather. So while it's nice to have a strong correlation like we do on the Gulf Coast, it doesn't mean it works perfectly. Things like the NAO, AO, EPO, PNA, etc., etc., etc. need to be taken into account. But the greatest impacts from the PDO, despite the correlation map will obviously be on the West Coast.

With that in mind, you can see the last 12 months of temperatures in the Western US at the left. It has been cooler on the West Coast for sure over the last year, and I think it's fairly clear that some of this is at least attributable to us being in a negative phase of the PDO. And, hey, the PDO was negative last winter...and it was fairly cold for awhile in the Delaware Valley. So again, a negative PDO doesn't mean we should expect warmer winters the next several years. But it will be interesting to see how it impacts the US, given that it's been 40 years since we've seen it this strong.