Frontal Dynamics Research Articles

Multiple timescale variations in fronts in the Seto Inland Sea, Japan

Abstract. The Seto Inland Sea (SIS) is a critical semi-enclosed coastal sea in Japan, characterized by intricate coastlines and narrow straits that give rise to various fronts. Despite extensive research on tidal fronts, knowledge gaps persist regarding their spatiotemporal dynamics, particularly in certain poorly documented regions. Additionally, the understanding of thermohaline fronts, which emerge during winter, requires further investigation. We aimed to enhance our understanding of tidal and thermohaline fronts in the SIS by analyzing their dynamic processes, including intra-tidal and spring–neap tidal cycles, seasonal variations, and anomalous frontal variability. Using a gradient-based algorithm with an advanced contextual feature-preserving median filter, we processed the high-resolution sea surface temperature dataset to detect and quantify tidal and thermohaline fronts. Our analysis revealed the presence of numerous tidal fronts, predominantly influenced by the M2 tide, across the SIS, with substantial spatial variations in amplitude due to complex coastlines and narrow straits. Intra-tidal movements of tidal fronts corresponded to ebb and flood currents, while spring–neap tidal cycles and seasonal shifts influenced frontal positions and intensities. Additionally, thermohaline fronts were identified in certain regions during winter, characterized by large horizontal temperature and salinity gradients. This study enhances the understanding of tidal and thermohaline fronts in the SIS, emphasizing the importance of intra-tidal and wind-driven influences on frontal dynamics. However, limited observational coverage and resolution emphasize the need for further research to explore long-term temporal changes and better grasp the influences of ambient currents and wind patterns. Such insights are vital for effective coastal management and environmental monitoring in the SIS region.

From source to sink: part 2-seasonal dispersion of microplastics discharged in the NW Mediterranean Sea by the Rhone River in southern France.

As the largest individual contributor of freshwater inflow to the basin, the Rhone River is likely to be one of the main sources of microplastics (MPs) to the Mediterranean Sea. In order to predict the fate of MPs discharged by the Rhone River, an innovative 3D Lagrangian dispersion of its particles associated with vertical velocities was modeled in Mediterranean ocean currents. Through winter and summer scenarios, the seasonal variability of transfers and the corresponding accumulation areas were depicted in the Northwestern Basin according to hydrodynamic conditions on the continental shelf of the Gulf of Lion and to the frontal dynamics from the Pyrenees to the North Balearic fronts. Our results indicated that MP transfers were driven by mesoscale and sub-mesoscale structures, resulting in steep concentration gradients across fronts during summer, while winter energetic mixing favored a more efficient and homogeneous spreading. After a year of drift, high MP retention (up to 50%) occurred in the coastal zone of the Gulf of Lion near the river mouth, with a large contribution of sinking MPs and an increase in stranding during the highest freshwater inflows of the winter season. Conversely, up to 60% of the floating MPs were exported to the Algerian Basin and then to the Eastern Mediterranean. This west-to-east transfer led to significant stranding on the islands, prevailing on the northern coasts of the Balearic Islands in winter (6% of floating inputs) and on the western coasts of Corsica and Sardinia in summer (13%). The southern Mediterranean coasts, from Algeria to Tunisia, represented also a major sink for floating debris with stranding ranging from 9 to 35% of MPs discharged in winter and in summer, respectively. We estimated that 3.5 to 5 t of the Rhone MPs remained in the surface layer at the end of the year, with high concentrations in the Ionian Sea.

Drivers of Cold Frontal Hourly Extreme Precipitation: A Climatological Study Over Europe

AbstractIn the mid‐latitudes extreme precipitation events are strongly associated with cold fronts. By exploring drivers across different scales and relating them to precipitation, we aim to improve our understanding of processes influencing cold frontal extremes. Using hourly ERA5 data over Europe and the North Atlantic, cold fronts are detected and the associated conditions are identified. Quantile regression models are employed to find drivers of frontal precipitation and to quantify these relations. Additionally, we use composites to study the synoptic conditions and meso‐scale structure of extreme events. We find that humidity close to the detected fronts, convergence and the low level jet speed contribute most to the formation of extreme precipitation. Synoptic conditions favoring the formation of extreme events are also identified. These results improve our understanding of cold frontal processes leading to precipitation. Additionally, they provide the foundation for a process‐based evaluation of frontal dynamics in climate models.

Evidence for Kilometer‐Scale Biophysical Features at the Gulf Stream Front

AbstractUnderstanding the interplay of ocean physics and biology at the submesoscale and below (<30 km) is an ongoing challenge in oceanography. While poorly constrained, these scales may be of critical importance for understanding how changing ocean dynamics will impact marine ecosystems. Fronts in the ocean, regions where two disparate water masses meet and isopycnals become tilted toward vertical, are considered hotspots for biophysical interaction, but there is limited observational evidence at the appropriate scales to assess their importance. Fronts around western boundary currents like the Gulf Stream are of particular interest as these dynamic physical regions are thought to influence both productivity and composition of primary producers; however, how exactly this plays out is not well known. Using satellite data and 2 years of in situ observations across the Gulf Stream front near Cape Hatteras, North Carolina, we investigate how submesoscale frontal dynamics could affect biological communities and generate hotspots of productivity and export. We assess the seasonality and phenology of the region, generalize the kilometer‐scale structure of the front, and analyze 69 transects to assess two physical processes of potential biogeochemical importance: cold shelf filament subduction and high salinity Sargasso Sea obduction. We link these processes observationally to meanders in the Gulf Stream and discuss how cold filament subduction could be exporting carbon and how obduction of high salinity water from depth is connected with high chlorophyll‐a. Finally, we report on phytoplankton community composition in each of these features and integrate these observations into our understanding of frontal submesoscale dynamics.

Eddy‐Induced Dispersion of Sea Ice Floes at the Marginal Ice Zone

AbstractOcean heat exchanges at the marginal ice zone (MIZ) play an important role in melting sea ice. Mixed‐layer eddies transport heat and ice floes across the MIZ, facilitating the pack's access to warm waters. This study explores these frontal dynamics using disk‐shaped floes coupled to an upper‐ocean model simulating the sea ice edge. Numerical experiments reveal that small floes respond more strongly to fine‐scale ocean currents, which favors higher dispersion rates and weakens sea ice drag onto the underlying ocean. Floes with radii smaller than resolved turbulent filaments (∼2–4 km) result in a wider and more energetic MIZ, by a factor of 70% each, compared to larger floes. We hypothesize that this floe size dependency may affect sea ice break‐up by controlling oceanic energy propagation into the MIZ and modulate the sea ice pack's melt rate by regulating lateral heat transport toward the sea ice cover.

Inferring microplastics origins in the Mediterranean Sea by coupling modelling and in-situ measurements

This work focuses on the dynamics and retention of microplastics (MP) in the Mediterranean. MP manta-net trawls were performed in autumn 2019 north of the Balearic Islands and along the Balearic Front (BF). Lagrangian modelling was used to find the MP collected origin during the campaign. These combined results show that North of Mallorca is a temporary retention zone of 3 months variability, with MP origin being the Northern Current (NC) and the Gulf of Lion (GOL). Retention processes were less clear along the BF, due to frontal dynamics together with the strong northerly winds. However, it appears that the origin can differ between the North (i.e. the large North-Westerly basin, including the GOL and the NC path) and the South (short distances around the zone) of this front. In both areas, the wind and the current variability are strongly conditioning the existence and position of the MP concentration zones.

New Insights Into the Role of Atmospheric Transport and Mixing on Column and Surface Concentrations of NO2 at a Coastal Urban Site

AbstractWe use a multi‐year record of Pandora‐derived NO2 total column abundance in Boston to examine the influence of atmospheric transport on column NO2 and its surface concentrations during the warm season in a coastal urban environment. We derive tropospheric NO2 estimates from the total column with a measurement‐model fusion approach using near‐real‐time estimates of stratospheric NO2 from NASA's Goddard Earth Observing System Composition Forecast model system and find the average influence of stratospheric NO2 at this urban site can be 30%–70% depending on season and time of day. Sea breeze days tend to exhibit rapid temporal variability in the column that which can go in the opposite direction of changes in surface NO2 concentrations. By comparing tropospheric NO2 with surface concentrations, we constrain the role of boundary layer entrainment processes in the evolution of surface NO2 concentrations, while highlighting the value of column measurements in identifying sea breeze frontal dynamics. We estimate an apparent equal mixing layer height of NO2 and infer that surface NOx emissions remain concentrated near the surface regardless of atmospheric stability regime. When comparing the Pandora‐ to TROPOMI‐derived column NO2 measurements, we find that sea breeze days present a unique challenge likely due to higher spatial heterogeneity in NO2 and the meteorology involved that is not well represented in retrieval inputs. Our observations provide new insights into column and surface variability of NO2 which will be relevant to interpreting geostationary observations, especially in coastal urban locations.

Spatial and temporal variability of suspended sediment fronts over the Yangtze Bank in the Yellow and East China Seas

Suspended sediment fronts (SSFs) are an important sub-mesoscale phenomenon prevalent in estuaries, coasts, and continental shelf seas, affecting ocean hydrodynamics, fisheries, and ecology. Using archived (2011–2021) Geostationary Ocean Color Imager (GOCI) data, the spatial distributions and long-term variations in SSFs over the Yangtze Bank were quantitatively investigated by calculating frontal probability (FP). The findings indicate that the majority of SSFs occurred in regions inside the 50-m isobath. Their distributions were considerably limited by depth gradients. The FP values were divided into three main levels according to their statistical characteristics. The boundaries of moderate-FP zones (FP = 3%) were aligned with the locations of tidal mixing fronts. Empirical orthogonal function (EOF) analysis revealed that frontal activity exhibited strong seasonal cycles, together with large interannual variations. Its spatio-temporal pattern was associated with multiple factors. Among them, topographic effects combined with tidal mixing dominated frontogenesis. The seasonal development of SSFs was mainly controlled by stratification and wind waves associated with monsoons and was partially affected by wind-driven regional circulations. The direct impact of the Yangtze River sediment discharge was restricted to waters inside the 30-m isobath. The El Niño-Southern Oscillation affected interannual frontal dynamics to a large extent. After El Niño years, due to weakening winter winds and warmer sea surfaces, frontal activity was suppressed by the intensification of stratification and weakening of vertical mixing. In contrast, frontal activity increased after La Niña years. These findings will broaden the understanding of SSF variability and its ecological effects over the Yangtze Bank and in other systems.

Shelf break exchange processes influence the availability of the northern shortfin squid, Illex illecebrosus, in the Northwest Atlantic

AbstractThe United States Northern Shortfin squid fishery is known for its large fluctuations in catch at annual scales. In the last 5 years, this fishery has experienced increased availability of Illex illecebrosus along the Northeast US continental shelf (NES), resulting in high catch per unit effort (CPUE) and early fishery closures due to quota exceedance. The fishery occurs within the Northwest Atlantic, whose complex dynamics are set up by the interplay between the large‐scale Gulf Stream, mesoscale eddies, Shelfbreak Jet, and shelf‐slope exchange processes. Our ability to understand and quantify this regional variability is requisite for understanding the availability patterns of Illex, which are largely influenced by oceanographic conditions. In an effort to advance our current understanding of the seasonal and interannual variability in this species' relative abundance on the NES, we used generalized additive models to examine the relationships between the physical environment and hotspots of productivity to changes in CPUE of I. illecebrosus in the Southern stock component, which comprises the US fishery. Specifically, we derived oceanographic indicators by pairing high‐resolution remote sensing data and global ocean reanalysis physical data to high‐resolution fishery catch data. We identified a suite of environmental covariates that were strongly related to instances of higher catch rates. In particular, bottom temperature, warm core rings, subsurface features, and frontal dynamics together serve as indicators of habitat condition and primary productivity hotspots, providing great utility for understanding the distribution of Illex with the potential for forecasting seasonal and interannual availability.

Biophysical Dynamics at Ocean Fronts Revealed by Bio‐Argo Floats

AbstractThe straining regions of the ocean in between mesoscale eddies contain large vertical velocities that may be important in regulating phytoplankton accumulation rates. We analyze time series of variables measured by ocean Bio‐Argo floats (mixed layer depths [MLDs], chlorophyll, and carbon concentrations) in conjunction with variables derived from satellite altimetry (strain rates, Lyapunov exponents, vertical velocities) to determine the evolution of mixed layer phytoplankton biomass in response to straining by the mesoscale geostrophic flow. A Lagrangian (water parcel following) framework is justified by restricting the analysis to profiles whose value of a Quasi‐Planktonic Index—an index quantifying averaged distance between a float trajectory and a surface geostrophic trajectory over three consecutive time steps—is less than 5 km. Bin‐averaged Lagrangian derivatives of phytoplankton biomass and chlorophyll concentration are positive for elevated strain rate and upwelling quasigeostrophic vertical velocities. Lagrangian derivatives of MLD and phytoplankton carbon averaged at straining fronts (in rotated along‐ and across‐front coordinates) have features in common with submesoscale dynamics, including increasing phytoplankton carbon (and chlorophyll) and a shoaling mixed layer over the front. To elucidate a mechanism, we average time derivatives of modeled cell division rates, finding the pattern approximately matches the pattern of phytoplankton accumulation rates and is controlled primarily by the term modulating light stress, suggesting that frontal dynamics cause accelerations of division rates by increasing available light. Regions of increasing chlorophyll are also approximately co‐located with upwelling quasigeostrophic velocity, suggesting non‐Lagrangian behavior of floats causes some imprint of larger scale, more persistent mesoscale signals.

Submesoscale variability in a mesoscale front captured by a glider mission in the Gulf of Finland, Baltic Sea

Modern research methods enable unfolding the structure of the water column with higher resolution than ever, revealing the importance of submesoscale. Submesoscale processes have intermediate space and time scales of <5 km and a few days in the Baltic Sea. A glider mission was conducted in the Gulf of Finland in May 2018. The appearance of a mesoscale front as a response to the persisting NE–E winds was observed. Within the front, smaller scale features at a lateral scale of a km were apparent. The tracer patterns indicated the presence of two adjacent motions – cold (warm) water penetrating upward (downward) on the lighter (denser) side of the front. We suggest they were traces of ageostrophic secondary circulation emerging while the loss of the upwelling-favorable forcing arrested the strengthening of the front. The analysis showed favorable conditions for the baroclinic and wind-driven instability. Such circulations could work to equalize the differences in cross-front direction, affecting the stratification and acting against the persistence of the mesoscale front. The spatial spectra of isopycnal tracer variance revealed the depth-dependence of the spectral slopes at the lateral scales of 1–10 km in the upper part of the water column. The differing of the slopes in the density layers associated with the mesoscale front indicates that frontal dynamics contribute to the energy cascade.

Quantitative EEG as a Biomarker in Evaluating Post-Stroke Depression.

Introduction: Post-stroke depression (PSD) has complex pathophysiology determined by various biological and psychological factors. Although it is a long-term complication of stroke, PSD is often underdiagnosed. Given the diagnostic role of quantitative electroencephalography (qEEG) in depression, it was investigated whether a possible marker of PSD could be identified by observing the evolution of the (Delta + Theta)/(Alpha + Beta) Ratio (DTABR), respectively the Delta/Alpha Ratio (DAR) values in post-stroke depressed patients (evaluated through the HADS-D subscale). Methods: The current paper analyzed the data of 57 patients initially selected from a randomized control trial (RCT) that assessed the role of N-Pep 12 in stroke rehabilitation. EEG recordings from the original trial database were analyzed using signal processing techniques, respecting the conditions (eyes open, eyes closed), and several cognitive tasks. Results: We observed two significant associations between the DTABR values and the HADS-D scores of post-stroke depressed patients for each of the two visits (V1 and V2) of the N-Pep 12 trial. We recorded the relationships in the Global (V1 = 30 to 120 days after stroke) and Frontal Extended (V2 = 90 days after stroke) regions during cognitive tasks that trained attention and working memory. For the second visit, the association between the analyzed variables was negative. Conclusions: As both our relationships were described during the cognitive condition, we can state that the neural networks involved in processing attention and working memory might go through a reorganization process one to four months after the stroke onset. After a period longer than six months, the process could localize itself at the level of frontal regions, highlighting a possible divergence between the local frontal dynamics and the subjective well-being of stroke survivors. QEEG parameters linked to stroke progression evolution (like DAR or DTABR) can facilitate the identification of the most common neuropsychiatric complication in stroke survivors.

Submesoscale Frontal Instabilities Modulate Large‐Scale Distribution of the Winter Deep Mixed Layer in the Kuroshio‐Oyashio Extension

AbstractIn the mid‐latitude North Pacific, the wintertime ocean heat loss reaches the maximum in the Kuroshio‐Oyashio Extension (KOE) region. However, the mixed layer depth (MLD) there is shallow (<50 m), flanked by a zonal band of deep MLD (>100 m) on either side. Such an observed mixed layer pattern is not reproduced by coarse‐resolution or mesoscale eddy‐resolving (1/10°) models. What causes the observed MLD shoaling along the KOE frontal region? Here we show that the mixed layer shoaling in the front is well reproduced by a 1/30°, submesoscale‐permitting ocean general circulation model, indicating that it is closely related to the submesoscale frontal dynamics. The geostrophic strain rate is strong along the sharp KOE front, favoring submesoscale frontal instabilities. The surface buoyancy loss during winter reduces stratification and enhances the fronts (i.e., with steeper, northward rising isopycnals). The available potential energy stored in the narrow fronts can be released in the form of eddy kinetic energy by the mixed layer baroclinic instabilities (MLI). The associated vigorous ageostrophic motions associated with MLI efficiently slump the vertically oriented isopycnals and restratify the mixed layer. Thus, the MLD is shallow in the KOE frontal region despite the strong surface heat loss and wind stirring during winter. The submesoscale processes revealed here have important implications for improving climate models.

Diagnosing Frontal Dynamics From Observations Using a Variational Approach

AbstractIntensive hydrographic and horizontal velocity measurements collected in the Alboran Sea enabled us to diagnose the three‐dimensional dynamics of a frontal system. The sampled domain was characterized by a 40 km diameter anticyclonic eddy, with an intense front on its eastern side, separating the Atlantic and Mediterranean waters. Here, we implemented a multi‐variate variational analysis (VA) to reconstruct the hydrographic fields, combining the 1‐km horizontal resolution of the Underway Conductivity‐Temperature‐Depth (CTD) system with information on the flow shape from the Acoustic Doppler Current Profiler velocities. One advantage of the VA is given by the physical constraint, which preserves fine‐scale gradients better than the classical optimal interpolation (OI). A comparison between real drifter trajectories and virtual particles advected in the mapping quantified the improvements in the VA over the OI, with a 15% larger skill score. Quasi‐geostrophic (QG) and semi‐geostrophic (SG) omega equations enabled us to estimate the vertical velocity (w) which reached 40 m/day on the dense side of the front. How nutrients and other passive tracers leave the mixed‐layer and subduct is estimated with 3D advection from the VA, which agreed with biological sampling from traditional CTD casts at two eddy locations. Downwelling warm filaments are further evidence of subduction, in line with the w from SG, but not with QG. SG better accounted for the along‐isopycnal component of w in agreement with another analysis made on isopycnal coordinates. The multi‐platform approach of this work and the use of variational methods improved the characterization and understanding of (sub)‐mesoscale frontal dynamics.

Variability and Intensity of the Sea Surface Temperature Front Associated With the Kuroshio Extension

Seventeen years of satellite observational data are used to describe the variability in sea surface temperature (SST) fronts and associated features, e.g., frontal intensity and probability, in the northwestern Pacific Ocean. Compared with the SST gradient and frontal probability, the frontal intensity is less impacted by background noise in satellite observations and precisely represents the variability in frontal patterns. The seasonal variability in frontal intensity is prominent, and the corresponding seasonality varies spatially. Fronts are more common during winter in the northern region around the Oyashio Current but are most common during spring in the Kuroshio Current and to the south of the Kuroshio Extension. The meridional migration of fronts is associated with the seasonal movement of the North Pacific subtropical gyre. Though overall averaged frontal intensity along the Kuroshio Extension is most prominent in the northwestern Pacific Ocean, the associated variability in fronts is highly complex. The current study reveals that seasonality itself cannot capture the whole picture of frontal features because eddy-induced intraseasonal variability and basin-scale signal-induced interannual variability can modulate frontal dynamics. In particular, the monthly frontal intensity in regions where the seasonal cycle is not significant depends on the Pacific Decadal Oscillation and North Pacific Gyre Oscillation. Furthermore, the oscillation of the Kuroshio Extension and associated mesoscale eddies can impact its intraseasonal variability. The comprehensive analysis of frontal intensity in the Kuroshio Extension is helpful for delineating regional dynamics and has the potential to improve our understanding of controls on marine primary production.

Potential vorticity fronts and the late-time evolution of large-scale quasi-geostrophic flows

The late-time behaviour of freely evolving quasi-geostrophic flows with initial characteristic length scale $L_0$ larger than or equal to the deformation radius $L_D$ , $L_0/L_D \geq 1$ , is studied. At late time the flows are dominated by large multi-level vortices consisting of ascending terraces of well-mixed potential vorticity (PV), i.e. PV staircases. We examine how the number of mixed PV levels depends on the initial conditions, in particular $L_0/L_D$ . For sufficiently large values of $L_0/L_D \approx 5$ , a complete staircase with regular steps forms, but as $L_0/L_D$ decreases, the staircase becomes more irregular, with fewer mixed levels and the appearance of a large step centred on zero PV, corresponding to large regions of near-zero PV separating the multi-level vortices. This occurs because weak PV features in the initial field with scales smaller than $L_D$ undergo filamentation and are coarse-grained away or homogenised. For all values of $L_0/L_D$ considered, inverse cascades of potential energy commence at sufficiently late times. The onset of these cascades, even when the flow is initialised well within the ‘asymptotic model’ (AM) regime, suggests that the AM regime is not self-consistent: when potential vorticity fronts are well-resolved, frontal dynamics eventually drive ongoing flow evolution.

Separating Energetic Internal Gravity Waves and Small‐Scale Frontal Dynamics

AbstractOceanic fronts with lateral scales less than 20 km are now known to be one of the major contributors to vertical heat fluxes in the global ocean, which highlights their potential impact on Earth's climate. However, frontal dynamics with time scales less than 1 day, whose contribution to vertical heat fluxes is thought to be significant, are obscured by energetic internal gravity waves (IGWs). In this study, we address this critical issue by separating IGWs and frontal dynamics using an approach based on their respective vertical scales of variability. Results using a numerical model with a horizontal grid spacing of 500 m confirm that it is possible to recover frontal dynamics at short time scales as well as associated intense vertical velocities and vertical heat fluxes. This opens up new possibilities for a more accurate estimation of the vertical exchanges of any tracers between the surface and the ocean interior.

New geomorphic evidence for a multi‐stage proglacial lake associated with the former British–Irish Ice Sheet in the Vale of Pickering, Yorkshire, UK

ABSTRACTFormer proglacial lakes provide insight into the temporal and spatial evolution of past ice sheet dynamics and the glaciohydrological systems associated with them. Glacial Lake Pickering was a proglacial lake in North Yorkshire impounded by the British–Irish Ice Sheet. However, geomorphic evidence for its existence and extent is weak and contested. This paper presents, for the first time, systematic mapping of geomorphic features from high‐resolution (50 cm to 5 m) digital terrain model data and undertakes palaeogeographical reconstructions to establish Palaeolake Pickering's extent and levels. Results show only weak evidence for a 70‐m lake. This and the fact that the present landscape configuration is unable to impound such a large lake suggests that it either did not exist or is much older and that considerable landscape erosion has since taken place. There is good geomorphological evidence to support lake levels between 45 and 20 m. The new palaeogeographical reconstructions show these lake levels were initially controlled by the balance between the North Sea and the Vale of York Lobes of the British–Irish Ice Sheet with lower and later levels impounded by the Filey moraine. A multi‐stage Palaeolake Pickering with various extents and levels possibly spanning more than one glacial period has implications for other British–Irish Ice Sheet palaeolake extents reconstructed only from undifferentiated lacustrine sediments and their maximal elevation. The existence of an extensive multi‐phase proglacial Palaeolake Pickering along with other similar ones in Eastern England would have impacted mass loss and frontal dynamics via subaqueous melting and iceberg calving, potentially playing a role in controlling the dynamic behaviour of the North Sea Lobe.

Frontal Dynamics in the Alboran Sea: 2. Processes for Vertical Velocities Development

AbstractSignificant lateral density gradients occur throughout the year in the Alboran Sea, giving rise to two main fronts: the Western Alboran Gyre Front (WAGF) and Eastern Alboran Gyre Front (EAGF), where large vertical velocities often develop. To improve the understanding of the processes that underlie the development of the vertical velocities in the fronts, the periods of development were analyzed in the perspective of the frontogenesis, instabilities, non‐linear Ekman, and filamentogenesis mechanisms, using multi‐platform in‐situ observations and a high‐resolution realistic simulation in spring 2018. The spatio‐temporal characteristics of the WAGF indicate a wider, deeper, and longer‐lasting front than the EAGF. Additionally, the WAGF shows stronger and deeper upwelling and downwelling regions. The WAGF vertical velocities (up to |55| m/day) are amplified by an across‐front ageostrophic secondary circulation generated by: (a) frontal intensification explained by frontogenesis, which shows a sharpening of buoyancy gradients associated with the Atlantic Jet, (b) nonlinear Ekman effects, that are enhanced by the persistent western wind blowing along the frontal direction, and (c) submesoscale instabilities (symmetric and ageostrophic baroclinic instabilities). The EAGF vertical velocities (up to |38| m/day) are amplified by two asymmetrical ageostrophic cells developed across the front with a narrow upwelling region in the middle. The cell's circulation is explained by frontal intensification produced by filamentogenesis through a cold filament advection to the Mediterranean Sea interior, that is characterized by pointy isopycnals at the center of the filament. This mechanism is observed in both the model and glider observations.

A Lagrangian approach to the Atlantic Jet entering the Mediterranean Sea: Physical and biogeochemical characterization

The temporal evolution of the physical and biogeochemical properties of the Atlantic Jet (AJ) along the first ∼ 75 m of the water column during a 4-day journey was analysed by following the trajectory of a drifter dragged by the jet from the Strait of Gibraltar towards the Alborán Sea. Three stages were differentiated based on the evolution of several variables (e.g., velocity, temperature, nutrients, fluorescence). (i) Within the Strait of Gibraltar, the water column was primarily influenced by the tidal cycle, leading to a nutrient-enrichment of surface waters. However, due to the short residence time, the phytoplankton community that was mainly dominated by diatoms, did not demonstrate significant changes. (ii) Once outside the Strait, the drifter trajectory was mainly influenced by the frontal dynamics associated with the AJ. The drifter moved forward along the jet but also laterally across it and was continuously attracted to the mainstream (maximum current speed) or detached to its southern edge (minimum current speed). Due to the associated upwelling processes induced by the intensification of the current along the mainstream, the water column was characterized by colder, nutrient-richer water and lower fluorescence values. Conversely, along the southern edge of the jet, the water column was characterized by higher temperature, low nutrient concentration, and higher fluorescence. Along the first stations of this stage, diatom total abundance and biovolume continuously increased, reaching values ∼ 12-times higher than the initial concentrations. (iii) In the last stage, the water parcel was still influenced by the frontal dynamics but with less intensity. Additionally, the colder and denser water of the AJ and the associated phytoplankton community subducted progressively as it moved into the region surrounded by warmer waters. Concomitantly, fluorescence and diatoms total abundance and biovolume decreased and were influenced by the decline of nutrient availability and the increase of mesozooplankton. Our results reveal the coupled processes induced by the entrance of the AJ in the Alborán Sea and highlight the strong control of the physical environment over the ecological processes in this region. • The temporal evolution of the Atlantic Jet was analysed along a Lagrangian experiment . • The physical environment strongly controls the ecological processes. • Nutrient availability was determined by several hydrodynamic processes. • Diatoms, proving their adaptability, dominated the phytoplankton community.