Vanishing plankton may be quieting the Atlantic’s own El Niño

In the vast machinery of Earth’s climate system, some of the most consequential gears are barely visible to the naked eye. Phytoplankton, the microscopic drifting plants that anchor the marine food web, have long been treated by physical oceanographers as a passive backdrop to the drama of winds and currents. A new modeling study published in the journal Ocean Science turns that assumption on its head, showing that the amount of chlorophyll a, the pigment that phytoplankton use to harvest sunlight, can measurably reshape the temperature structure of the equatorial Atlantic Ocean and even modulate the strength of its most important year-to-year climate fluctuation, the Atlantic Niño.

The research, led by Arthur Prigent of the Abdus Salam International Centre for Theoretical Physics in Trieste, now at the University of Brest, together with Riccardo Farneti, Manfredi Manizza, and Rodrigue Anicet Imbol Koungue, used a suite of ocean model simulations in which the monthly climatology of chlorophyll a was multiplied by scaling factors ranging from 0.01 to 2. The control simulation, based on a satellite-derived climatology from SeaWiFS imagery, represented realistic ocean color. The most extreme sensitivity experiment, dubbed the clear-ocean run, effectively stripped the water of its phytoplankton, allowing sunlight to penetrate far deeper than it would in the green, plankton-rich waters of the tropical Atlantic upwelling system.

The physics at play hinges on where solar energy is deposited in the water column. In the model, incoming shortwave radiation is split into three wavelength bands: infrared, red visible, and blue-green visible light. Infrared is absorbed within centimeters regardless of biology, and red light penetrates only about four meters in all experiments. But blue-green light, which carries roughly a fifth of the surface energy, is strongly attenuated by chlorophyll. When chlorophyll is present, its e-folding penetration depth in the ATL3 region, the eastern equatorial Atlantic box where Atlantic Niños live, hovers around 14 meters in the doubled-chlorophyll run. In the clear-ocean run it stretches to about 40 meters, meaning a substantial fraction of solar heating bypasses the surface layer entirely and warms the subsurface instead.

Counterintuitively, removing the plankton warmed the eastern equatorial Atlantic sea surface rather than cooling it. Relative to the control run, the clear-ocean simulation produced a statistically significant warming of 0.15 degrees Celsius in the ATL3 region, along with surface warming along the Angolan and Namibian coasts and in the Senegalo-Mauritanian upwelling system. The subsurface response was even more dramatic: temperatures within about 50 meters of the surface, near the mean depth of the main thermocline, warmed by more than 2 degrees Celsius. This deep heating weakened the vertical temperature gradient around the thermocline, deepened both the thermocline and the mixed layer, and ultimately suppressed the wind-driven upwelling of cold water that normally cools the equatorial surface.

The mechanism the authors untangle is a subtle one rooted in the momentum budget of the mixed layer. Trade winds push surface water poleward away from the equator through Ekman transport, and this divergence is normally partly offset by an equatorward geostrophic transport driven by the zonal slope of the sea surface. Because both simulations were forced with identical atmospheric conditions, the reduced poleward transport in the clear-ocean run could not be blamed on weaker winds or a flatter sea surface slope. Instead, the culprit was the deepened mixed layer itself. A deeper mixed layer integrates a stronger compensating geostrophic transport, which cancels more of the wind-driven divergence, weakens the net meridional transport away from the equator, and thereby reduces the upwelling of cold subsurface water into the surface layer.

These mean-state changes rippled into the ocean’s variability on multiple timescales. The amplitude of the seasonal sea surface temperature cycle in the ATL3 region, which swings from above 28 degrees Celsius in February through April down to 25 degrees or less when the Atlantic cold tongue develops in July through September, shrank by 14 percent in the clear-ocean run, from 3.42 to 2.93 degrees Celsius. Most of that reduction came from warmer conditions during the cold-tongue season, precisely when chlorophyll concentrations in the region naturally peak. The finding highlights how tightly the biology and physics of this upwelling system are interwoven: the very season when plankton bloom most vigorously is the season when their optical influence on the cold tongue is strongest.

More striking still was the response of interannual variability. The Atlantic Niño, the tropical Atlantic’s analogue of the Pacific El Niño-Southern Oscillation, involves a coupled feedback in which sea surface temperature anomalies, zonal wind stress, and ocean heat content reinforce one another. These warm and cold events, concentrated in May through July with a secondary peak in November and December, steer rainfall over West Africa, influence the Indian monsoon, modulate the sea-air exchange of carbon dioxide, and stress marine ecosystems. In the clear-ocean simulation, the standard deviation of ATL3 sea surface temperature anomalies dropped from 0.31 to 0.27 degrees Celsius, a statistically significant reduction of 12.9 percent, with the largest suppression, 13.47 percent, occurring in the May through July peak season of the Atlantic Niño.

The subsurface story proved equally important. Temperature variability in the upper 150 meters of the equatorial Atlantic was also significantly reduced in the low-chlorophyll run. Because all simulations shared identical winds, thermocline depth variations were similar across experiments and could not explain the difference. Instead, the weakened vertical temperature gradient around the thermocline meant that the same vertical displacements of water produced smaller temperature anomalies, while the weakened upwelling further muted the communication between subsurface and surface. Notably, the response across the full range of experiments was nonlinear: a quadratic fit explained 98 percent of the variance in the seasonal-cycle peak of variability. Halving chlorophyll reduced variability by only 3.27 percent, while increasing it by 50 and 100 percent raised variability by just 2.29 and 3.76 percent. Variability falls faster under declining plankton than it rises under growing plankton, an asymmetry with real-world implications.

Those implications stem from what satellites are actually observing. Analyzing the Copernicus-GlobColour merged satellite product from 1998 to 2024, the team found a marked decline in chlorophyll a over the eastern equatorial Atlantic, driven mainly by shrinking yearly maxima during the July through September bloom season, while the minima held steady. This decline echoes recent independent reports of decreasing ocean greenness and net primary production in low and mid-latitude waters. Because the study’s experiments show that lower chlorophyll weakens sea surface temperature variability, the authors suggest the observed biological decline may have contributed to the weakening of tropical Atlantic sea surface temperature variability documented since the year 2000, a puzzle that has intrigued climate scientists for years.

The study comes with honest caveats. The ocean model was forced by a prescribed atmosphere rather than fully coupled to one, which likely dampens the magnitude of the response, although previous work in the tropical Pacific suggests the sign of the effect is robust in ocean-only frameworks. And because no biogeochemical model was coupled to the ocean, the experiments could not capture the feedback loop in which warm events suppress nutrient supply and plankton, which in turn alters light penetration and damps the temperature anomaly itself, a damping mechanism demonstrated for the Pacific and likely active in the Atlantic as well. The authors also flag the neighboring Benguela and Dakar Niño regions, where chlorophyll is likewise declining, as priorities for future coupled ocean-atmosphere-biogeochemistry simulations. What is already clear, however, is that the ocean’s color is not cosmetic. The microscopic life tinting the tropical Atlantic green appears to help set the amplitude of one of the planet’s major climate rhythms, and as the water clears, that rhythm may be quietly fading.

Subject of Research: Effects of chlorophyll a concentration on the mean state and interannual sea surface temperature variability of the equatorial Atlantic Ocean

Article Title: Chlorophyll a concentration effects on equatorial Atlantic Ocean mean-state and interannual variability

Article References: Prigent, A., Farneti, R., Manizza, M., & Imbol Koungue, R. A. (2026). Chlorophyll a concentration effects on equatorial Atlantic Ocean mean-state and interannual variability. Ocean Science, 22(5), 2973-2991. https://doi.org/10.5194/os-22-2973-2026

Image Credits: AI Generated

DOI: 10.5194/os-22-2973-2026

Keywords: chlorophyll a, phytoplankton, Atlantic Niño, equatorial Atlantic, sea surface temperature, ocean modeling, upwelling, mixed layer, thermocline, ocean color, climate variability, biophysical feedback