The tropical Pacific experienced 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 chance of La Niña is slightly lower than last month and it is expected to 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. Rising warm air in the tropics drives global atmospheric circulation and therefore the jet stream, storm tracks and resulting temperature and rainfall patterns. The fluctuating sea surface temperature of ENSO changes where the strongest rising air movement occurs, thus altering global atmospheric circulation. ENSO is predictable several months in advance, and combining this predictability with our knowledge of how it alters global patterns gives an early picture of possible upcoming weather and climate patterns.

Then this ebony bird tempted my sad imagination to smile

Let’s take a look at the current conditions in the tropical Pacific. The threshold for La Niña is a sea surface temperature in the Niño 3.4 region of the central equatorial Pacific that is at least 0.5 °C below the long-term average. (Currently, the long-term period is 1991-2020.) The last 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 also impacted NOAA’s data center – see footnote for details on our new data sources this month. CPC’s Michelle and Caihong Wen conducted extensive testing and found that the temporary replacement devices in the past were very similar to the ones we normally use. We are therefore confident that ENSO-neutral conditions still apply.

As you can see above, there has been a region of cooler-than-average water in the central-eastern tropical Pacific in recent weeks, but it’s not quite crossing the line yet. Some aspects of the tropical atmosphere are still reflected neutrally. There was a region of above-average trade winds in the east-central Tropical Pacific and slightly reduced precipitation over the central Pacific, but overall there was no strong, distinct pattern of above-average trade winds or increased precipitation over Indonesia as we would expect during an established La Niña . In fact, a few weeks of average to weaker trade winds in September caused the surface to warm slightly.

And the raven that never flutters still sits, Despite it sits

If it seems like we’ve been stuck in neutral here for longer than we expected – that’s because it is! Last winter, models predicted that La Niña would develop rapidly after El Niño ended in 2023-24. So what happened? Why are we still waiting? Didn’t I just say that ENSO is predictable?

ENSO is predictable, but only broadly, i.e. at or above the seasonal average. Reliable indicators are the signals that tell us that El Niño or La Niña is imminent, such as a large amount of cooler or warmer water beneath the ocean surface or a particularly strong and long-lasting shift in the trade winds. Additionally, our computer climate models, which take current conditions into account and make predictions based on mathematical and physical equations, are quite good, especially after the spring season (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 in advance (at best). In borderline situations where we are close to our ENSO limits, they tend to have a disproportionate impact. These small fluctuations can tip the balance in one direction or the other. In this case, they resulted in a slower and weaker La Niña development. Nevertheless, many of our models maintain that La Niña will evolve soon.

Once upon a time there was a dreary midnight as I pondered, weak and weary

However, forecasters have estimated the overall likelihood of La Niña to develop slightly lower compared to last month. This means that while La Niña is still favored, we are less confident that La Niña will emerge.

Additionally, should La Niña form, it is very likely that it will 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 formed this late in the year, two in September-November and two in October-December. These were all either weak or on the borderline between weak and moderate. ENSO events are strongest in winter, so there is less time for this La Niña to develop from where we are now.

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

Now my soul became stronger; Then I won’t hesitate any longer

Next month, Nat will review what the models have to say about winter conditions, including the extent to which they reflect the expected impacts of La Niña. Stay up to date here on the ENSO blog – we’re all a treat, not a trick!

footnote