La NiƱa continues in the tropical Pacific, with both the ocean and atmosphere clearly reflecting La NiƱa conditions. The current forecast favors the continuation of La NiƱa through the summer (59% chance), with a slightly lower chance into the fall (50-55% chance). A third-year La NiƱa would be pretty unusualāweāve only seen two others since 1950. Iāll run the numbers to see how current conditions add up and whatās factoring into the odds for La NiƱa later this year.
Paint by numbers
Letās take stock of current ENSO conditions (ENSO=El NiƱo/Southern Oscillation, the whole ocean-atmosphere El NiƱo/La NiƱa system) in the tropical Pacific. In March, the sea surface temperature in the key ENSO monitoring region (NiƱo-3.4) was still well within the La NiƱa range, about 1.0 Ā°C cooler than the long-term (1991-2020) average, based our most reliable historical record, ERSSTv5. Remember: the La NiƱa threshold is a temperature anomalyāa difference from the long-term averageāin the NiƱo-3.4 region of -0.5 Ā°C or lower. March 2022 was the 6th most negative March sea surface temperature anomaly in NiƱo-3.4 since 1950.
As you can see from the graph, March 2022 was also tied for the coldest of the nine second-year La NiƱa events on record, for this time of year.
One plus one
While the ocean surface temperature confidently indicates that La NiƱa is still going strong, itās not unusual for this time of year, and wouldnāt necessarily tell us much about how long this La NiƱa might last. Things start to get interesting when we look at the atmosphere, though, providing a bit more insight into why forecasters are favoring La NiƱa to continue through the summer.
ENSO Blog frequent flyers will know that ENSO is a coupled ocean-atmosphere system. Changes in the tropical Pacific Ocean temperature influence the circulation of the atmosphere (the Walker circulation); those atmospheric changes in turn affect the ocean temperature, and so on. For example, La NiƱa features cooler-than-average surface water in the central and eastern equatorial Pacific and warmer-than-average surface water in the far western Pacific. This cool-versus-warm pattern leads to less rising air and storms over the central Pacific and more over Indonesia, amping up the normal Walker circulation and driving stronger near-surface winds along the equatorāthe trade winds. Stronger trade winds further cool the surface and keep even more warm water piled up in the far western Pacific, reinforcing La NiƱaā¦ you see where Iām going! For more details, visit Michelleās post on the mechanics of ENSO.
Baby, donāt lose my number
Enough preambleāwhatās the Walker circulation doing right now? I thought youād never ask. Itās really feeling its oats these days, as several different atmospheric measurements tell us.
First, letās talk Equatorial Southern Oscillation, an index that measures the relative sea level pressure in the far western Pacific vs. that in the eastern Pacific. When the EQSOI is positive, it indicates lower-than-average pressure over the west (more rain and clouds) and higher-than-average pressure over the east (less rain and clouds), i.e., evidence of a stronger Walker circulation. In March, the EQSOI measured 1.4, the 6th strongest since 1950.
As I mentioned above, stronger trade winds are key to the La NiƱa feedback between the ocean and atmosphere. The trade winds were enhanced through March, and remain stronger than average into mid-April. You want a number, you say? Okay! Thereās an index that measures the near-surface winds in the central Pacific region of 5Ā°Nā5Ā°S, 175Ā°Wā140Ā°W; it was 4.3 meters per second (9.6 miles per hour) faster than average in March. This is the strongest March value on record, but thereās a catchāthis record only goes back to 1979.
One more index! The central Pacific was much less cloudy and rainy than average in March. We monitor cloudiness via satellite, by looking at how much radiation is leaving the Earthās surface and reaching the satellites. Less radiation making it to the satellite means more clouds are blocking the path.
The index that measures outgoing radiation (and therefore cloudiness), the CPOLR, tells us that this March featured the least amount of clouds for any March on record over the central Pacific. Weāre number 1! Again, though, like the winds, this record only goes back to 1979, when the satellite measurement era began. So, a grain of salt with your records.
One last measurement todayāletās look under the surface of the tropical Pacific Ocean. The amount of cooler-than-average water under the surface increased in March. This cooler subsurface water provides a supply of cooler water to the surface, contributing to ENSO forecastersā prediction that La NiƱa will remain into the summer. Index-wise, last month the water under the surface was the 9th coolest March since 1979.
Add it all up
There are two main information sources for ENSO forecasters: current atmosphere-ocean conditions and computer model predictions. Computer models have a harder time making successful long-range predictions in April, during the spring predictability barrier, although they remain a critical tool. Current model predictions are mostly split between staying in La NiƱa or transitioning to neutral in summer. Looking out to next fall, the North American Multi-Model Ensemble is leaning toward La NiƱa conditions.
There is a lot of uncertainty in the current forecast, which is reflected in the probabilities. The odds for La NiƱa to remain through the next few months are fairly confident, bolstered by the cooler subsurface water and the current strong Walker circulation. The chance of a third-year La NiƱa has a slight edge for the fall, over the chance of neutral conditions. El NiƱo is unlikelyāless than 10% chance. None of the models are predicting El NiƱo for the fall, and none of those March atmospheric conditions I described earlier have been followed by El NiƱo later in the year. Itās not impossibleānature is full of surprisesābut very unlikely.
La NiƱa influences the hurricane season (more storms in the Atlantic, fewer in the eastern Pacific), has links to springtime tornado activity (complicated links!), and can increase the chance of drought in some regions. Given all these important relationships, we will closely watch the forecast and look forward to starting to emerge from the spring predictability barrier in the months ahead.
This post first appeared on the climate.gov ENSO blog and was written by Emily Becker.