Maria Josefina Olascoaga

We simulate the dynamics of pelagic 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 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 2 years before it was detectable via satellite imagery on the basin’s western side. This finding supports anecdotal evidence of 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 Sargassum in the 2011 bloom differ from those in the Sargasso Sea, which might otherwise be considered a natural origin.

We study the properties of, and investigate the stability of a baroclinic zonal current in, a thermal rotating shallow-water model, sometimes called Ripa's model, featuring stratification for quasigeostrophic upper-ocean dynamics. The model has Lie--Poisson Hamiltonian structure. In addition to Casimirs, the model supports weak Casimirs forming the kernel of the Lie–Poisson bracket for the potential vorticity evolution independent of the details of the buoyancy as this is advected under the flow. The model sustains Rossby waves and a neutral model, whose spurious growth is prevented by a positive-definite integral, quadratic on the deviation from the motionless state. A baroclinic zonal jet with vertical curvature is found to be spectrally stable for specific configurations of the gradients of layer thickness, vertically averaged buoyancy, and buoyancy frequency. Only a subset of such states was found Lyapunov stable using the available integrals, except the weak Casimirs, whose role in constraining stratified thermal flow remains to be understood. The existence of Lyapunov-stable states enabled us to a priori bound the nonlinear growth of perturbations to spectrally unstable states. Our results do not support the generality of earlier numerical evidence on the suppression of submesoscale wave activity as a result of the inclusion of stratification in thermal shallow-water theory, which we supported with direct numerical simulations.

We examine hurricane and ocean reanalysis data to understand how rapid intensification (RI) of tropical cyclones is influenced by the upper ocean density structure, focusing on barrier layer (BL) thickness and thermocline depth in the eastern Caribbean Sea and the nearby western tropical North Atlantic. This was done using transition path theory (TPT), complemented with basic statistical analysis. The framework for TPT is established using Markov chains, which are constructed by discretizing pertinent data time series, namely, intensity of weather systems, changes in intensity, translational speed, and BL thickness and thermocline depth along system paths. These elements are considered trajectories within abstract state spaces, in which evolution is portrayed as a memoryless stochastic process. RI imminence is rigorously framed using a newly derived TPT statistic, which gives the time distribution to first reach a target---the RI state---from a source---for instance, the state determined by a certain BL range and system intensity---conditional on connecting paths exhibiting minimal detours. We find a higher frequency of RI in the eastern Caribbean and adjacent Atlantic influenced by river runoff, predominantly in tropical storms and category 2 hurricanes. RI is often associated with a well-developed BL; however, intensifying systems require an increased translational speed. The TPT analysis reveals a stronger link between RI and thermocline depth than with BL presence, with the likelihood of RI increasing for hurricanes with a thin BL, particularly category 1. Across strength categories, a deep thermocline consistently increases the probability of RI, a factor overlooked by basic statistical methods. Translational speed emerges as an important factor, with faster and stronger hurricanes being more prone to RI, while slower systems are less likely.

The proliferation of pelagic Sargassum spp. (Sargassum) in the tropical Atlantic has significant ecological and socio-economic impacts. While large-scale ocean circulation patterns influence the basin scale distribution of Sargassum, the role of mesoscale eddies in their local accumulation and transport was not quantitatively assessed so far. This study investigates the relationship between mesoscale eddies and Sargassum dynamics using satellite observations. By analyzing 13 years of remote sensing observations, we demonstrate that both cyclonic and anticyclonic long-lived mesoscale eddies can trap and transport Sargassum. However, results show that in cyclonic eddies Sargassum cover is higher and tends to accumulate during their lifetime while within anticyclonic eddies the Sargassum cover is usually weaker and tend to decrease. These findings align with recent studies highlighting the role of eddies in shaping the distribution of floating debris and provide an important observational basis for the development of Sargassum drift models.

Sargassum has historically been found in the subtropical North Atlantic gyre where it provides important habitat for diverse marine species. However, since 2011 with the development of the Great Atlantic Sargassum Belt in the equatorial Atlantic, abundance has greatly increased, resulting in shorelines mass stranding. These coastal inundations of Sargassum have major impacts on the ecology, economies, and health of affected areas.Understanding the Sargassum raft's motion is required to be able to predict the areas that could be affected by Sargassum. The motion of the rafts is fundamentally unlike Lagrangian (i.e., infinitesimally small, neutrally buoyant) particle motion since they represent finite-size, buoyant objects subjected to the action of ocean currents, wind, and waves. In this talk, we will present a Maxey-Riley model for the motion of Sargassum rafts that takes their inertial nature into account as well as the elastic interactions within a raft and physiological changes affecting the structure of the rafts. This will be accompanied by a discussion of results from field and laboratory experiments used to validate the model. Joint work with F. J. Beron-Vera and G. Bonner.

The surge of pelagic \emph{Sargassum} in the Intra-America Seas, particularly the Caribbean Sea, since the early 2010s has raised significant ecological concerns. This study emphasizes the need for a mechanistic understanding of \emph{Sargassum} dynamics to elucidate the ecological impacts and uncertainties associated with blooms. By introducing a novel transport model, physical components such as ocean currents and winds are integrated with biological aspects affecting the \emph{Sargassum} life cycle, including reproduction, grounded in an enhanced Maxey--Riley theory for floating particles. Nonlinear elastic forces among the particles are included to simulate interactions within and among \emph{Sargassum} rafts. This promotes aggregation, consistent with observations, within oceanic eddies, which facilitate their transport. This cannot be achieved by the so-called leeway approach to transport, which forms the basis of current \emph{Sargassum} modeling. Using satellite-derived data, the model is validated, outperforming the leeway model. Publicly accessible codes are provided to support further research and ecosystem management efforts. This comprehensive approach is expected to improve predictive capabilities and management strategies regarding \emph{Sargassum} dynamics in affected regions, thus contributing to a deeper understanding of marine ecosystem dynamics and resilience.

The surge of pelagic Sargassum in the Intra-America Seas, particularly the Caribbean Sea, since the early 2010s has raised significant ecological concerns. This study emphasizes the need for a mechanistic understanding of Sargassum dynamics to elucidate the ecological impacts and uncertainties associated with blooms. By introducing a novel transport model, physical components such as ocean currents and winds are integrated with biological aspects affecting the Sargassum life cycle, including reproduction, grounded in an enhanced Maxey–Riley theory for floating particles. Nonlinear elastic forces among the particles are included to simulate interactions within and among Sargassum rafts. This promotes aggregation, consistent with observations, within oceanic eddies, which facilitate their transport. This cannot be achieved by the so-called leeway approach to transport, which forms the basis of current Sargassum modeling. Using satellite-derived data, the model is validated, outperforming the leeway model. Publicly accessible codes are provided to support further research and ecosystem management efforts. This comprehensive approach is expected to improve predictive capabilities and management strategies regarding Sargassum dynamics in affected regions, thus contributing to a deeper understanding of marine ecosystem dynamics and resilience.

The Malvinas Current (MC) in the Southwestern Atlantic is characterized as a system of multiple Lagrangian jets. This is done by applying geodesic theory of Lagrangian Coherent Structures (LCSs) on altimetry‐derived velocity. The number and spatial disposition of jet‐cores organizing the MC are found to vary in time influenced by topographic waves propagating along the current. The Lagrangian analysis reveals that permanent jet‐cores tend to approach one another upstream, yet do not merge into a single jet as the Eulerian analysis of the data suggests. Independent support for the existence of multiple jets is provided by trajectories of surface drifters from a deployment of unparalleled dimensions for the region and ocean color images. The drifters develop the characteristic boomerang‐shaped patterns into which passive tracers straddling jet‐cores deform and exhibit Lagrangian metrics that are largely consistent with those derived from altimetry data. Ocean color imagery reveals similar “chevrons” and suggest upwelling along the jets, which is consistent with a new solution of arrested topographic wave dynamics. This observation and the finding that the MC jet system is influenced by propagating waves suggests that the MC variability may have major implications in the modulation of primary productivity on synoptic timescales. Drifters flowing into the open ocean, exhibit organizing patterns that are largely shaped by mesoscale circulation and essentially differ from those observed along the MC. Evidence of this observation is provided on the basis of LCS analysis and the calculation of a quasi‐objective Lagrangian metric derived directly from drifter trajectories.