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

The research, led by Ingo Richter of the Japan Agency for Marine-Earth Science and Technology together with colleagues from institutions in Japan, the United States, Norway, and Brazil, rests on an unusually broad foundation of evidence. The team compared simulations of a pre-industrial climate with projections of a high-emissions future, known as scenario ssp585, drawn from 23 state-of-the-art climate models participating in the Coupled Model Intercomparison Project Phase 6, the same modeling framework that underpins the assessments of the Intergovernmental Panel on Climate Change. By contrasting these two worlds, one without human interference and one heated by sustained emissions, the researchers could isolate how the machinery of Atlantic variability itself changes when the planet warms.

The headline finding confirms a trend that earlier studies had already hinted at: the variability of sea-surface temperatures along the equatorial Atlantic, the signature of the Atlantic Zonal Mode, weakens under global warming. In plain terms, the Atlantic Niño becomes less intense, swinging less dramatically between warm and cold phases. Off the equator, by contrast, the picture is different, with sea-surface temperature variability tending to increase slightly in the flanking regions of the tropical Atlantic. That split result matters, because the equatorial mode is the one most tightly linked to the seasonal rains that farmers and water managers from Senegal to Angola depend upon, and its weakening has been flagged in previous work as a potential source of forecast uncertainty.

What makes the new analysis distinctive is its dissection of why the weakening occurs. The Atlantic Niño, like its Pacific counterpart, is sustained by the Bjerknes feedback, a self-reinforcing loop in which a warm anomaly in the eastern equatorial ocean weakens the prevailing easterly trade winds, which in turn suppresses the upwelling of cold subsurface water, further warming the surface. The study finds that this feedback is losing its grip. Contrary to some earlier projections, the team found that the mean-state equatorial Atlantic thermocline, the sharp vertical boundary between warm surface water and cold deep water, actually shoals slightly under radiative forcing, a change that should, on its own, strengthen the mode by making the surface more sensitive to subsurface dynamics. The decline of the Atlantic Niño therefore cannot be blamed on a deepening thermocline. Instead, the culprits are a weakening of the mean upwelling that supplies cold water to the surface and a reduced sensitivity of surface winds to sea-surface temperature anomalies, both of which blunt the Bjerknes feedback at its most critical links.

The second major discovery concerns the changing character of the mode’s forcing. In the pre-industrial simulations, the Atlantic Zonal Mode is driven primarily by dynamic forcing, meaning anomalies in surface wind stress that stir the ocean and rearrange its heat content. In the high-emissions future, the composites of Atlantic Niño events show that this dynamic pathway diminishes, while thermodynamic forcing takes on a more prominent role. Thermodynamic forcing operates through the exchange of heat at the ocean surface, particularly through latent heat flux, the energy carried away by evaporation, and through shortwave radiation, the sunlight that warms the upper ocean. In a warmer world, the Atlantic Niño becomes less a story of winds pushing water around and more a story of clouds, evaporation, and radiant heat reshaping the sea surface.

That shift in mechanism is tied to a striking change in the Atlantic’s relationship with the Pacific. The analysis reveals that the influence of the El Niño-Southern Oscillation on the Atlantic Zonal Mode strengthens under global warming, to the point that a positive correlation emerges between the two phenomena, with Pacific events preceding their Atlantic counterparts by roughly half a year. In today’s climate, the connection between El Niño and the Atlantic Niño has long been described as inconsistent and fragile, appearing in some decades and vanishing in others, a puzzle that has occupied tropical climate scientists for years. The new projections suggest that the greenhouse-warmed atmosphere will knit the two basins together more tightly, transmitting Pacific signals across Central America and the tropical atmosphere with greater reliability.

Two processes appear to explain this tightening bond. First, El Niño itself grows stronger in the high-emissions simulations, and a more powerful Pacific oscillator naturally broadcasts a louder signal into neighboring basins. Second, and perhaps more intriguingly, the weakening of the coupled air-sea feedbacks within the equatorial Atlantic leaves that ocean more susceptible to external forcing. A system whose internal feedbacks have gone quiet is one that listens more attentively to remote voices. When the Atlantic’s own Bjerknes feedback can no longer dominate its variability, the Pacific’s influence, arriving through atmospheric bridges such as shifts in the Walker circulation and changes in tropical tropospheric temperature, finds less resistance and leaves a clearer imprint on Atlantic sea-surface temperatures.

Paradoxically, this foreign domination may carry a silver lining for forecasters. A simple linear analysis performed by the team indicates that, in some of the models, the association of the Atlantic Zonal Mode with El Niño makes the Atlantic mode more predictable, even as its amplitude fades. The logic is straightforward: if a large fraction of Atlantic variability can be traced back to a Pacific precursor that emerges about six months in advance, then forecast systems that skillfully predict El Niño gain, for free, a measure of skill in predicting the Atlantic Niño. Seasonal prediction centers in Africa, Europe, and the Americas could eventually exploit this teleconnection to extend the useful lead time of rainfall outlooks for the Sahel, the Guinea Coast, and northeastern Brazil, regions where the Atlantic mode’s influence on the West African monsoon and coastal precipitation is well documented.

The study also underscores how much remains uncertain. The 23 models do not speak with one voice; the emergence of the ENSO-Atlantic correlation is clear in the ensemble but varies in strength from model to model, and tropical Atlantic simulations remain haunted by persistent mean-state biases that have plagued coupled models for decades. The researchers cross-checked their projections against observational and reanalysis products, including the ERA5 atmospheric reanalysis, the ORAS5 ocean reanalysis, and the HadISST sea-surface temperature dataset, all of which are publicly available, but the fundamental limits of simulating a basin as small and as seasonally locked as the equatorial Atlantic still apply. Whether the real ocean will follow the models’ script is a question that only the coming decades of observation can answer.

Still, the implications are hard to ignore. A tropical Atlantic that dances increasingly to the Pacific’s tune would alter the risk landscape for coastal fisheries, which are disrupted when the cold tongue fails to deliver its usual nutrient-rich upwelling, and for agricultural planners who rely on the statistical rhythms of Atlantic variability. It would also complicate attribution studies, since an Atlantic event that once looked like a local fluke may in the future be the distant echo of an El Niño half a world away. The research, published in Climate Dynamics as volume 64, article 450, offers both a warning and a tool: the Atlantic Niño of the future may be weaker and less self-reliant, but by borrowing predictability from the Pacific, it may also become a signal that humanity can see coming with more confidence than ever before.

Subject of Research: Projected changes in tropical Atlantic sea-surface temperature variability and its coupling to ENSO under global warming

Article Title: Strengthened tropical Pacific influence on tropical Atlantic variability in global warming projections

Article References: Richter, I., Chang, P., Kataoka, T., Kido, S., Keenlyside, N., Kosaka, Y., Okumura, Y., Tokinaga, H., Tozuka, T., & Vilela, I. (2026). Strengthened tropical Pacific influence on tropical Atlantic variability in global warming projections. Climate Dynamics, 64(11), Article 450. https://doi.org/10.1007/s00382-026-08390-y

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08390-y

Keywords: Atlantic Niño, Atlantic Zonal Mode, ENSO, CMIP6, Bjerknes feedback, tropical Atlantic, sea-surface temperature, climate change, thermocline, seasonal prediction, Walker circulation, Climate Dynamics

The technical approach behind the study is as important as its findings. The researchers combined satellite altimetry-based sea level anomalies from the Copernicus Marine Environment Monitoring Service with atmospheric and wave parameters from the ECMWF’s ERA-Interim reanalysis, storm surge outputs from the MOG2D-G barotropic model, and hourly astronomical tides from the FES30 global tide model. Wave run-up was modelled as a function of deep-water significant wave height, wavelength, and the subaerial coastal slope. Because these datasets arrived at different temporal resolutions, ranging from daily sea level fields to six-hourly surge and hourly tides, the team interpolated everything to a common hourly time step. That harmonisation matters: extreme coastal water levels are born from compound interactions that evolve on hourly scales, and a daily average would wash out precisely the peaks that flood homes and erode shorelines.

Once reconstructed, the ECWL record revealed a clear hierarchy of drivers. Tidal forces and wave run-up dominate the regional differences in extreme water levels, directly shaping coastal erosion and flooding, with Guinea-Bissau standing out as a hotspot because its wide, low-lying coastal plains amplify every centimetre of elevated water. Sea level anomalies, though small in mean terms at roughly 0.03 metres across countries, act as a slowly shifting baseline that reflects the cumulative state of the ocean and climate system, exacerbating the impacts of tides and waves when they coincide. The dynamic atmospheric contribution, essentially the storm surge signal driven by surface winds and pressure, remained comparatively minor, averaging around 0.02 metres across most of the coast.

The trend analysis added a sobering dimension. Sea level anomalies rose at an average of 3.08 millimetres per year along the coast, with local maxima reaching 4.23 millimetres per year, while total ECWL climbed at an average of 2.65 millimetres per year. Wave run-up, by contrast, declined slightly on average, at roughly 0.47 millimetres per year, though with substantial local variability. The dynamic atmospheric contribution was essentially flat. In other words, the steady rise in West Africa’s flood baseline is being driven primarily by the ocean itself, not by changing waves or winds, and that baseline keeps lifting the floor beneath every high tide and every swell event.

The spatial contrasts are striking. Mauritania and Senegal both show mean ECWL values above 2 metres with a consistent 98th percentile of 2.9 metres, indicating a stable but high extreme threshold. Guinea-Bissau records the highest 98th percentile in the region, far above its mean of 2.42 metres, signalling occasional very high extremes. Ghana presents perhaps the most dramatic gap: a mean of just 1.59 metres against a 98th percentile of 4.01 metres, meaning that when extremes arrive there, they arrive as sharp, exceptional spikes. Liberia shows the lowest exposure overall, while Nigeria sits in the middle with a mean near 1.57 metres and a 98th percentile of 2.6 metres.

Duration matters as much as height, and here the study quantified how long water stays dangerously high. Defining an extreme ECWL day as one exceeding the 98th percentile of a country’s distribution, the team found that Guinea-Bissau and Guinea logged the most extreme days, at 23 and 22 respectively in a single year, while Sierra Leone, Liberia, Togo, Benin, and Cameroon each recorded 15 to 17 days. Senegal, The Gambia, Ivory Coast, Ghana, and Nigeria hovered around 13 to 14 days. For communities in the Guinea-Bissau and Guinea cluster, flooding is not a brief shock but a prolonged condition, demanding continuous monitoring and rapid-response mechanisms rather than one-off defences.

Seasonality emerged as a second organising principle. Countries along the northwestern coast, including Mauritania, Senegal, The Gambia, Guinea-Bissau, and Guinea, see sea level anomalies peak around November at 0.10 to 0.14 metres, with tides peaking in September, while the lowest values arrive in May and June. Further south and east, the pattern shifts: Sierra Leone, Liberia, Ivory Coast, and Ghana peak earlier, with wave run-up reaching up to 1.33 metres in Ivory Coast in August, and Ghana recording run-up values as high as 1.44 metres. These rhythms track the migration of the Inter-Tropical Convergence Zone and the seasonal swell climate, in which long-period northwest swells and southwest trade-wind waves between 1 and 3 metres shape the coast from June to October.

To translate these patterns into policy, the researchers applied a k-means clustering algorithm to annual tide and wave run-up data, sorting the region’s countries into three hydrodynamic families. Cluster 1, Guinea-Bissau and Guinea, combines high tides of 0.88 and 0.79 metres with lower run-up, pointing to tidal barriers as the most logical investment. Cluster 2, spanning Liberia, Ivory Coast, Ghana, Togo, and Benin, features low tides between 0.26 and 0.34 metres but the region’s highest run-up values, from 1.05 to 1.25 metres, arguing for shoreline defences engineered against wave attack. Cluster 3, comprising Mauritania, Senegal, The Gambia, Sierra Leone, Nigeria, and Cameroon, shows a balanced interplay of moderate tides and run-up, calling for hybrid strategies. The clustering framework allows scarce adaptation resources to be matched to the actual physics of each coastline rather than applied uniformly.

Perhaps the most consequential finding concerns climate teleconnections. When the team correlated ECWL anomalies with major climate indices, the Atlantic Niño emerged as the clear regional heavyweight, with a moderate positive correlation of 0.5 and spatially coherent influence along the central and eastern coastline. The mechanism is physically intuitive: warm sea surface temperature anomalies in the tropical Atlantic alter the land-sea thermal gradient, strengthening the West African Monsoon and shifting the Intertropical Convergence Zone northward, which boosts rainfall, river discharge, and ultimately coastal water levels. By contrast, ENSO showed only a weak correlation of 0.15, the North Atlantic Oscillation a weak negative one of −0.17, and the Southern Annular Mode a negligible −0.04. Spatial correlation maps confirmed the pattern, with the Atlantic Niño reaching correlations of 0.49 in its warm phase and −0.47 in its cool phase, while ENSO, SAM, and NAO produced inconsistent, patchy signals across the coast.

The authors are careful to note that their zero-lag Pearson correlations indicate relative climatic influence rather than predictive or causal relationships, and that non-linear or lagged effects remain unexplored. Even so, the practical message is hard to ignore: flood prediction systems for West Africa should be tuned to the Atlantic Niño, a regional index, rather than leaning on global modes like ENSO that dominate headlines elsewhere. With urbanisation rates in countries such as Ghana, Nigeria, and Côte d’Ivoire projected to approach 70 percent by 2050, and sea level rise steadily raising the baseline beneath every tide, the study offers something the region has lacked, a physics-based, country-by-country blueprint for where the water comes from, when it rises, and what kind of defence actually fits each shore.

Subject of Research: Drivers of extreme coastal water levels and coastal flooding in West Africa

Article Title: Understanding the drivers of Extreme Coastal Water Levels in West Africa from oceanic and climatic processes

Article References: Brempong, E. K., Almar, R., Angnuureng, D. B., Abessolo, G. O., Dada, O. A., & Cissé, C. O. T. (2026). Understanding the drivers of Extreme Coastal Water Levels in West Africa from oceanic and climatic processes. Discover Oceans, 3(1), Article 23. https://doi.org/10.1007/s44289-026-00136-2

Image Credits: AI Generated

DOI: 10.1007/s44289-026-00136-2

Keywords: West Africa, extreme coastal water levels, Atlantic Niño, sea level rise, coastal flooding, wave run-up, tides, storm surge, ENSO, Guinea-Bissau, coastal erosion, flood management