Weather
The El Niño of 2023–24 is weakening. Forecasters estimate an 85% chance that El Niño will end and the tropical Pacific will transition to neutral conditions by the April–June period. There’s a 60% chance that La Niña will develop by June–August. Overall, the forecast this month is very similar to last month, and we continue to expect La Niña for the Northern Hemisphere fall and early winter (around 85% chance).
La Niña and El Niño are opposite phases of the El Niño-Southern Oscillation climate pattern. “ENSO” for short. Just like El Niño, La Niña changes the ocean and atmospheric circulation in the tropics. Those changes start in the Pacific Ocean and then ripple around the world in predictable ways. So, the arrival of La Niña gives us an early picture of potential upcoming climate conditions.
Why are our probabilities relatively high, even though we’re still solidly in the grip of the “spring predictability barrier,” a time of year when forecasts are often trickier? What could La Niña mean for summer and fall climate? And what might we expect for the global average surface temperature, after a record-setting year? So many questions! The hooks are baited, let’s cast our lines.
Tropical fishes
First things first: current ENSO conditions. The sea surface temperature anomaly in the Niño-3.4 region of the tropical Pacific is our primary metric for ENSO (anomaly = departure from the long-term average, long-term in this case is 1991–2020). Since El Niño’s peak in November–December 2023 at about 2.0 °C (3.6 °F), this anomaly has been dropping steadily, but, at 1.2 °C, it is still well above the El Niño threshold of 0.5 °C (0.9 °F).
Looking at the atmosphere over the tropical Pacific, however, we find that the expected El Niño pattern—weaker-than-average trade winds, more rain and clouds in the central tropical Pacific, drier conditions over Indonesia, reflecting a weaker Walker Circulation—has largely disappeared. This is not unexpected; as ENSO events decay, sometimes the atmosphere and the ocean are on somewhat different schedules. (This is also the case when they begin.) What it tells us is that the ocean-atmosphere coupling, an essential component of ENSO, has likely ended. That provides confidence that the warm sea surface temperature anomaly will continue to diminish, likely crossing into neutral (between 0.5 and -0.5 °C) by April–June.
Creatures of the deep
More evidence that El Niño is likely to give way to neutral soon, with La Niña right on its tail, can be found under the surface of the tropical Pacific. We keep a close eye on the temperature of the water in the upper 300 meters (~1000 feet) of the equatorial Pacific because this water provides a source to the surface. Since January, two upwelling Kelvin waves—blobs of cooler water that travel from the west to the east under the surface—have been moving through.
The more recent upwelling Kelvin wave will continue to shift eastward and rise up, providing a source of cooler-than-average water to the surface.
Sailfish
As I mentioned above, La Niña causes changes in global atmospheric circulation, making certain temperature and rainfall patterns more likely. We’ll dig into this a bit more after El Niño ends, but one potential La Niña impact has been getting some notice recently: La Niña tends to encourage a more active Atlantic hurricane season. It does this by reducing vertical wind shear—the change in wind from near the surface to high up in the atmosphere—over the Atlantic Ocean, making it easier for hurricanes to grow. Considering that the tropical Atlantic Ocean is already very warm, you can bet that NOAA’s hurricane outlook team is paying close attention to the likelihood of La Niña. NOAA’s early seasonal hurricane outlook will come out next month, and we’ll have a post about hurricanes on the ENSO Blog in June.
Shark tank
Speaking of the bathwater Atlantic, let’s revisit the topic of the global average surface temperature. This metric isn’t particularly relevant to anyone’s day-to-day operation—when’s the last time you woke up in the morning and thought “I’ll just check the global mean surface temperature forecast for today!”—but it’s a critical monitoring tool for climate change.
El Niño’s warmer-than-average tropical Pacific tends to contribute to higher global average surface temperature, while La Niña’s cooler tropical Pacific usually contributes to relatively cooler years. However, emphasis is on the relative since more recent La Niña events have been among the top ten warmest years ever. One can see that much of the global oceans are warmer than average, going beyond El Niño.
Like with ENSO, we track the global surface temperature anomaly as the departure from the long-term average. Unlike ENSO, a few different “long-term” base periods are used by different researchers and in different situations, including 1991–2020 (recent normal), 1901–2000 (the 20th century), and 1850–1900 (the pre-industrial era). However, so long as you pay attention to which base period is being used, the message is still the same—the global average temperature anomaly is breaking records.
According to NOAA’s National Center for Environmental Information, “the February global surface temperature was 2.52 °F (1.40 °C) above the 20th-century average of 53.8 °F (12.1 °C), making it the warmest February on record [dating back to 1850] and the ninth consecutive month of record-high global temperatures.”
Could a developing La Niña return the global average surface temperature closer to normal? Not very likely. We are just a few months in, and NCEI’s Global Annual Temperature Outlook already predicts “a 45% chance that 2024 will rank as the warmest year on record and a 99% chance that it will rank in the top five.” For more info on how NCEI makes this prediction, check out this post.
The forecast from the North American Multi-Model Ensemble (NMME), a collection of state-of-the-art climate models from U.S. and Canadian centers, predicts only a slight reduction in the global surface temperature anomaly over the next several months. Note that the NMME prediction uses a base period of 1850–1900 to provide an estimate of the increase in global temperature over “pre-industrial” times.
It could be another very interesting year, climate-wise. Stay tuna-ed for more from us on ENSO and global climate!
