Why is the Northwest Warm and California Dry? Part 1.

Many Northwest residents are asking the same questions:

Why is it so warm this winter?
Why so little snowfall in the mountains?
Is the warmth and snow drought connected with human-caused global warming?
Is there a connection with the cold and snow over the eastern U.S. and our anomalous weather?
Or with the drought in California?
What is the role of blob (the region of warm water off our coast the last year)?

Today the Seattle Times front page headline called out the poor Cascade snowpack and showed a picture of the Snoqualmie Summit ski area that would bring depression to any ski enthusiast. And media all over the country are asking similar questions.

Picture courtesy of the Seattle Times

Fortunately, there are some solid answers to these questions.  Combining some basic meteorology, logic, and the results from a number of new research studies, this and my next blog will attempt to provide a coherent picture of what we know.

But for the impatient and the lovers of executive summaries here is the gist of it:

Yes, they are all interconnected.
Natural variability is the probable cause.
There is no reason to expect that anthropogenic (human-caused) global warming has much to do with it.

So what is going on this winter?

It is quite easy to explain the proximate cause of the warmth over the Northwest, the drought over California, and the cold/snow over the eastern U.S.    They are all caused by the same basic phenomenon:  a high amplitude upper level pattern with a persistent ridge over the West Coast and a trough over the eastern U.S.

You can see this in a map of the difference from normal (the anomaly) for the last 90 days for 500 hPa heights (think of is like pressure at around 18,000 ft).  Above-normal heights (ridging or high pressure) are indicated by red colors and below normal heights (troughing) by blue colors.  You can see how disturbed the atmosphere is over and near North America.

Want to see what this pattern looks like on a specific day?...here is the forecast for this Thursday, a day that will probably hit 60F in Seattle and bring another snowstorm to the Northeast!  Big ridge over the western U.S. and trough in the east.  I am tired of this pattern.

In general, ridges bring dry conditions and warmth, while troughs are associated with cooler, wetter situations.  Here is the surface air temperature anomaly (again difference from normal) for the last 90 days across the U.S>.  Warm in the west under the influence of the ridge and cool in the east with the trough.

Why do ridges and troughs influence weather?  Because ridges have sinking air, which kills clouds and air warms as it sinks.  Also, the southwesterly flow on the west side of ridges brings up warm air from the south.   Troughs have rising motion, which cools the air and causes clouds and precipitation, and the northwesterly flow on the west side of troughs bring cooler air from the north.

Here in the Northwest we have gone between dry, warm periods and wet, warm periods.  And it all has to do with the position of the persistent western  ridge.  Look at this schematic based on the upper level map I showed earlier. Nice ridge along the west coast.  The arrows indicate the flow direction at this level and note the winds are parallel to the lines.  West of the ridge axis there is moist southwesterly flow.   If the ridge shifts a bit to the east or if its amplitude weakens a bit, the Northwest gets into the moist southwesterly flow--putting us into a wet, atmospheric river situation. Stronger ridge or positioned as shown below, we are warm and dry.   California, located farther south, is far less likely to be hit by the moist, southwesterly flow and thus they are in a big drought.

You can see this effect on the precipitation at Seattle-Tacoma Airport the past 4 weeks (see below).  Long period of no rain (big ridge) interrupted by shorter period of intense rain (atmospheric rivers on the western side of the ridge). The blue line is the average situation.

Could this contrasting pattern be caused by global warming?  

Averaged over the U.S., the temperature departures from normal are about zero (warmer west, cooler east).  There is no uniform warming going on--so it doesn't appear to be associated with a large scale global warming signal.  Folks on the East Coast might be complaining about global cooling with record-breaking snow hitting the Northeast, particularly around Boston.

Some investigators have claimed this pattern of enhanced ridging in the west and stronger troughing in east is the result of global warming, but such causality is very unlikely.  First, the papers claiming such a connection have been pretty much disproven, with serious faults in their methodology being shown.  (Many of these papers were cited in newspapers and by politicians last year when the polar vortex was in vogue).  Second, their is no long-term trend in such wave patterns, which one would expect if global warming was the culprit.  Third, global warming simulations of the future climate under global warming do not indicate higher amplitudes of atmospheric waves.  There are other reasons, but you get the picture.  It is unlikely that global warming caused by increases in CO2 are causing this pattern.

But what is the origin of this bizarre pattern of ridging in the west and troughing in the east?  I will take that up in the next blog!

The Pacific Northwest Weather Workshop

Interested in attending the big local weather workshop of the region?  The Pacific Northwest Weather Workshop will be held in Seattle at the NOAA facility on February 27-28th.   Everyone is invited and the majority of talks are accessible to laypeople.  To attend you have to register or they won't let you in the gate.  There will be a major session on the Oso landslide.  There is a registration fee that covers refreshments and food, and special student pricing.  If interested, check out this website.


Climate Change and the Pacific Northwest

I will be giving a provocative talk on this subject on March 11th at 7:30 PM Kane Hall on the UW campus in Seattle.  Sponsored by local public radio station KPLU, tickets for this event can be secured at this web site.