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.