The tropical Pacific reflected neutral conditions in September – neither El Niño nor La Niña. Forecasters continue to favor La Niña later this year, with about a 60% chance of it developing in September-November. However, the probability of La Niña is slightly lower than last month, and it will likely be a weak event.

Let my heart be still for a moment and explore this mystery

La Niña is the cool phase of the El Niño/Southern Oscillation (ENSO), a pattern of alternating warmer (El Niño) and cooler surface waters in the tropical Pacific Ocean. Rising warm air in the tropics is the driving force behind global atmospheric circulation, and with it the jet stream, storm tracks and resulting temperature and rainfall patterns. ENSO’s varying sea surface temperatures change where the strongest rising air movement occurs, altering global atmospheric circulation. ENSO is predictable several months in advance, and coupling that predictability with our knowledge of how it is changing global patterns gives an early view of possible future weather and climate patterns.

Then this ebony bird tempted my sad imagination to smile

Let’s take a look at recent conditions in the tropical Pacific Ocean. The threshold for La Niña is a sea surface temperature in the Niño-3.4 region of the central equatorial Pacific Ocean that is equal to or more than 0.5°C below the long-term average. (Currently the long term is 1991–2020.) The most recent weekly measurement of the temperature difference from the average in the Niño-3.4 region was -0.3 °C, and the September average was also -0.3 ° c.

Hurricane Helene’s devastating impact on the Asheville, NC region impacted NOAA’s data center. See the footnote for details on our new data sources this month. Michelle and CPC’s Caihong Wen have conducted extensive testing and found that the temporary replacements are historically very close to the replacements we typically use, so we are confident that ENSO neutral conditions are still present.

There has been an area of ​​cooler than average water in the eastern central tropical Pacific in recent weeks, as you can see above, but it hasn’t quite crossed the threshold yet. Some aspects of the tropical atmosphere also still reflect neutrally. There was a region of stronger than average trade winds in the east-central tropical Pacific and some reduced rainfall in the central Pacific, but overall there was not a strong, clear pattern of stronger than average trade winds or increased rainfall. over Indonesia, as we would expect during an established La Niña. A few weeks of average to weaker than average trade winds in September even allowed the surface to warm up a bit.

And the Raven, never fluttering, still sits, still sit

If it seems like we’ve been in neutral here longer than we expected, that’s indeed the case! Last winter, models predicted that La Niña would develop rapidly after the end of El Niño in 2023-2024. So what happened? Why are we still waiting? Didn’t I just say that ENSO is predictable?

ENSO is predictable, but only in the big picture, i.e. seasonal averages or longer. The signals that tell us that El Niño or La Niña is approaching, such as a large amount of cooler or warmer water beneath the ocean’s surface, or a particularly strong, long-lasting change in the trade winds, are reliable indicators. Furthermore, our computer climate models, which look at current conditions and make predictions based on mathematical and physical equations, are quite good, especially after the spring barrier (a time of year when predictions are particularly difficult).

However, small short-term fluctuations, such as the weaker equatorial trade winds in September, cannot be predicted more than a few weeks (at best) in advance. They tend to have a disproportionate impact during borderline, more marginal situations where we hover near our ENSO thresholds. These small fluctuations can tip the scales one way or the other. In this case, they led to a slower and weaker La Niña development. That said, many of our models remain stable for La Niña to develop soon.

Once upon a time there was a gloomy midnight, while I was thinking, weak and weary

However, forecasters have downgraded the overall likelihood of La Niña developing slightly compared to last month. That means that while La Niña is still favored, we have less confidence in La Niña’s emergence.

If La Niña does develop, it is very likely to be a weak event, with a maximum between -0.9 and -0.5 °C, in line with model predictions. In the historical record beginning in 1950, only four La Niña events have occurred this late in the year, with two in September-November and two in October-December. These were all weak or borderline between weak and moderate. ENSO events are strongest in the winter, so there is less time for this La Niña to grow from where we are now.

The strength of an ENSO event, as measured by sea surface temperature anomalies, matters because stronger events change atmospheric circulation more consistently, leading to more consistent impacts on temperature, rainfall, and other patterns. A weaker event makes it more likely that other weather and climate phenomena could play the role of spoiler. But even a weak La Niña could impact the seasonal outlook because it can still give the global atmosphere a boost.

Presently my soul grew stronger; please don’t hesitate any longer

Next month, Nat will look at what the models have to say about winter conditions, including how well they reflect the expected impact of La Niña. Stay tuned to the ENSO Blog: we’re all spoiled, no tricks!

Footnote