What Happened to La Niña?

A few days ago, reader Gary commented on the relatively warm winter that Alaska has had so far, and asked about the status of La Niña.  It's worth considering this, because La Niña is still ongoing in the equatorial Pacific Ocean, and La Niña tends to produce cold winters in Alaska (but not always, of course).

The current La Niña - which is now waning and is likely to dissipate this spring - has been a strong one, and this is the third La Niña winter in a row.  The chart below shows the Multivariate ENSO Index since 1950; the MEI is the single most comprehensive index we have to measure the behavior of La Niña and El Niño:

 

Neither of the two past winters was particularly cold for Alaska as a whole: only November was notably cold last winter, and February was the only month with significantly unusual cold in winter 2020-21.  Both winters ended up close to the 1991-2020 average, judging by the November-March average for the state overall.

But this winter we've seen not just an absence of cold but a considerable degree of warmth, particularly in January.  It was the 4th warmest January since 1990, and the 2nd warmest since 1950 for a La Niña winter (based on MEI data since 1950).

 

Here's a chart of November-January statewide average temperatures since 1950, with the ENSO classification indicated by colors.  For the classification, I simply divided the Nov-Jan MEI index into equal thirds.

There are several remarkable things to notice here.  First, this winter so far (the right-most blue marker) has indeed been much warmer than the average La Niña winter of the past 70 years, although winter 2000-2001 was much warmer still; it's not unprecedented for La Niña's cold signal to "fail" completely.

Second, the linear trend lines denote the changing "normal" for the three ENSO categories, and we see that this winter (through January) has been as warm as the typical El Niño winter would be in the current climate.  (Note that last winter was cold for Nov-Jan, but warmth in March largely reversed that anomaly for the overall winter average.)

Third, it's very interesting to see that neutral ENSO winters seem to have warmed much less than either El Niño or La Niña winters.  We see the same result in a chart for November-March (of course this winter is not yet plotted here):

According to this analysis, neutral ENSO winters are now just as "cold" as La Niña winters.  Interestingly, there's also a suggestion that neutral ENSO winters used to be the warmest of the three categories, although the uncertainty is large on these linear regression estimates because of the small sample size in each category, and the large year-to-year variance.

The surprising difference in the neutral-ENSO trend is accentuated even more if we only use data since the 1976-77 Pacific climate shift, when Alaska winters suddenly warmed up in association with the PDO phase reversal.  In this short and highly uncertain trend analysis (see below), the neutral-ENSO trend is negative, but that's clearly because the very warm winters of the late 1970s and the 1980s saw a lot of ENSO-neutral conditions.  We would probably wish to avoid attributing the warmth back then to the neutral ENSO state, because the persistently positive PDO regime likely had more to do with it.

The temperature data from Fairbanks tell a very similar story: November-January this winter was as warm as a "typical" El Niño winter; winter 2000-2001 was much warmer still; and neutral-ENSO winters have warmed much less than either El Niño or La Niña winters.

 

Incidentally, it's interesting also to see the consistency between the independent trend estimates for La Niña versus El Niño winters.  There's a hint that El Niño winters have warmed slightly more than La Niña winters, but there's not much in it.

Here are some other climate ranking maps for January: it was relatively wet for the eastern and northern parts of Alaska, significantly less windy than normal in much of the south and west, and cloudier than normal almost everywhere.

Sea-level pressure was below-normal across the Bering Sea and most of Alaska, which explains the relatively cloudy, damp, and warm weather.  La Niña winters more typically see above-normal MSLP in the Bering Sea and Aleutians.

 

La Niña also more commonly brings low MSLP to the Arctic, and therefore a positive AO phase, but the AO remained negative in January (although much less so than in December).  The enhanced north-south pressure gradient produced above-normal winds for the Arctic coast: Utqiaġvik appears to have had one of its windiest Januarys on record.