📄 earth science

Mesoscale eddies contribution to nuisance flooding in the tropical island of Guadeloupe

This study demonstrates that mesoscale anticyclonic eddies significantly contributed to a nuisance flooding event in Guadeloupe in 2022 by elevating coastal sea levels by over 10 cm, highlighting their role as a critical and previously underappreciated driver of coastal flooding.

Original authors: Rémy Asselot, Rémi Thieblemont, Deborah Idier, Thibault Laigre

Published 2026-09-24

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CC BY 4.0

Original authors: Rémy Asselot, Rémi Thieblemont, Deborah Idier, Thibault Laigre

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Rising seas are a slow-moving crisis for coastal communities, but the immediate threat often comes from a different kind of water: the nuisance flood. These are the days when the ocean simply laps over the curb or seeps into a basement, not because of a storm or a hurricane, but during a calm high tide. As the global ocean warms and expands, these quiet intrusions are becoming more frequent, turning daily life into a struggle against water that shouldn't be there. For decades, scientists have understood that tides, long-term sea level rise, and weather patterns drive these events. However, a new layer of complexity has emerged in the tropical waters of the Caribbean, where the ocean itself is churning with massive, rotating currents that behave like invisible weather systems. These swirling bodies of water, known as mesoscale eddies, are vast enough to cover entire islands, yet they often go unnoticed by those standing on the shore. Understanding how these underwater giants interact with the coastline is now critical for predicting when the water will rise just a few centimeters too high.

In the tropical island of Guadeloupe, a team of researchers from the French Geological Survey set out to solve a specific mystery: why did the city of Pointe-à-Pitre experience repeated flooding between late July and early August 2022, despite the absence of any storms? The water was calm, the wind was light, yet the sea level rose enough to breach drainage systems and flood streets. To find the cause, the scientists built a detailed statistical model of the local sea level, breaking it down into its constituent parts. They accounted for the predictable rhythm of the tides, the steady climb of long-term sea level rise, and the usual seasonal shifts in water height. Crucially, they also added a new variable to their equation: the influence of the massive eddies swirling in the open ocean nearby. By using satellite data to track these rotating currents, they could calculate how much each eddy pushed or pulled the water level at the coast.

The investigation revealed a clear culprit. During the flooding period, a giant anticyclonic eddy—a massive, clockwise-rotating body of water—was drifting just off the coast of Guadeloupe. This eddy was not a fleeting event; it lingered for several days, acting as a slow-moving wall that elevated the entire sea surface. On August 2nd, 2022, the day the flooding was most severe, this eddy contributed an extra 10.44 centimeters to the sea level. To put this in perspective, the researchers found that this single eddy was responsible for 27 percent of the daily maximum water height. The only factor that contributed more was the tide itself, which accounted for 59 percent. This discovery upended the traditional view that tides and storms are the only significant drivers of coastal flooding. In this instance, the eddy was the second most important force, effectively raising the baseline water level so that even a moderate high tide could spill over the edges of the land.

The researchers noted that the eddy's influence was not a sudden spike but a sustained pressure. Unlike the tide, which rises and falls twice a day, the eddy's contribution grew steadily as the swirl moved closer to the island, reaching its peak and then slowly receding over several days. This behavior suggests that these eddies can set the stage for flooding long before the water actually breaches the shore. The study also highlighted that this phenomenon is not unique to that single week; the team analyzed data from 2011 to 2022 and found that while such large contributions are rare, they do happen, particularly during the summer months when these eddies are most active. In fact, the 2022 event represented the most extreme eddy contribution recorded in their decade-long dataset, surpassing even the effects of major climate events like El Niño in other parts of the world.

This work suggests that coastal risk assessments in tropical regions may have been missing a key piece of the puzzle. For years, planners have focused on tides, storm surges, and the slow creep of rising seas, often overlooking the chaotic, rotating currents of the open ocean. The study indicates that these eddies can temporarily raise sea levels by amounts comparable to decades of projected global sea level rise, all while the weather remains perfectly calm. While the researchers acknowledge that their method is a simplified representation and that satellite technology has limits in detecting smaller eddies, the evidence is clear: these underwater features are a critical driver of nuisance flooding. As the climate changes, understanding and predicting the movement of these massive eddies will become just as important as tracking the weather, offering a new way to protect vulnerable island communities from the quiet, creeping waters of the future.

Technical Summary: Mesoscale Eddies Contribution to Nuisance Flooding in the Tropical Island of Guadeloupe

Problem Statement
Nuisance flooding, defined as high-tide flooding occurring under calm weather conditions, is increasing globally due to sea level rise (SLR). While the roles of tides, long-term SLR, and extreme weather events (storms, cyclones) are well-documented, the contribution of mesoscale ocean dynamics—specifically eddies—to coastal sea level variability and nuisance flooding remains poorly quantified, particularly in tropical island environments. In Pointe-à-Pitre, Guadeloupe, a notable flooding event occurred between mid-July and mid-August 2022. This event coincided with high seasonal anomalies and the presence of a large anticyclonic eddy, suggesting a potential link between mesoscale eddies and coastal inundation that lacks a formal framework for assessment.

Methodology
The authors developed a statistical reconstruction of coastal sea level at the Pointe-à-Pitre tide gauge (2011–2022) to explicitly incorporate mesoscale eddy contributions. The reconstruction integrates five components:

Tidal Prediction: Derived from SHOM operational forecasts using MAS software.
Long-term SLR: Estimated via a quadratic fit to tide gauge data to account for acceleration.
Eddy Contribution: Derived from satellite altimetry (Copernicus DUACS vDT2024) using the autonomous eddy identification scheme of Faghmous et al. (2015). The contribution is parameterized as a Gaussian decay function of eddy amplitude ( $A$ ) and the distance ( $d$ ) between the eddy center and the coastline, scaled by the eddy radius ( $R$ ): $\eta_{eddy} = \alpha A \exp(-d^2 / \beta R^2)$ . Anticyclonic eddies are assigned positive values (raising sea level), while cyclonic eddies are negative.
Seasonal Residuals: A monthly climatology derived from residuals after removing tides, SLR, and eddy terms, with corrections to avoid double-counting eddy-induced seasonality.
Non-seasonal Residuals: Modeled probabilistically using a Gaussian distribution based on monthly and sea-level decile bins (following Dusek et al., 2022) to generate an ensemble of 100 members.

The study focused on the "nuisance flooding period" (July 28 – August 2, 2022), excluding data influenced by storms and cyclones identified via the IBTrACS dataset.

Key Results

Reconstruction Accuracy: The reconstructed sea level showed strong agreement with observations ( $R^2 = 0.89$ for the full period; $R^2 = 0.94$ for the flooding period), with a mean bias of $2 \times 10^{-4}$ mm and an RMSE of 0.041 m.
Eddy Impact during the 2022 Event: A large anticyclonic eddy passed near Guadeloupe, reaching a maximum amplitude of 12.13 cm on August 2, 2022, with its center located ~66 km from the tide gauge.
- The eddy contribution peaked at 10.44 cm on August 2, 2022.
- This contribution represented 27% of the observed daily maximum sea level (0.49 m) and was the second most significant driver after the tide (59%).
- During the flooding period, the mean hourly contribution of eddies (38.4%) was nearly equivalent to that of tides (42.9%).
Temporal Dynamics: Unlike the semi-diurnal tidal signal, the eddy contribution evolved smoothly over several days, providing a sustained elevation of the background sea level. The relative contribution of eddies to the daily maximum sea level increased from 16% to 33% as the eddy approached the coast.
Climatology: Over the 2011–2022 period, eddy contributions exceeding 5 cm occurred on only 1% of days. However, a distinct seasonal cycle exists, with positive (anticyclonic) contributions peaking in the summer months (June–August), coinciding with the timing of the 2022 event.

Significance and Claims
The paper claims that mesoscale eddies represent a critical and previously underappreciated driver of present and future nuisance flooding in tropical island environments. The study demonstrates that:

Magnitude: Eddy-induced sea level anomalies can reach magnitudes (10.44 cm) comparable to long-term SLR trends over decades or major climate phenomena like ENSO, capable of setting the stage for flooding even under moderate tidal conditions.
Mechanism: Anticyclonic eddies can raise the background sea level, allowing moderate tidal peaks to exceed flooding thresholds that would otherwise be safe.
Temporal Scale: These events occur on multi-day timescales (several days to a week), distinct from instantaneous storm surges or seasonal cycles.
Implication for Risk Assessment: Current coastal risk assessments and future adaptation strategies, which typically focus on tides, storm surges, and long-term SLR, may need to integrate offshore mesoscale ocean dynamics to accurately predict nuisance flooding frequency and severity in a changing climate.

The authors note limitations, including the reliance on satellite altimetry which may underestimate smaller or short-lived eddies, and the simplified Gaussian decay formulation which does not explicitly resolve complex wave dynamics or coastal geometry. However, the results provide a first-order quantification of eddy impacts, highlighting their potential to act as decisive factors in extreme coastal sea level events.

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