Saturday, December 30, 2017

Not Your "Average" La Niña Pattern

Because of the presence of La Niña conditions in the tropical Pacific, La Niña is an easy target to blame for the wacky weather of the past month or two, but is it the real culprit?  Let's have a look.   

La Niña is a component of the natural see-saw of oceanic and atmospheric conditions in the tropical Pacific characterized by anomalously cold ocean temperatures in the central and eastern tropical Pacific.  These anomalously cold temperatures are evident in sea surface temperature anomalies from earlier this month.  Note in particular, the tongue of anomalously cold water extending along the equator from South America to the dateline. 

Source: Climate Prediction Center
If one looks at composites (or averages) of La Niña winters, one finds patterns similar to those illustrated in the bottom panel below with high pressure over the central Pacific near about 150ºW, cold conditions over northwest North America, wet conditions in the Pacific Northwest, and dry conditions across the southern tier of the United States.  

Another way to look at this is in terms of hight anomalies at upper levels.  Areas in warm colors below correspond to anomalous ridging, and areas in cool colors anomalous troughing.  At upper-levels, the primary circulation features are an anomalous trough near Hawaii, ridging in the North Pacific, and troughing over northwest North America. 

The pattern over the past month hasn't really looked like that.  Instead, we've had anomalous ridging along the west coast of North America. 

In addition, while it has been dry across the southwest US and most of the southern US, it has also been dry in the Pacific Northwest.  The only areas of anomalously high precipitation is in the northwest U.S. Rockies and adjoining plains and in lake-effect areas.  

Thus, this is a pattern that doesn't fit the average La Niña pattern all that well.  That isn't to say La Niña isn't playing some role.  It could be playing an important role, with our use of relationships based on averaging past events the real problem.  On the other hand, it is also possible that we need to look at what is happening from a broader, global perspective.  Some discussion of this topic is provided by the California Weather Blog.  This is an area of active research, and one that will probably get even more attention after this winter, which has generated some remarkable weather extremes across the United States.  


  1. Thanks for linking to that California Weather Blog. Lots of good info there, though not exactly encouraging. Our only hope at this point might be for you to take a trip.

  2. I have tried to find correlations between some of the other indices (such as the NAO, AO etc) and winter precip in parts of the western U.S. but have not found much of any significance. There is some definite correlation (plus or minus) with ENSO of course, but even so this correlation is relatively low with R values between 0 and 0.25 for most areas (1.0 would be a perfect correlation). For California, positive phase (El Niño) seems to be associated with greater variability in winter precipitation, with the La Niña phase more "predictable", e.g. less variable, and having a lower seasonal mean in southern half of the state. In Utah, areas roughly north/west of I-70 appear slightly favored by La Niña but this correlation is really only significant in the far north (R value ~ 0.20). Southern and eastern Utah are slightly favored by El Nino for winter precip but with a low correlation (< 0.10). The bottom line is that any season really can bring any given type of pattern.

  3. In Colorado, neither La Nina nor El Nino conditions produce statistically decipherable impacts to snowpack. Its spaghetti.