La Nina Criteria Observed In Pacific Ocean

The Weekly Climate Prediction Center ENSO Update has been published, and it confirms what observations have been showing over the past couple of weeks that the La Nina is strengthening. Now, for the first time, we had SST anomalies across Nino Region 3.4 be below -.5 degrees Celsius over the past week, which indicates on a weekly basis the criteria for a La Nina. Of course, to be a technical La Nina, SST anomalies there have to be sustained below -.5 degrees Celsius SST anomalies on a 3-month rolling average for 5 consecutive months, but this recent cooling would seem to indicate that La Nina conditions are more likely to begin developing. 

There was some cooling observed across all Nino regions this past week, though some of the strongest was in Nino 3.4, which will have more noticeable implications for the downstream pattern across the US. Over the past four weeks we can see that sea surface temperatures cooled dramatically right along the equator in the Pacific, creating that strong Nina signal. 

Similarly, we see that subsurface ocean temperatures remain relatively cool across the Pacific. However, the CPC notes in their update that "the below-average temperatures have weakened across the western Pacific at depth." 

This was a trend that appeared to continue into the most recent update from July 12th. 

Accordingly, though we have seen cooling across the Pacific in key Nino regions that will bring greater La Nina influence to the pattern in the coming weeks and months, this rate of strengthening appears unlikely to continue. We do not see evidence of any significant Kelvin Waves since an upwelling wave began in March, though we could be seeing the very early signs of a warmer downwelling wave forming in the western Pacific over the past couple of weeks. This could slow down further La Nina development as well if it develops and propagates across the Pacific. 

The result is that even with these recent cooler trends over the past week, the Climate Prediction Center is actually less certain that a true La Nina will develop and last through the winter. In fact, La Nina probabilities are now shown peaking in November/December/January at just over 60%, while by February/March/April there is an almost equal chance of neutral conditions versus a La Nina. 

Similarly, CFSv2 American climate is now showing it as quite unlikely that a strong La Nina develops. There remains a decent spread, with the average showing a weak/moderate Nina developing, though a couple members show a strong one developing and a couple others show no Nina developing whatsoever (with little further cooling from where we are currently). 

The result is lower confidence in meaningful La Nina development through the coming months and into winter despite more clear development over the past week. The SOI has begun to bounce back modestly, indicating that we should begin to feel the impacts of a more Nina-like pattern moving into August. Similarly, the AAM is likely to remain solidly negative through the next couple of weeks, resulting in a more Nina-like global pattern as well. 

However, without a strong La Nina developing, there will certainly be room for other oscillations and pattern drivers to influence the pattern. The MJO is not shown as being particularly active after the next week or so, but if that oscillation became more active we could see further pattern influence (it will help the Southwest warm later in Week 1 it appears, though). Similarly, a lingering positive PDO and stronger positive Kaplan AMO could have influences on the pattern moving forward that will need to be monitored. The QBO is going through an unprecedented breakdown that could further complicate long-range forecasts and patterns. Thus, though on a weekly basis we moved closer to La Nina conditions, that by no means is entirely settling what our patterns are likely to look like moving into August and the fall/winter, and instead confidence remains average to below average with further forecast volatility expected.