The last three years in the Pacific region have been dominated by La Niña, the cold sister of El Niño. Both together are part of ENSO, the El Niño Southern Oscillation. It swings back and forth between these two extreme states in multi-year cycles. Now the pendulum is beginning to swing from cold back toward warm.
The signs point to El Niño
A month ago, there were already first signs of warming in the eastern Pacific off Peru and Ecuador, and this development has since continued and accelerated. The temperature deviation of the surface water here is only already 4 to 5 degrees. In the middle of the equatorial Pacific, the last remnants of cold water from the past three La Niña years are still visible.
Fig. 1: Deviation of sea surface temperature from the norm in the Pacific Ocean; Source: NOAA
According to current calculations, the signs are now clearly pointing to El Niño. The warm water anomaly will continue to spread westward into the central Pacific in the coming months, and it looks like a strong El Niño event for the end of the year.
Record global sea surface temperature
But it's not just in the Pacific that sea surface temperature trends are interesting, but also on a global scale. Currently, we are at an absolute record level in a comparison of the years from 1981 onwards; in large areas, the sea water in the uppermost layer is clearly too warm. The previous peak in this respect was the year 2016, but at that time there was also a strong El Niño at the beginning of the year. But this is not yet the case, and nevertheless the current sea surface temperature (SST) between 60° south and 60° north has already exceeded that of 2016 for several days! This lead is expected to increase as El Niño picks up speed. Higher water temperatures also mean more evaporation and more water vapor in the atmosphere.
Fig. 2: Course of the mean global sea surface temperature for the years 1981-2023; Source: NOAA, University of Maine
Fig. 3: Current global deviation of sea surface temperature from the norm; Source: NOAA
Traces of record-breaking Cyclone Freddy
Cyclone Freddy left its mark on the weather statistics and broke several records. In addition to the devastation on land and the resulting humanitarian catastrophe, its traces are still visible in the form of an anomaly in sea surface temperature. Between Madagascar and Southeast Africa, the water is currently colder than usual. This is due to Freddy, which stayed here for many days and mixed the water due to the high wind speeds. Cooler water from deeper layers was able to reach the surface.
Fig. 4: Deviation of sea surface temperature from the norm in the Indian Ocean; Source: NOAA
La Niña goes, El Niño comes
During La Niña, water temperature at the surface of the equatorial Pacific is below average, and warm water is displaced into the western Pacific. ENSO has numerous effects on weather phenomena around the globe (teleconnection); for example, the Atlantic hurricane season is often more active than normal in La Niña years. In addition, the very large cool water surface has a dampening effect on the global mean temperature; this cooling effect is on the order of -0.1 degrees. That doesn't sound like much, but it's actually critically important.
Fig. 1: Weekly anomaly in surface water temperature in the Pacific Ocean. Recent onset of warming in the East Pacific.; Source: NOAA
2023 still started with La Niña, even currently the equatorial Pacific is colder at the surface than the long-term average. The first computer models had been hinting at a change for some time, but the trend is now clear. Practically all relevant models calculate an end of La Niña in the next weeks and after a neutral phase during the later spring as well as the summer a transition to El Niño. In fact, the observational data now also show clear signs of the change in the eastern Pacific; in the last two weeks there has been a warming of the surface water off the coast of Peru and Ecuador – the deviation from the norm is now already positive here.
Fig. 2: Current ENSO forecasts of the various computer models for the coming months. ; Source: IRI
It is therefore to be expected that in the course of March larger and larger parts of the equatorial Pacific will warm up and this unusually long La Niña phase will end. During late summer and fall, ENSO swings to the warm side and El Niño sets in. From the current perspective, this occurs close in time to the peak of the Atlantic hurricane season. However, it is not yet possible to say how strong the effects of this will be. In any case, the global cooling effect now ceases and turns into the opposite. El Niño years are on average warmer than average on a global scale. Models see a moderate to strong El Niño by the end of the year and for 2024, and the effects will most likely be felt at the global level. According to a joint analysis by the Potsdam Institute for Climate Impact Research (PIK), the Justus Liebig University in Giessen and two Chinese universities, the scientists expect a significant increase in the global mean temperature, especially for 2024. As a result, there is a high probability that 2024 could become the warmest year since records began. According to this study, the global mean temperature could even be 1.5 degrees above the pre-industrial mean for the first time. It will be exciting to see if this prediction comes true. But also disturbing. Either way, we're experiencing it first row footed!