Tracing the origin of tropical North Atlantic Sargassum blooms to West Africa

Abstract

We simulate the dynamics of pelagic \emph{Sargassum} rafts as systems of finite-size floating particles, governed by a Maxey--Riley law with nonlinear elastic interactions. Using surface ocean currents and wind data from reanalysis systems for clump transport, we computed trajectories within a domain covering the tropical and subtropical north Atlantic. The subsequent motion is reduced using Ulam's discretization method into a time-inhomogeneous Markov chain that simulates a background \emph{Sargassum} concentration. Bayesian inversion, combined with nonautonomous transition path theory, was used to infer the origin of the first significant recorded bloom in the tropical North Atlantic, which unfolded in April 2011. Both methodologies independently identified the bloom's origin as near the West African coast, up to two years before it was detectable via satellite imagery on the basin's western side. This finding supports anecdotal evidence of \emph{Sargassum} strandings on the Ghanaian coast in 2009. Moreover, it correlates with unusual environmental conditions -- such as increased nutrient loads from significant upwelling linked to a pronounced Dakar Niña and Saharan dust deposition -- that promote bloom proliferation. Additionally, it aligns with the observation that the species of \emph{Sargassum} in the 2011 bloom differ from those in the Sargasso Sea, which might otherwise be considered a natural origin.

Figures (5)

• Figure 1: Forward evolution of the discrete representation of the probability density depicted in the top-left panel under left multiplication by the nonautonomous transition matrix \ref{['eq:Pext']} constructed using Sargassum clump trajectories obtained by integrating the eBOMB model equations \ref{['eq:eBOMB']}, with ocean currents and winds as produced by reanalysis. The initial density is randomly distributed on boxes intersecting with the first documented major Sargassum bloom location in the tropical North Atlantic. A second-root transformation is applied to each distribution.
• Figure 2: Posterior distribution of the location of the origin of the first documented major Sargassum bloom intersecting with the ocean surface's negion partitioned by the set ($B$) of blue boxes, after the bloom has been observed in $t_{k^B} = \text{April}/2011$. Results shown correspond to two initialization times $t_0$ of the time-inhomogeneous Markov chain, obtained via a reduction of the motion described by Sargassum trajectories as simulated using eBOMB. Indicated in red is the maximum likelihood estimator of the location of the bloom’s origin, $\hat{b}$.
• Figure 3: Snapshots of time-dependent effective transition currents into the region spanned by the Sargassum bloom observed in April/2011, for an initialization 2.25 years before this observation of the time-inhomogenous Markov chain constructed by discretizing Sargassum trajectories as produced by eBOMB. A fourth-root transformation is applied to the current magnitude.
• Figure 4: Similar to Fig. \ref{['fig:tpt']}, but with the source set ($A$) spanning the Saragasso Sea (left panels) and a region including the the Gulf of Guinea (right panels), as informed by Bayesian inference of the bloom's origin. The latter is informed by Bayesian inference, highlighting the top 50th percentile of the posterior distribution for the bloom's origin, as depicted in Fig. \ref{['fig:bayes']}, right panel.
• Figure 5: Sea-surface temperature (top-left), nitrate (top-right), and phosphate (bottom-left), as produced by a biogeochemical hindcast, and dust extinction aerosol optical depth (bottom-right), as produced by a reanalysis system, all averaged within [21$^\circ$W,17$^\circ$W] $\times$ [9$^\circ$N,14$^\circ$N] in spring.