First, though, let’s bid La Niña adieu. The most recent weekly measurement of the sea surface temperature in the Niño-3.4 region (our primary monitoring region for La Niña and El Niño) was a mere -0.2°C (-0.4˚ F) compared to the long-term average. (To calculate a change in temperature in degrees Celsius to Fahrenheit, multiply by 1.8.) Also, the latest monthly value from the OISSTv2.1 dataset was -0.4°C (-0.7˚ F) for this same region. The threshold for La Niña is cooler than -0.5 °C, so we can say the ocean surface has moved away from La Niña.
Sea surface temperatures in the tropical Pacific Ocean from early January through early March 2023 compared to the long-term average. East of the International Dateline (180˚), waters remained cooler than average, a sign of La Niña. Graphic by Climate.gov, based on data from NOAA’s Environmental Visualization Lab. Description of historical baseline period.
The atmosphere is a little more complicated, as it tends to be! We still see some indications of a La Niña-like strengthened Walker circulation, with more rain and clouds than average over Indonesia, less over the central Pacific, and enhanced trade winds. However, without the cooler sea surface characteristic of La Niña, it’s likely this pattern will diminish in the coming weeks.
One measure of the atmospheric component of ENSO is the Equatorial Southern Oscillation Index (EQSOI), which measures the relationship between surface air pressure in the western and eastern Pacific. When this index is positive, it indicates that the Walker circulation is amped up. In February, after nearly a year at or above 1.0, the EQSOI was just 0.1 This tells us that at least one element of the atmospheric La Niña response has weakened.
We’re often going on and on here at the ENSO Blog about how ENSO is a seasonal phenomenon, meaning the ocean and atmosphere criteria must be met for several consecutive months in order to qualify as La Niña or El Niño. The same is not true for neutral conditions, however. Once the tropical Pacific ocean-atmosphere system is showing signs of decoupling, such as a monthly Niño-3.4 index value warmer than -0.5 °C, we can say that neutral conditions have likely arrived.
The forecaster consensus is indeed very confident that neutral conditions will remain through the spring. We know what you’re really interested in, though—will El Niño develop? If we can anticipate an El Niño, we can anticipate an increased likelihood of its impacts on weather and climate. In contrast, a continuation of neutral conditions means the tropical Pacific Ocean will not be an actor on the world’s climate stage. The lack of El Niño or La Niña means that there is no seasonal-scale influence from the Pacific to push around the global atmospheric circulation and influence seasonal climate patterns.
Many of our computer climate models are predicting a transition into El Niño sometime later this year. However, right now is a very tricky time of year for the models, due to the “spring predictability barrier.” ENSO events peak in the winter and tend to decay and transition in the spring, so models often don’t have a lot of strong signals to go on. (Why do ENSO events peak in the winter?? This is a really complicated topic that we don’t have a simple, satisfactory answer to… (yet!)
Maulucioni, CC BY-SA 4.0 , El_Niño_1982-83 via Wikimedia Commons