From NOAA (Anthony Barnston):
Neutral ENSO conditions reigned during June and so far in July, as the sea surface temperature in the Niño3.4 region, often used to measure the El Niño/La Niña (ENSO) state, was between -0.5°C and 0.5°C away from average. ENSO-neutral means forecasters can relax a bit, and phones are no longer ringing off the hook now that this whale of an El Niño finally bit the dust.
However, we can’t rest too much. During the last 60 or so years, strong El Niño years have often been followed by La Niña years. There are known physical reasons for this, which we have covered (here and here). Although the record is short, it appears the stronger the El Niño, the greater the chance for a La Niña the next year. And we just had a pretty strong El Niño!
Sure enough, forecasters currently believe there is an approximately 50% chance that sea surface temperatures for the Niño3.4 region will drop to at least -0.5°C below average (qualifying as a La Niña condition) during the August–October season, and a 55-60% chance for La Niña for the fall. Although 55-60% is far from a sure thing, it means that getting some grade of La Niña, even if just a weak one, is more likely than not.
At the moment, there is a good supply of cooler-than-average water below the surface in the central and eastern tropical Pacific. This cold water has been making contact with the surface along portions of this vast stretch of ocean right along the equator, and the Niño-3.4 index dropped into weakly negative territory by late June as a result.
Weakened Probabilities for La Niña
However, the 55-60% chance for La Niña represents a noticeable drop in probability from last month, which had peak probabilities of about 75%. Why the decrease? A typical cop-out answer would be “The ENSO forecast models now show a decrease in the likelihood, and also in the peak La Niña strength, and we trust them.”
But why are they showing the decrease? What are the models seeing that the sea gull of average intelligence flying over one of the TAO or Argo buoys in the eastern tropical Pacific might not see? Or that some egghead scientist sitting on the 13th floor of an ivory tower at a high-powered university overlooking a green hill might miss in eyeballing a dozen busy-looking charts and graphs?
First, let’s probe the obvious, and look at the recent monthly rate of decrease of the ocean temperature in the Niño3.4 region since the peak of the 2015-16 El Niño, in comparison with what happened following other past El Niño events—in particular, the very strong 1997-98 and 1982-83 events. The figure below shows the result.
We can see that between May and June this year, the temperature decrease lost pace with that of 1998, after more closely paralleling it up through May. Note that a significant La Niña developed by late summer 1998. However, this year’s rate of decrease has been steeper, so far, than the one in 1983, and a weak La Niña condition even emerged in late 1983 (but it didn’t last long enough to make in on NOAA/CPC’s official list). So far, we’re on a trajectory this year that is somewhere in between those two cases.
But looking carefully at the behavior of 1998, 1983 and other significant post-El Niño cases like 1973, 1992, 1958 and 1988, we see that monthly fluctuations in behavior may not mean much in the grander scheme of things. Remember that ENSO usually acts more on a seasonal time scale than a monthly one, although ENSO-relevant events can occur within a single month from time to time, such as when the MJO interacts with ENSO. We may not have cooled at a very fast rate between May and June, but could easily make up for it in the coming months. So then, why are the models weakening their forecasts at this point in time?
One likely answer is the delay in the expected onset of La Niña-like ocean-atmosphere coupling in the eastern tropical Pacific Ocean. What is coupling? To set the stage, cooler than average water currently sits beneath the surface, and is now impacting the surface across the eastern Pacific Ocean, as seen in the figure below.
With below-average water temperature along the equator in the eastern tropical Pacific, we would expect some atmospheric coupling, such as enhanced trade winds that blow from the colder-than-average water in the east toward the warmer-than-average waters of the central and western Tropical Pacific. Sinking motion (higher surface pressure) tends to occur over anomalously cool water, with rising motion (lower pressure) over warm water. And surface winds blow from high toward low pressure. If the trade winds were to increase, they would cause upwelling of the even more anomalously cold waters from below the surface, which would then cause even stronger trade winds, and so on—the typical La Niña feedback loop. This is nothing more than a strengthening of the Walker circulation that typifies La Niña.
Enhanced Trades—Where Are You??
But the first nod toward above-average trade winds has not yet been observed, as we can see in the figure below. Trades are very close to average across most of the tropical Pacific.
To get a La Niña started, someone—the ocean or the atmosphere—has to make the first move. The ocean has done that—the ocean surface has cooled somewhat, and appears to be “waiting” for some stronger surface winds to upwell still colder water. To get the La Niña to grow, we need more upwelling. It is the atmosphere’s turn to play ball. Is it going to kick in or isn’t it?
Climate forecast models, which look at the latest observations, are beginning to wonder the same thing. Therefore, they recently have become less bullish on the La Niña—both for the rate of onset and the peak strength later in 2016. But they are not forecasting a cancellation of the event, so we need to keep our eyes peeled over the coming weeks and months to see if we get some action.