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High- and medium-resolution satellite optical imagery show that submesoscale cyclonic eddies (SCEs) trap coastal waters and induce their rapid cross-shelf transport. Due to the presence of a rigid boundary, the convergence is observed in the coastal part of SCEs. It causes accumulation of suspended matter, which spins inward in a spiral motion toward the SCE core. Small SCEs with a radius of 1–10 km transport waters with local anomalies in the concentration of chlorophyll, total suspended matter and temperature to a distance of up to 150 km and are observed for more than 10 days. Lagrangian calculations based on realistic NEMO numerical model are used to estimate the fate of the coastal waters in such SCEs. The eddy entrains the largest number of particles during its separation from the coast when its vorticity reaches the maximum. Then, the SCE weakens, which is accompanied by the flattening of initially risen isopycnals and deepening of the trapped coastal waters. The described mechanism shows that coastal SCEs may cause intense short-period cross-shelf transport of the biological and chemical characteristics, and is another process affecting the functioning of the marine ecosystems. [ABSTRACT FROM AUTHOR]

Eddies play a crucial role in shaping ocean dynamics by affecting material transport, and generating spatio‐temporal heterogeneity. However, how eddies at different scales modulate biogeochemical transformation rates remains an open question. Applying a multi‐scale decomposition to a numerical simulation, we investigate the respective impact of mesoscale and submesoscale eddies on nutrient transport and biogeochemical cycling in the California Current System. First, the non‐linear nature of nutrient uptake by phytoplankton results in a 50% reduction in primary production in the presence of eddies. Second, eddies shape the vertical transport of nutrients with a strong compensation between mesoscale and submesoscale. Third, the eddy effect on nutrient uptake is controlled by the covariance of temperature, nutrient and phytoplankton fluctuations caused by eddies. Our results shed new light on the tight interaction between non‐linear fluid dynamics and ecosystem processes in realistic eddy regimes, which remain largely under‐resolved by global Earth system models. Plain Language Summary: Eddies play a crucial role in shaping ocean dynamics by affecting material transport, and generating spatio‐temporal heterogeneity. However, how eddies at different scales modulate biogeochemical transformation rates remains an open question. We address this question in the California Current System, an eddy‐rich, highly productive region. Our method is based on a state‐of‐the‐art high resolution numerical simulation, and employs a multi‐scale decomposition. With this approach, we highlight several key findings that expand our understanding of ocean dynamics and biogeochemistry. First, the transport of nutrients by mesoscale and submesoscale eddies can greatly differ along a gradient of surface nutrient concentrations and circulations. Second, biological nutrient uptake is reduced by about 50% in the presence of eddies. Third, this eddy effect on nutrient uptake is controlled by the covariance of temperature‐nitrogen‐phytoplankton fluctuations at the scale of eddies. These findings challenge the adequacy of non‐eddy resolving models to accurately represent large‐scale biogeochemical transformations. Since the dynamics of pelagic ecosystems is govern by a variety of non‐linear processes, from food‐web interactions to respiration and microbial dynamics, eddy rectification effects could greatly alter ecosystem dynamics and marine habitats, especially in environments naturally sensitive to multiple stressors such as ocean acidification, warming and de‐oxygenation. Key Points: In the California Current, subduction by submesoscale eddies near the coast and mesoscale eddies offshore reduces surface nutrientsIn the presence of submesoscale eddies, the non‐linear nature of nutrient uptake decreases primary production by up to ∼50%The amplitude and sign of eddy nutrient uptake is controlled by the covariance of temperature, nutrient and phytoplankton fluctuations [ABSTRACT FROM AUTHOR]

The North Pacific Gyre Oscillation (NPGO), Pacific Decadal Oscillation, and El Niño‐Southern Oscillation are climate fluctuations in the Pacific. Mesoscale eddies dominate the ocean kinetic energy spectrum. By analyzing the altimetry data, a close linkage between these climate fluctuations and eddy propagation speed is found. Propagation speeds of both cyclonic and anticyclonic eddies in the regions of 10° ∼ 20°N, 120°–180°E and 35° ∼ 45°N, 160°E−160°W are faster in the NPGO negative phase than positive phase, while those in the area of 25° ∼ 35°N, 150°E−170°W are opposite. These differences between positive and negative phases are the response of large‐scale circulation modulated by NPGO. When these climate fluctuations support the same westward/eastward anomaly of circulation, the eddies can propagate much faster/slower than that only considering NPGO. Causality analysis verifies that climate fluctuations can have strong influences on eddy movement. These climate fluctuations can affect the water mass, heat and salt transport by eddy advection. Plain Language Summary: Satellite altimetry is used to investigate the effects of three climate fluctuation including North Pacific Gyre Oscillation, Pacific Decadal Oscillation, and El Niño‐Southern Oscillation on the westward movement of the eddies. It is found that the climate fluctuations affect the eddy westward propagation by regulating the strengthen of large‐scale ocean circulation. Moreover, when the three climate fluctuations support the same westward/eastward anomaly of eddy movement, the intensity of the regulation is greater than that of the single one. Causality analysis also suggests the potential importance of large‐scale climate fluctuations in modulating the eddy motion. This study further indicates that mass, heat and salt transport can be affected by eddy advection modulated these large‐scale climate fluctuations. Key Points: Mesoscale eddies can move faster/slower as a result of the large scale ocean circulation induced by the climate fluctuationsCausality analysis verifies that climate fluctuations can modulate eddy propagation speedLarge scale climate fluctuations can affect the mass, heat and salt transport by eddy advection in the different climate phases [ABSTRACT FROM AUTHOR]

We propose an isoneutral eddy transport diagnostic framework and apply it in the Southern Ocean to analyze eddy potential vorticity (PV) transport dynamics. We leverage a thickness‐weighted spatiotemporal scale separation to define the generalized eddy flux. A local sampling method is employed and validated to estimate the transport tensor. We introduce Leonard's decomposition in Large Eddy Simulations (LES) to split the process into the Reynolds and non‐Reynolds parts. We also use the stationary‐transient decomposition. The non‐Reynolds term and the stationary effect contribute significantly to the transport properties. The Reynolds and transient parts have distinctions, that show oceanic mesoscale dynamics are not synergistic in time and spatial scales. We investigate the spatial scale dependence of each part and find that the transient and stationary components of the Reynolds (non‐Reynolds) term have consistent (dissimilar) scale dependence. The scale dependence of the generalized eddy flux and the transport eigenvalues is controlled by the non‐Reynolds term. We further investigate the eigenvalues in terms of the potential enstrophy and anisotropy. The potential enstrophy in the Southern Ocean holistically has a forward cascade on the researched scales. The Reynolds and transient effects efficiently promote the forward enstrophy cascade. The non‐Reynolds and stationary effects stimulate drastically anisotropic eddy transport. The PV gradient barrier mechanism dominates the drastic anisotropic case. When the anisotropy is weak, a special anisotropic case with a locally balanced potential enstrophy budget overwhelms the case of real isotropy. We also introduce several LES concepts, hoping to enlighten any potential ocean modeling and parameterization. Plain Language Summary: In this study, a 2D spatial filter is applied to define the sub‐grid scale signals or approximately mesoscale eddying signals as the deviation from the filtered large‐scale field. Then, a Reynolds temporal average is used to divide the eddying effect into transient and stationary parts. Since we use a spatial filter to separate the scales, the form of the eddy flux needs to be generalized and a non‐Reynolds part emerges. This part is invisible under the traditional Reynolds average (e.g., zonal average, time average, ensemble average). We introduce Leonard's decomposition from large eddy simulations to decompose the generalized eddy flux into non‐Reynolds and Reynolds parts and to categorize the interactions of eddies with various sizes. We utilize a flux‐gradient relationship to link the generalized eddy flux with the large‐scale tracer gradient through a locally sampled anisotropic transport tensor. Both decompositions and the transport tensor help provide insights into the multiscale nature of mesoscale eddy PV transport dynamics and prompt a theoretical understanding of designing potential LES‐related parameterizations for ocean models. Key Points: A spatiotemporal thickness‐weighted isoneutral eddy transport diagnostic frame‐work is proposed and applied in the Southern OceanThe stationary‐transient and Leonard's decomposition are leveraged to analyze the multiscale effects on eddy potential vorticity transportThe Reynolds and transient effects promote the forward enstrophy cascade, and the non‐Reynolds and stationary effects stimulate anisotropy [ABSTRACT FROM AUTHOR]

The strong interaction between the Brazil Current and the adjacent shelf is clearly visible in satellite‐derived products (sea surface temperature, salinity, and chlorophyll‐a concentration). Assessments of circulation features and cross‐shelf exchanges from these products are, however, limited to the surface layer. Here we analyze the regional circulation and dynamics using the results of a suite of process‐oriented, high‐resolution numerical experiments. Passive tracers and Lagrangian floats characterize the exchanges between the shelf and the open ocean, identifying regions of high variability, and assessing the contribution of small‐scale eddies to the cross‐shelf mass exchanges. We estimate that 0.2–0.4 Sv of the shelf transport variability between 34°S and 25°S comes from ocean internal variability which represents ∼50%–70% of the total variability. Between 25°S and 21°S, internal ocean variability represents more than 90% of the shelf transport variability. We find that generation of cyclonic eddies is more frequent (>15% of the time) at the shelfbreak bights. The core of these eddies contains fresher, colder, and more nutrient‐rich shelf waters. Maps of satellite chlorophyll‐a concentration suggest that the horizontal and vertical exchanges of mass associated with these eddies are a critical element of the primary production cycle. Plain Language Summary: We use the results of a suite of ocean model experiments to investigate the variability of the shelf circulation off southern Brazil and Uruguay and the exchange between the shelf and the open ocean. Ocean internal variability (e.g., from eddies) represents more than 50% of the total shelf transport variability and reaches values up to 90% in the northern shelf region. Supported by maps of satellite chlorophyll‐a and using model passive tracers and Lagrangian floats, we also find that eddies along the shelfbreak play an important role in the advection of colder, fresher, and more nutrient‐rich shelf water to the open‐ocean. This is particularly significant where the shelfbreak changes its orientation (shelfbreak bights) where cyclonic small‐scale eddies are detected more than 15% of the time. Key Points: The variability of the shelf circulation and cross‐shelf exchanges off Southern Brazil and Uruguay is investigated using model experimentsMore than 50% of the total shelf transport variability is explained by internal ocean variabilityEddies along the shelfbreak play an important role in the advection of colder, fresher, and nutrient‐rich shelf water to the open‐ocean [ABSTRACT FROM AUTHOR]

Simple analytic models developed in this study are applied to long‐term averages of reanalysis surface salinity data to quantify two fundamental properties of ocean currents. The first model is based on the new Freshening Length schema and its application to the Irminger Current yields a ratio of about 5 between the turbulent entrainment rates of surrounding fresher surface waters west and east of Greenland. The second model is based on the steady solution of the advection‐diffusion equation subject to suitable boundary conditions. The application of this model to the spreading of fresh, snow‐melt, water from the delta of the Po river in the northwest Adriatic Sea into the rest of the Sea yields a ratio of 8 × 104 m between the eddy exchange coefficient and the speed of advection in the Sea. Plain Language Summary: Differences in ocean water salinity were used for over a century to quantify the horizontal fluxes in and out of evaporative, motionless, basins such as the Mediterranean Sea. In the present study we develop simple expressions based on analytic models that extend the century‐old approach to ocean currents where the water is constantly moving rather than remaining stagnant. The models developed here are combined with long‐term data of sea surface salinity along two currents—the salty Irminger Current that flows around the southern tip of Greenland and the flow of fresh snow‐melt water from the Po river into the Adriatic Sea. The models and climatological data used here yield quantitative estimates of two basic parameters: (a) the rate at which a high‐salinity current detrains salt to the surrounding ocean. (b) The balance between the slow downstream propagation and eddy (turbulent) exchange coefficient. The models developed in this study can be applied to other currents and regions of the world ocean. Key Points: In some oceanic circumstances, changes in Sea Surface Salinity (SSS) gradient provide a simple, reliable and robust diagnostic of ocean currentsChanges in the entrainment rate of surrounding water into a current, correspond to observable changes in SSS gradientReanalysis SSS data quantify the ratio between the speed of advection and the eddy exchange coefficient in slow currents [ABSTRACT FROM AUTHOR]