Pages

Tuesday, April 10, 2018

West Coast Precipitation Trends

There is so much talk about heavy precipitation and drought along the U.S. West Coast these days, as well as claims about long-term trends, that I thought it would be useful to look at the actual data.

In the figures below, I show the winter precipitation (November through March) for the states of Washington, Oregon, and California for 1918 to 2018 using the NOAA Climate Division data available from the NOAA ESRL website.  The West Coast has a Mediterranean climate,  which means the year is divided into wet and dry seasons, and November through March includes the overwhelming majority of precipitation everywhere along the coast.


First, California.   Quite a bit of up and down, with 1977 being the lowest (about 7 inches) and 1982 the highest (about 33 inches).  Little overall trend.   With an average of only 20 inches during the winter season, lots of variability, a huge population, and an enormous agricultural establishment based on irrigation, you can see why California has invested in a  massive multi-year reservoir capacity.


Oregon also shows little long-term trend in precipitation, with 1977 (a year of a huge persistent ridge) being the driest of the last century.  Mean precipitation is higher than California (about 27 inches compared to California's 20 inches), the population in much less, and there is far less irrigated agriculture.  Thus, they can get away with less reservoir capacity than California.


And then there is Washington State.   Winter average precipitation is about 42 inches and the lowest it has fallen to is about 22 inches, far above the minima of California and Oregon.  Plenty of variability year to year.  There appears to be a slight upward trend in winter precipitation over the past century.


 So what should you conclude from all this?   

(1)  With all the talk of droughts and floods in the media, there is really very little long-term trend in winter precipitation over the U.S. West Coast.

(2)  The West Coast has a lot of year-to-year variability, so having dry and wet years is not unusual.

Why so much winter variability in precipitation along the West Coast?

Several reasons.  The West Coast is greatly influenced by atmospheric rivers, which bring periods of very heavy precipitation.  These features are intermittent and just getting one or two more or less can make a huge difference in seasonal precipitation.

Another reason is El Nino and La Nina.  The oscillation between them, a mode of natural variability, can produce wet and dry years in various portions of the coast. 

The bottom line:  West Coast precipitation is characterized by wet and dry years, precipitation increases northward, and little overall trend has occurred over the past century.  Not sexy, but the truth, nevertheless.



from Cliff Mass Weather and Climate Blog https://ift.tt/2Hm2rZ2

No comments:

Post a Comment