Monday, September 16, 2019

Why have our low temperatures been so HIGH this summer?

Have you noticed something meteorologically very strange this summer?

The overnight low temperatures have been very warm for pretty much the entire summer.

I can demonstrate this in several ways.    Let's start with a plot of the temperatures at SeaTac Airport for the last 3 months, with the average highs (purple) and lows (cyan) shown.  Mama mia!  The observed minima were above normal of most days--often by 3-6 F.   Sometimes even more.


A plot of the difference between the observed and normal temperatures for the last 90 days shows the same thing (see below)--but with a few wrinkles.   The coastal zone is all warmer than normal, with some places 3-5F above normal.  But go inland and many areas have normal temperatures--some even below normal temperatures.  Why is that?

Scott Sistek on his KOMO-News blog ran some of the numbers, finding that this summer we had 79 days in a row at or above 55F...absolutely SMASHING the previous record of 52 days set in 2013. 

The high minimum temperatures have made my tomato plants very, very happy--but why have we been so warm in the morning?

I think I know the answer--- warmer than normal sea surface temperatures over the eastern Pacific.   Also known as the BLOB--or in this case BLOB junior.

Here is a plot of the sea surface temperature anomaly (the difference of sea surface temperatures from normal) for the past 90 days.  You see the warm area off our coast?   A large area of 2C above normal--some a bit more?   That is the key feature.  2C is about 3.6F above normal....just in the neighborhood of how much warmer than normal our minima have been.

Now physically this all makes a lot of sense.    On most days, he air over the western WA lowlands was over the Pacific Ocean a few hours earlier, with the temperature of the surface air determined by the temperatures of the ocean's surface.

But there is more--the amount of moisture that air can pick up from the ocean depends on sea surface temperature---since warm air is able to hold more water vapor.  In fact, the amount of moisture air can hold goes up exponentially with temperature--that means REALLY quickly.  This figure illustrates this fact (saturation mixing ratio is the maximum amount of water vapor a sample of air can hold at a certain temperature).  So having air above warm water means the air can pick up more water vapor from the water.
Why does this matter?  Because water vapor is a potent greenhouse gas that warms the surface of the earth by reducing the loss of heat to space (sort of like an atmospheric blanket).  And a measure of the amount of low-level moisture in the air--the surface dew point--HAS been higher than normal this summer in western Washington.

So how can we really clinch this relationship between warm sea surface temperatures in the eastern Pacific and our minimum temperature "heat wave"? 

There is a way: we can play God using our WRF weather prediction model.  I asked UW Research Scientist Dave Ovens to do an experiment.  Redo an extended (7.5-day) forecast but replace the atuall warm sea surface temperatures with normal (or climatological) values of sea surface temperature.

Here are the results for a 7.5 day run verifying 5 AM last Friday--specifically the differences in surface air temperature between the current sea surface temperatures and normal sea surface temperatures.  Warmer air temperatures over ocean, which extend into western Oregon and Washington.  The Cascades seems to be holding back the influence from the interior.  Bingo.



Dew point temperature (again a measure of water vapor content) differences.... same story.  Much more oven the ocean, but with higher values getting into the coastal zone (and a bit farther in eastern WA). Bingo2.

So I think we know the culprit for our warm minimum temperatures...it is the area of warm water over the eastern Pacific.   A.K.A. the BLOB.

What caused the BLOB?  A combination of persistent high pressure that reduced vertical mixing in the ocean and southerly winds.  Essentially, a highly anomalous atmospheric circulation over the North Pacific.   That also can be inferred by the SST anomaly map shown in the third figure, since the north Pacific seems to be a big outlier.

Global warming could be making a small contribution.  A map of the surface temperature change from 1930 to now (from the NASA GISS site) indicates a warming of .2-.5C (.35-.9F) over the period.   Some, but not all, of that temperature increase might well be associated with global warming.



9 comments:

  1. Thank you Cliff. To the best of my knowledge this increase in low temperatures at Seatac, for example, has been going on for many years. I'm fairly certain a portion of this temperature ascent is caused by human-caused climate change. Some of the increase is likely the result of urban heat sources. I'll await a peer-reviewed scientific paper for the solid answers.

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  2. I'm guessing that the increased rainfall in the NW this summer has been due to the increased water vapor carried into our region from the blob. From a wildfire perspective and, maybe, an agricultural perspective that seems beneficial. I know that our summer in the southern Willamette Valley was quite pleasant. I'm wondering how the blob will affect our winter weather.

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  3. What will be interesting is how much of an effect the warm water will have on our winter storms by adding additional water vapor and moisture. The warmest water looks to be in the prime NW windstorm breeding ground. With a more neutral ENSO winter, has me believe we could see some potent storms develop.

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  4. I did not observe overnight minimum daily temperatures that were consistently notably above average in NW Bellingham this summer. The warmest overnight minimum temperatures at my location were around 61F on a few nights. The summer of 2015, when "The Blob" was really making an impact featured far more anomalously high overnight minimum daily temperatures. I recall at least a couple of nights featuring minimum temperatures of 65F-66F. Quite unpleasant without air conditioning. This summer was not comparable. For what it's worth, in my period of record, I have never recorded a calendar month in which the monthly minimum temperature was above 49F.

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  5. Would it be safe to say that all these systems that have been coming through lately would mix out this blob?

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  6. Is this due to the fires in the Amazon Rain Forest Dr. Mass? Or flaring of Natural Gas?

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  7. We had a lot of cloudy nights.... Insulated us a bit

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  8. Cliff, It's fun to gain insight into how the systems influence each other and work together!

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  9. Out here in the Columbia Gorge, we had a cooler than usual summer (translation: not as broiling) with cool mornings. I track electricity use every day, and propane use with each delivery.

    Plenty of mornings needed us to turn on the propane fireplace that we typically only use in winter for supplementary heat; its fan takes a lot of juice, so electricity consumption for the July 20-August 20 period rose 7% even though we never used the heat pump for cooling. When the propane tank gets refilled, I expect usage to be greater than last summer's.

    Spring was late, and we also had more rain than usual this summer. It followed a brutal February during which 6 feet of snow fell at our place in one month, compared with about two feet for the entire prior winter. Fall is arriving early. We're getting ready for another heavy winter.

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