Wednesday, October 14, 2020

Our Mornings Have Been Warm: Blame the Blob!

Why Still Here?

During the past two months, our morning low temperatures have been much warmer than normal. Yet our daily highs have not been that unusual.

Why?  

A big reason is the blob--an area of warm water over the northeast Pacific Ocean. 

Below are the temperatures for the past month at SeaTac Airport, with the normal highs (purple line) and lows (cyan line) shown as well.  Averaged over the period, SeaTac's high temperatures have been near or slightly above normal, with both days above and below normal.

But look at the minimum temperatures!!   On NO day have SeaTac's minimum temperature reached normal values, and on many days the low temperatures were 5-8 F above normal!    No wonder my tomatoes are still alive and flourishing.


You can see the effect in regional temperature maps.  Here is the difference from normal of the minimum temperatures over the past month for Washington State.
Wow.  The coastal zone of our state was 6-8 F above normal, and the warming was about half as large east of the Cascades.

But now look at the difference from normal for the same period of maximum temperature.  Quite different.  First, the differences are less and the smallest warming is over the western half of the state.  Why?

The answer  to this strange temperature pattern is the warm water over the eastern Pacific, commonly referred to as the BLOB.  

Here is the difference from normal of the sea surface temperatures over the northeast Pacific today.  The waters off our coast are quite warm--as high as 3-5C (5-9F) above normal!

Even averaged over the past month, the northeast Pacific is quite warm  by 2-3C (see below).

A warm Pacific warms the low level air moving into area, particularly west of the Cascade crest,  and particularly for minimum temperatures.  Maximum temperatures are less affected, because greater vertical mixing during the day (because of solar heating at the surface) brings down air from aloft that is less influenced by the sea surface temperatures.

Reading your minds, you are probably asking, why has the eastern Pacific sea surface been so warm recently?  Glad you asked.  

The origin can be understood by looking at the difference from normal over the past month at upper levels (500 hPa, about 18,000 ft) and considering the heights of that pressure surface above sea level... which you can think of in terms of pressure (see below).

Much higher than normal heights-- a ridge- is over the continent and much lower than normal heights--a trough--are found over the Gulf of Alaska.   This highly anomalous and persistent pattern has resulted in enhanced southwesterly flow in the lower atmosphere that brings warm water northward and lessens cold water upwelling along the coast.



Considering we are about to enter a La Nina winter, one would expect that this pattern to break down.    My tomatoes will be sad if it does.


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6 comments:

  1. That wind storm yesterday blew a large section of our roof off. It bent back and folded over on itself. The roof had problems already, but there must have been a big gust for that to happen. At least it wasn't raining at the time.

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  2. Judging by the temperature predictions for next week, the blob might not be around much longer. By next Wednesday, Oct 21, the daytime high temperature is only supposed to be 53, and the low temps in the low to mid 40s.

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  3. Let me be the first to ask if this is because of...wait for it...Global Warming? Blob, Shmob, just admit the obvious, Dr. Mass!

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  4. Wouldn't the smoke and persistent fog over that time period also add to the warmer minimum temps? Seems like that would have a bigger impact?

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  5. Well, here in Sacramento, I am looking at the best tomatoes of this season (and hoping they quickly ripen). Fingers crossed the blob stays around long enough for those bad boys to reach 'ready to eat'--and also that we get a LOT of precipitation this winter. After a maddening summer of heat/smoke/ash/Covid, we need the rain to wash away months of stress, and breathe life back into our landscape! As always, a great write-up Cliff. Thank you!

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  6. Any chance of a history of Blobs? What are the frequency, duration & strength of Blobs over time. And how do those phenomena relate (or don't) to changes in, for example, to arctic sea or air temperatures?

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