What kind of weather pattern produces lots of smoke in Puget Sound?

Even with all the talk about smoke here in Puget Sound, little real research has clarified the weather patterns associated with our smoky days.

OK....let's address this deficiency!

I will do this by using a technique called compositing--- finding a series of times of big, recent smoke events and then averaging weather maps for these times.  Common features should reinforce themselves, showing us the essential circulation patterns accompanying smoke.

The times I will composite are the most smoky times during the past three years:

8/21/2018    1500 UTC 

8/15/2018   0300 UTC 

8/4/2017 1200 UTC

8/9/2017 1200 UTC

9/7/2017 0000 UTC

8/23/2015 2100 UTC

First, let's composite the a representative mid-level parameter-- 500 hPa heights (think of it like pressure around 18,000 ft)--see below.  Mama mia, there is quite a signal!  Big ridge of high pressure over the West Coast, with troughs (low pressure or height) over the eastern Pacific and the eastern U.S.

If you really want to see, the wave-like anomalous upper circulation, here is the same map with the mean values for the period taken out (these are called anomalies from climatology).  Amazing.   When we have smoky period, anomalous high pressure is found over southeastern BC.  Such high pressure is associated with warm, sinking air aloft.   In contrast, the eastern U.S. has a cool trough when we get smoky.

What about at the surface?   There we tend to get lower heights (or pressure) over western Washington due to the warm air aloft (see below).  We call that  feature a thermal trough.  In such a pattern, the normal east Pacific high pressure area is pushed offshore, other high pressure builds inland, and the onshore flow of clean ocean air is interrupted.  In fact, with high pressure inland, we tend to get offshore (easterly flow), which pushes inland smoke over Puget Sound.

This pattern--high pressure aloft centered over southern BC, thermal trough over western WA, lack of onshore flow, and modest offshore flow-- is very good for pushing  smoke over western WA.    

But the question you are asking is whether this pattern has become more frequent in time and whether global warming could be the cause.  A very good question.

To help answer your question, I have plotted for over many years the values of the 500 hPa heights (think pressure at roughly 18,000 ft) over exactly the region associated with our heat waves:  the area centered on SE British Columbia for July and August (see below).  Higher heights (or pressure) is associated with our smokiest periods. (see below)

No wonder this was the big year for smoke!  This year had the highest pressure since the late 1940s.  Heights appear to have risen over the past decades, which is expected as the region (and the planet) have slowly warmed.  Warming causes air to expand, which rises the heights aloft.  This year's spike up seem anomalous and unprecedented, and this is not reason to  expect a repeat next year.

 

 Why do I say that?  Because I have done extensive work (with graduate Matt Brewer) on exactly this issue.   Heights aloft are controlled by the mean conditions (which are slowly rising with the warming earth) and transient high pressure areas, like the one over southeast BC that was associated with the smoke.    Our research (based on past trends and climate models for the next century) suggests that transient high pressure area should be attenuated under global warming--which would work against the fires and smoke.   On the other hand, global warming would help reduce relative humidities and thus encourage fire.

from Cliff Mass Weather and Climate Blog https://ift.tt/2Q3eEpn