Ismael Hernández-Carrasco

Fronts, eddies, jets, and meanders populate the ocean surface and play a key role in organizing transport and mixing across a wide range of scales. From a Lagrangian perspective, frontal structures are conveniently detected as surface filaments that act as barriers to horizontal transport. However, the degree to which these structures maintain coherence below the surface and the relationship between horizontal and vertical motions in their vicinity remain poorly understood. Here we analyze two such filaments in the western Mediterranean Sea, south of the Ibiza Channel, using a high‐resolution ocean simulation and Lagrangian diagnostics specifically designed to characterize submesoscale transport. Backward Finite‐Size Lyapunov Exponents (FSLE) computed at successive depths reveal nearly vertical, curtain‐like structures extending several tens of meters into the water column, with intensity decreasing with depth. Vertical-velocity sections across the filaments show coherent downward motion aligned with the filament cores, while Lagrangian trajectories indicate rapid horizontal convergence of fluid particles accompanied by sustained downward displacement. The barrier effect of Lagrangian Coherent Structures (LCSs) is commonly associated with persistent dynamical features of large-scale flows. Here, however, we demonstrate that FSLE filaments generated by rapidly fluctuating submesoscale dynamics can also act as effective transport barriers. Taken together, these analyses reveal a clear coupling between horizontal convergence and vertical motion in FSLE filaments, consistent with subduction‐like processes, and highlight the interplay between mesoscale and submesoscale transport. The results demonstrate that the surface signatures of Lagrangian filaments identified through FSLE techniques correspond to three-dimensional transport structures that regulate both lateral aggregation and vertical connectivity in the upper ocean.

Oceanographic variability plays a critical role in shaping the migratory behavior of the Atlantic bluefin tuna ( Thunnus thynnus ). This study investigates the thermal preferences of individuals migrating through the Strait of Gibraltar and the Gulf of Cádiz during pre‐ and post‐spawning movements. Comparing data measured from electronic tags and satellite‐derived sea surface temperature (SST) fields, we identified distinct SST preferred ranges associated with each migratory phase: 17.4°C–19.3°C inbound to the Mediterranean and 18.4°C–22°C outbound to the Atlantic. Based on these preferences, we developed a SST adequacy index reflecting thermal suitability, which we applied to 13 years of high‐resolution SST satellite data (2010–2022). Spatial and temporal patterns of SST adequacy were then classified using Self‐Organizing Maps, revealing dynamic hotspots of thermal suitability along migratory corridors. Our findings highlight the nonrandom, environmentally modulated nature of ABFT migrations and offer a framework to advance towards climate informed fisheries assessment, a current request of the International Commission for the Conservation of Atlantic Tunas.

Understanding the transport pathways of floating material at the ocean surface is important to improve our knowledge on surface circulation and assessing its environmental impacts. Numerical experiments through Lagrangian particle simulations are widely used to investigate the dispersion of floating material, typically relying on velocity fields from ocean circulation models. However, the contribution of different ocean dynamics (at different temporal and spatial scales) to the net Lagrangian transport remains unclear. Here we focus on tidal forcing, only included in recent ocean models, to explore its effect on particle dispersion at the ocean surface. By comparing a twin simulation with and without tidal forcing, we conclude that tide‐induced dynamics play an important role in horizontal Lagrangian pathways. We focus on the Azores Islands region and find that surface particles travel a longer cumulative distance and a lower total distance with than without tidal forcing. Additionally, tidal forcing leads to higher variability in surface particle accumulation patterns. The differences found in the surface particle accumulation patterns can be greater than 40%. These findings have important implications for virtual particle simulations, suggesting that considering tidal currents alone may not capture the full range of tide‐induced effects. A deeper understanding of the underlying dynamics is essential for accurately analyzing transport properties. Our outcomes can already help improve Lagrangian simulations made to understand the connectivity of marine species and for marine pollution applications, for example, ocean clean‐up strategies for plastics or oil spills, in the Azores Islands and regions with similar dynamics.

Plankton, plastics, nutrients, and other materials in the ocean can exhibit different dispersion patterns depending on their individual transport properties. These dispersion patterns can provide information on the effective timescales of interaction between different types of materials in a highly dynamic ocean environment, such as the Benguela system in the southeast Atlantic Ocean. In this study, we compare the timescales and spatial distribution of separation for zooplankton performing Diel Vertical Migration (DVM) while drifting with currents to those of other materials: (a) positively buoyant plastics or planktonic organisms passively floating near the ocean's surface; (b) nutrients or pollutants passively advecting in the three‐dimensional flow; and (c) sinking biogenic particulate matter. We apply the drift properties of each material type in Lagrangian flow modeling to simulate the movement of virtual particles across the Benguela system. Our results indicate faster separation between zooplankton performing DVM and the other particle types during the upwelling season in the austral spring and summer. We also observe a decrease in the separation timescales between zooplankton performing DVM and other particle types as the zooplankton migration depth increases. Despite the differences in separation timescales across seasons, different particle types can become trapped in coherent features such as eddies, fronts, and filaments, indicating prolonged exposure of zooplankton to prey and pollutants in these coherent ocean features.

Understanding the oceanic surface dispersion has important applications in ocean pollution scenarios. Of the different ocean pollution events, some of which have the most impact both on the marine environment and, on society and economy are: marine plastics, oil spills and Sargassum inundation events. Understanding the ocean dynamics that affect their trajectories is vital to simulate their pathways, and thus know their sources and sinks. This can then be used to implement clean-up strategies and to better manage MPAs. It can also help reduce the impact of ocean pollution on the marine environment and some major economic sectors like tourism. High frequency motions have an important impact on the surface dynamics, but high temporal resolution data is necessary to study their effects. New datasets and methodologies have allowed to obtain better representations of high frequency motions. Here, we specifically focus on the high frequency motions due to tides (like for example internal waves), as well as inertial oscillations. We simulate surface trajectories of plastic, oil and Sargassum using the OceanParcels Lagrangian simulator. We focus on three regions in the Atlantic Ocean: Açores Islands, North Atlantic and Tropical Atlantic, respectively. For the plastic simulations we look at the effect of tides by using velocity outputs from a high-resolution model which is a twin simulation without and with tidal forcing. For the oil spills and Sargassum outputs we use a new surface currents product generated by combining velocity data from drifters, high-frequency winds and altimetry to reconstruct high-frequency surface currents. We find that considering high-frequency motions is key to correctly simulate their surface trajectories, but that further work is necessary to understand the ocean dynamics at the fine-scales that can drive the variability in these Lagrangian trajectories.

Understanding the pathways of floating material at the surface ocean is important to improve our knowledge on surface circulation and for its ecological and environmental impacts. Virtual particle simulations are a common method to simulate the dispersion of floating material. To advect the particles, velocities from ocean models are often used. Yet, the contribution of different ocean dynamics (at different temporal and spatial scales) to the net Lagrangian transport remains unclear. Here we focus on tidal forcing, only included in recent models, and so our research question is: What is the effect of tidal forcing on virtual particle dispersion at the ocean surface? By comparing a twin simulation with and without tidal forcing, we conclude that tides play an important role in horizontal Lagrangian dynamics. We focus on the Açores Islands region, and we find that surface particles travel a longer cumulative distance and a lower total distance with than without tidal forcing and a higher variability in surface particle accumulation patterns is present with tidal forcing. The differences found in the surface particle accumulation patterns can be more than a 40\% increase/decrease. This has important implications for virtual particle simulations, showing that more than tidal currents need to be considered. A deeper understanding of the dynamics behind these tidal forcing impacts is necessary, but our outcomes can already help improve Lagrangian simulations. This is particularly relevant for simulations done to understand the connectivity of marine species and for marine pollution applications.

Understanding the pathways of floating material at the surface ocean is important to improve our knowledge on surface circulation and for its ecological and environmental impacts. Virtual particle simulations are a common method to simulate the disper

Plankton, plastics, nutrients, and other materials in the ocean can exhibit different dispersion patterns depending on their individual drifting properties. These dispersion patterns can provide information on the effective timescales of interaction between different types of materials in a highly dynamic ocean environment, such as the Benguela system in the southeast Atlantic Ocean. In this study, we compare the timescales and spatial distribution of separation for zooplankton performing Diel Vertical Migration (DVM) while drifting with currents to those of other materials: (i) positively buoyant plastics or planktonic organisms passively floating near the ocean's surface; (ii) nutrients or pollutants passively advecting in the three-dimensional flow; and (iii) sinking biogenic particulate matter. We apply the drift properties of each material type in Lagrangian flow modeling to simulate the movement of virtual particles across the Benguela system. Our results indicate faster separation between zooplankton performing DVM and the other particle types during the upwelling season in the austral spring and summer. We also observe a decrease in the separation timescales between zooplankton performing DVM and other particle types as the zooplankton migration depth increases. Despite the differences in separation timescales across seasons, different particle types can become trapped in coherent features such as eddies, fronts, and filaments, indicating prolonged exposure of zooplankton to prey and pollutants in these coherent ocean features.

Understanding the pathways of floating material at the surface ocean is important to improve our knowledge on surface circulation and for its ecological and environmental impacts. Virtual particle simulations are a common method to simulate the dispersion of floating material. To advect the particles, ocean models’ velocities are usually used, but only recent ones include tidal forcing. Our research question is: What is the effect of tidal forcing on virtual particle dispersion and accumulation at the ocean surface? As inputs we use velocity outputs from eNATL60, a twin simulation with and without tidal forcing. We focus on the Açores Islands region and we find: 1) Surface particles have a larger displacement, but a lower distance travelled with than without tidal forcing 2) Surface accumulation seasonal differences depend on the spatial scale of the ocean structures 3) A greater variability in surface accumulation is present with tidal forcing.