After the Pacific, a possible Niña Atlantic

The Pacific Ocean is switching from an El Niño event at the beginning of the year 2024 to a La Niña event expected at the end of this summer. But it seems, for NOAA scientists, that a similar phenomenon is also occurring at this same time in the Atlantic Ocean (¹).

Observation

First of all, it was observed that a large part of the North Atlantic has been extremely warm since the start of the year 2024.

On the other hand, since the beginning of June, sea ​​surface temperature (SST) in the central equatorial Atlantic was 0.5° to 1.0° Celsius colder than average for this time of year. If these cold conditions persist until the end of August, a phenomenon known as Atlantic Niña could occur. But what is the Atlantic Niña and what influence on the weather and climate around the tropical Atlantic? ?

What is La Niña (and El Niño) Atlantic ?

The Atlantic Niña is the cold phase of a natural climatic phenomenon called the Atlantic Zonal Mode. (Zonal meaning “along lines of latitude”). This phenomenon, just like the’ENSO (El Niño-Southern Oscillation), oscillates between cold and warm phases every few years. Generally, sea ​​surface temperatures in the eastern equatorial Atlantic present a rather surprising seasonal cycle : the warmest waters of the year occur in spring, while the coldest waters of the year — less than 25° Celsius — occur from July to August.

This summer cooling is due to the winds which act on the surface of the ocean. Earth has a year-round belt of precipitation around the tropics. Under the effect of greater solar heating, this belt of precipitation migrates northward during summer in the northern hemisphere. Regular rainstorms draw air from the southeast over the equatorial Atlantic. These regular southeasterly winds are strong enough to push surface water away from the equator, which brings to the surface the relatively cold waters of the deep layers of the ocean. This process, known as equatorial water upwelling (upwelling), forms a tongue of relatively cold water along the equatorial Atlantic during the summer months.

But for several years, this cold tongue is significantly warmer or colder than average due to fluctuations in the Atlantic zonal mode. Cold events are called “ Atlantic Niñas ” and hot events “ Atlantic Niños ”. In general, these sea surface temperature anomalies in the key monitoring area (black frame in illustrations), averaged over three months, must exceed ± 0.5° Celsius for at least two overlapping seasons to be able to speak of an Atlantic Niño or Niña.

From the beginning 2024 to August 2024

The year 2024 started with sea surface temperatures (SST) extremely hot in the eastern equatorial Atlantic in February and March, with temperatures exceeding 30° Celsius. This is the most significant heat episode since 1982. The rapid transition between warm and cold sea surface temperature anomalies was equally remarkable. Never before in observed records, the eastern equatorial Atlantic had never switched so quickly from one extreme phenomenon to another.

However, in a surprising way, cold anomalies observed in the eastern equatorial Atlantic in June/July 2024 coincide with a weakening of the southeast trade winds near the equator. Slackening equatorial winds is generally associated with a reduction in upwelling and warm anomalies, which was obviously not the case.

It is only south of latitude 5 degrees South that the southeast trade winds were stronger than usual. From now on, these atmosphere-ocean conditions, apparently unfavorable to the development of the Atlantic Niña, leave us perplexed. We will have to dig deeper to know the exact causes of this apparently unusual event..

Plus or minus 0.5°, what does it matter ?

One might think that a temperature gap of ±0.5° Celsius in the tropical Atlantic is not very important. In reality this difference may have a considerable impact on precipitation on surrounding continents. The decrease in precipitation in the Sahel region, increased precipitation in the Gulf of Guinea and the shift of the rainy season to northeastern South America, all these events have been attributed to the Atlantic Niño phenomenon.

Furthermore, Atlantic Niños have been shown to increase the likelihood of strong cyclones developing near the Cape Verde Islands. NOAA's seasonal forecast for above-normal hurricane activity in 2024 are based in part on the La Niña conditions expected in the equatorial Pacific and on high ocean temperatures in the tropical North Atlantic (²). It will be interesting to observe if this Atlantic Niña fully develops and, if applicable, whether it has a moderating effect on hurricane activity as the season progresses

NOAA will be monitoring this event in the coming weeks and will publish a follow-up article later this month to let you know if an Atlantic Niña has fully developed.. The agency will also examine some of the hypotheses put forward by scientists to explain how these phenomena trigger and how human-caused global warming could affect their frequency over the coming century.. Stay tuned !

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(¹) Source : NOAA
(²) After El Niño 2023 here is La Niña 2024
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