Why so Dry? Will it Continue?

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Announcement:  I will be teaching Atmospheric Sciences 101 during winter quarter

If anyone is interested, either a UW student or outside folks using the ACCESS program, I will be teaching this general introduction to weather in Kane Hall (210) at 12:30 PM.
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During the past week, there have been a number commenters asking the big W question.   Why have we been so dry?   Yes,  they understand that persistent high pressure (or ridging) over the Northwest and the northeast Pacific have been the proximate cause, but why is there such ridging?   And is it a random event or is there some underlying cause, such as global warming? 

Let's examine this issue here.

First, the proximate cause--the ridging or persistent high pressure.  Meteorologists often evaluate such ridging by looking a level in the mid-troposphere:  500 hPa--about 18,000 ft above the surface.

Below is a nice graphic produced by the NOAA Climate Prediction Center, showing heights at that level (analogous to pressure), with red/orange colors showing you anomalies (differences) from normal.  October and November is shown.   You can clearly see the wavelike nature of the flow aloft with alternating ridges and troughs (lower than normal heights).   A ridge sets up in the NE Pacific in late October and holds through November.     That is why we were dry, since ridges are associated with sinking motion and the lack of midlatitude cyclones and fronts.

During the past week, the ridge has shifted westward and an area of low pressure/heights has developed west of California.  This pattern has drenched California with the Northwest getting the meteorological "leftovers".   A bit of light rain, but not much of significance.  So far this month, Sea-Tac has only received .05 inches, over an inch below normal.

Now the first question many have is if there is a trend for such high pressure over the northeast Pacific.  If so, we would need to consider global warming as a contributor.   We can examine this issue easily by plotting the heights over a box encompassing the region for an extended period (see below)

Here are the results, plotting the 500 hPa heights over that box for November from 1948 to 2019.  Heights were record high in November, tying the extreme value observed in 1956.  But there is no clear evidence of a trend.  One way you can see this is by simply covering the last year in the plot.

Without a trend, there is little likelihood that there is a climate change origin to this feature.

So what is the cause?   First, it is possible that this persistent pattern is the result of chance, of the essentially chaotic nature of the atmosphere.  The atmosphere does not necessarily need a "cause" to produce persistent configurations.

But sometimes there is a physical origin of persistent patterns.  And one such mechanism deals with El Nino or La Nina.

El Nino is associated with persistent warm water anomalies in the central and eastern Pacific.   Such warm anomalies can shift the locations of thunderstorm areas in the tropical Pacific and in turn alter the upper level wave pattern in the Pacific.  La Nina is the opposite.

There are some interesting changes in the water temperatures and resulting thunderstorms/convection over the tropical Pacific.   But since this blog is getting long, my discussion of those anomalies will have to wait for a future blog.

But I will show you the prediction of the upper level pattern for the next week--the average 500 hPa heights and anomalies from normal over the north Pacific based on the European Center model.  Yellow/orange is above normal (ridging) and blue is below normal (troughing).  The ridging has moved back over us, which suggests below normal precipitation.   But substantial troughing is offshore--very reminiscent of a classic El Nino pattern.   But enough for now...