April 24, 2011

PDO and Long-Term Trends

Saturday's clear skies over nearly the entire state....you won't see this again soon!

After Saturday's spectacular sun and warmth, we have returned to normal again. Saturday was warmest day for the region so far this year, with temperatures climbing into the mid to upper 60s for most western WA locations away from the water. As expected some of the warmest temperatures were east of the Cascade foothills, where compressional heating from descending easterly flow and distance from the cold water allowed a boost into the lower 70s. North Bend, WA got to 71F for example.

Looking at the latest forecast model output, I am afraid that we have showers and cooler weather ahead of us this week.

Through April 20th, April temperatures for many observing sites (e.g., Sea-Tac) have been on the coldest on record--since roughly 1950. This is also true looking at the air temperatures approaching our region aloft in the lower atmosphere (850 mb--roughly 5000 ft--at Forks on the NW WA coast). Thus, by any measure this has been an unusually cold spring.

What does this all mean? What are the trends? Will moss take over our lawns forever? Several of you have complained that springs are getting worse lately--La Nina or not. So in this blog, lets look at some facts.

Here is a two-plot panel showing the trend in April temperatures in the lower atmosphere (mainly Forks, 5000 ft) and at Sea-Tac for the surface since 1948 (click to enlarge). The dots are the annual values and the other lines are curves fit to these variations with various degrees of smoothing. One is struck by the similarity of the long-term temperature changes in these two plots-- temperatures seemed to peak around 1990 and have cooled since. Temperatures were cooler in the early 70s back into the 60s.

Some researchers have suggested there is a multi-decadal variation in the atmospheric circulation over the eastern Pacific (period roughly 30 years) and have termed it the Pacific Decadal Oscillation (PDO for short). This oscillation influences temperature and snowpack over the NW. UW scientists (like Nate Mantua) have led the study of this feature and have even plotted values of a PDO index, shown in the figure below. Blue (negative values) indicate cold PDO (and generally colder and snowier over the NW), and red is warmer. According to this index, the PDO was associated with colder temperatures into the 50s into the mid-70s, and then began a warm-up phase. More recently, it looks like we may have headed back into a cold-phase PDO.

The trouble with this is that data before 1900 is really not good enough to tell whether this oscillation is robust over a long period and we really don't understand why it exists--just that there appears to be a natural mode of variation that predates any possible global warming forcing. As I have noted before, because of our position downstream of the Pacific, global warming will be weaker and start slower around here compared to most of the globe.

So where does this all leave us?

(1) It is clear from the above information and others, that the region has cooled the past few years during the spring. You are not crazy thinking things are getting worse.
(2) Such cooling is consistent with us going into a negative (cool) phase of the PDO.
(3) Natural variability like the PDO is probably dominant around here at this point in time...far more significant than any global warming signal.
(4) Eventually the PDO will switch into the warm phase and the global warming signal here will get stronger....so don't throw out your tee shirts yet. It is going to get warmer.

And one final note...the return to last week's pattern implies not only that we will go back to cool and wet weather, but the eastern part of the U.S. may be on tap to experience more savage storms. In fact, the computer models are predicting this.


  1. Let's not forget about 2009. Early spring might suck, but that year had the best late spring and summer in the history of the universe. Happy Easter!

  2. Cliff Thanks for sharing this information. This matter has some very real practical value regarding farm planing times and field drainage in western Washington.

  3. Great. Something to look forward too....

  4. England is supposed to have worse weather than here - yet:


  5. I would like to know what effect La Nina has on summer weather. Does La Nina make it cooler, wetter or dryer and warmer than normal?
    Dave S.

  6. Thanks Cliff. But the thought of a decade or more of cold springs due to the PDO cycle leaves me ... well, cold. I may have to set up a second home base in Hawaii.

  7. Are my eyes picking out a non-existent pattern, or is there a 2-3 year cycle in the April temperature graph?

  8. I tried to cross-correlate the PDO to glacier advance/retreat farther north in SE AK, but could only show some possible relationship. I don't know much about glaciers; perhaps the period on such things is much longer than decades. But I do believe that over the last 25 years, several glaciers in SE showed a historically rapid retreat. Now we'll have to see if they slow....

  9. OK, I'm really not clear on what you're saying here. "It's going to get warmer"...this summer? ....in 20 years?? Your definition of long term seems to be really long terms. I just want to have some idea of what this all means for this summer.

  10. I see that the decadal charts have big ups & downs from year to year within the decades - all those dots that the line smooths out.

    Do la Niña and el Niño have anything to do with these?

    Do the ups correlate to el Niño years and downs to la Niña years?

  11. As a kid in Seattle we spoke in hushed reverent tones regarding the summer of 1958 and summer of 1959. They were summers of water rationing and warm lakes. It was glorious. Then came the summer of the World’s Fair in 1962. Opinion shapers from the networks made fun of our ‘Country Fair’ and the summer that wasn’t. We used to say: ‘if you don’t like our weather just wait 3 months’– what with the advent of the possibility of the PDO rhythm we can say: ‘if you don’t like our weather just wait 20 years!’

  12. Great article as always, thank you. There's a cool website www.weatherdig.com where you can find a lot of historical weather data and trends for places all over the world in a way that is easy to read. Thought I'd share with you all.

  13. Cliff, this is really depressing! I can barely stand a normal year coming as I do from Connecticut where it is full-on summer by the end of May. I did not know the PDO had such a far-reaching effect... although I have some faith in the seeming tendency for a cool early spring to produce a warm summer. Tell me it's true!

  14. Could you please (oh please) do a post on what July/August/September 2011 may be like? Do the models speak yet. The tourists would really like to know which climate to pack for. Thank you so much.

  15. Great Blog Cliff, one of the few I hit every day.

    The graph of april temperature you showed is very informative. I'm curious to see these laid up against a historical graph for the month of August. Does a cold april necessarily predict a cold summer? ( I'm really hoping the answer is no! ) I also noticed that the swing of the PDO is getting larger over the short amount of information that you have. perhaps one effect of global warming would be that the severity of the PDO would increase. Just an intuitive thought, but chaotic systems to tend to jump around more as energy input increases.

    On another point. I noticed that the area radar maps for saturday night showed rain blanketing the entire region! I went outside to confirm a starlit sky cloudless sky. All of the internet based radar sources showed the same data. Is it calibration issue with the interpretation of the radar signal?

  16. Cliff, what does the same long-term charts look like for July/August? I also feel like our summers have been getting a bit cooler as well. I swear that the mid-90s had warmer summers.

    Also, seems like a some sort of on/off or 2-3 year cycle between warm/cool springs within the trend.

  17. Cliff, I would also love some sort of Cliff Mass awesome summary as to how the PDO affects all seasons in Western WA. Or maybe linking to a thorough summary already in existence?

  18. Does all our cool wet weather have anything to do with the drought and hot weather they are having in Texas? http://news.yahoo.com/s/nm/20110428/us_nm/us_usa_drought


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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