Thursday, January 8, 2015

PDO Influence Aloft

Tuesday's post on the persistent warmth aloft above Fairbanks (despite recent chill at the surface) sparked the question of how the PDO affects temperatures at different elevations.  This is a question that deserves more detailed investigation that I can give it now, but a simple chart tells the tale for early winter: see below.  The colored lines on the chart show the mean temperature for the November-December period at 500, 700, and 850 mb, for each year since 1948, and the black columns show the November-December mean PDO index.


The high correlation between temperatures at various levels in the lower half of the atmosphere is clearly evident, and it's also plain that the PDO has a strong connection to temperatures even up at 500 mb (~18000 feet).  Here are the Pearson correlation coefficients between the November-December PDO index and temperature:

Surface: +0.65
850 mb: +0.72
700 mb: +0.67
500 mb: +0.50

Interestingly the correlations are slightly higher at 850 and 700 mb than at the surface, and the correlation remains a fairly robust +0.50 at 500 mb.

The deep and coherent nature of the warm air over Fairbanks since the start of winter is illustrated by a time-height cross-section of temperature anomalies for the lowest 3000 m (see below).  It's clear that the recent cold spell at the surface was a very shallow phenomenon; warmth continues unabated aloft.  In fact, temperatures have warmed dramatically again in the past 24 hours, and this afternoon's balloon sounding reported a temperature of +7.6 °C at 850 mb, which is 19.2 °C above normal.  This is just about the greatest warm anomaly seen so far this winter at 850 mb (+19.8 °C was seen in November).



12 comments:

  1. I sometimes act like a parrot with a limited vocabulary. Warm PDO>more clouds>warm air. Clouds clear>colder>surface inversion.

    Cracker?

    Gary

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  2. Richard,

    Sam Friedman at the News-Miner here. I've been reading the blog for a while and would like to do a story about it for our spotlight series. Would you be interested in doing an interview? I'm at 459-7545.

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    1. Sam, thanks for reading. I sent you an email from my Prescient Weather address. Let me know if you don't see it.

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  3. Great post Richard. The correlation between PDO strength and surface temperatures is interesting. I think that because the low level inversion can hold on for several days after an airmass change takes place aloft explains the R-value discrepancy; i.e., an airmass change can take place and it goes unnoticed at the surface for some period of time – especially if there are no clouds. Just a guess though.

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    1. Brian, I agree. Also, because of the inversion, surface temperatures can be strongly influenced by small changes in wind and cloud cover, unlike temperatures aloft.

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  4. The last time Drudge headlined this Blog Rick had to shut down the comments/questions due to all the offers from Nigeria for financial aid, or whatever scat topic posters could come up with. Global Warming became a flame war for others.

    Gary

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  5. Looking at the first chart I'm struck by how positive the PDO is compared to the previous 60 years. I'm also intrigued by how much of the PDO indices are negative. This leads me to believe that something isn't right with the standardization. And if nothing is wrong then why has the PDO been so positive? Too bad there isn't more radiosondes scattered around the state.

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    1. My question: What's driving the warm air aloft? Pressure cells, flow, Jetstream looping, PDO, ENSO, MJO, Global climate change, or other factors in the alphabet soup of the current climate?

      Locally in Fairbanks we're in the Tanana Jet mode now with warm air and periodic clouds moving north over Alaska. Let it clear and without southerly flow we would be visited by the cold airmass over NW Canada. Wait a week-10 days and see what happens to our temps.

      Gary

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    2. Gary, I can't speak for Fairbanks but down here in Anchorage the temperatures have increased substantially during the winter months in the last few decades – particularly December and January. I pulled all of the PANC RAOBS and it appears that at 850 millibars, southerly and southeasterly winds in December and January occur about 5% more often since the PDO shift and northerly and northeasterly winds are about 5% less frequent. The S/SE wind percentage had a R-square of 0.64 with 850 temperature departure. Therefore, there is an increase in warm, southerly winds in the upper atmosphere. But, that doesn't answer your question as to "why?"

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    3. Thanks Brian as always for the fill in info. "Why" vs "Is"? Last I searched there were several recent papers that discussed the potential drivers of the weather patterns along the Pacific rim, mainly north of the Equator.

      Seems that agricultural and food production (both terrestrial and aquatic) are impacted by the recent trends in climate. That probably drives the studies more than the comfort of Alaskan residents.

      And I agree with your observations...If I look up see clouds periodically moving in from the S/SE. Good enough explanation of our weather for me.

      Gary

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    4. Eric, the PDO values come straight from the University of Washington at

      http://jisao.washington.edu/pdo/PDO.latest

      I'm not sure of the baseline period for the index, but I have no doubts about the validity of the calculations.

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  6. I'll add this new NWS WX viewer. Zoom to Alaska:

    http://www.wrh.noaa.gov/map/?wfo=&obs_type=weather&obs=true

    Gary

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