Eddy Generation Research Articles

Multiscale roughness influence on hydrodynamic heat transfer in a single fracture

This study investigates the multiscale roughness effects on hydrodynamic heat transfer (or heat transfer associated with water flow) in a single fracture, based on direct numerical simulations of fluid flow and heat transfer in fracture-matrix systems with or without secondary roughness considered. The simulation results show that the primary roughness controls the low velocity regions and the overall flow direction in the fracture, while the secondary roughness enhances the complexity of the flow near the local sharp cornered asperities and promotes the generation of eddies. The changes of flow behavior could in turn profoundly affect the water temperature distribution and local heat transfer coefficient in the fracture-matrix system. In addition, two competing mechanisms impacting the overall heat transfer coefficient (h) due to the multiscale roughness are revealed. On one hand, the increase in flow velocity induced by primary and secondary roughness increases h. On the other hand, the eddy zone induced by the secondary roughness can trap the high-temperature fluid and inhibit the heat transfer from the fracture surface to the ambient fluid, which could decrease h. The macroscopic effect of surface roughness on h is the result of the joint action of these two competing influence mechanisms.

Coherent Eddies Transporting Passive Scalars Through the Plant Canopy Revealed by Large-Eddy Simulations Using the Lattice Boltzmann Method

A double-distribution-function lattice Boltzmann model for large-eddy simulations of a passive scalar field in a neutrally stratified turbulent flow is described. In simulations of the scalar turbulence within and above a homogeneous plant canopy, the model’s performance is found to be comparable with that of a conventional large-eddy simulation model based on the Navier–Stokes equations and a scalar advection–diffusion equation in terms of the mean turbulence statistics, budgets of the second moments, power spectra, and spatial two-point correlation functions. For a top-down scalar, for which the plant canopy serves as a distributed sink, the variance and flux of the scalar near the canopy top are predominantly determined by sweep motions originating far above the canopy. These sweep motions, which have spatial scales much larger than the canopy height, penetrate deep inside the canopy and cause scalar sweep events near the canopy floor. By contrast, scalar ejection events near the canopy floor are induced by coherent eddies generated near the canopy top. The generation of such eddies is triggered by the downward approach of massive sweep motions to existing wide regions of weak ejective motions from inside to above the canopy. The non-local transport of scalars from above the canopy to the canopy floor, and vice versa, is driven by these eddies of different origins. Such non-local transport has significant implications for the scalar variance and flux budgets within and above the canopy, as well as the transport of scalars emitted from the underlying soils to the atmosphere.

Dynamics of Tidal and Residual Currents Based on Coastal Acoustic Tomography Assimilated Data Obtained in Jiaozhou Bay, China

AbstractTo investigate the dynamics of tidal and residual currents in a shallow bay with narrow bay mouth and large tidal flat area, this study conducted three sets of coastal acoustic tomography (CAT) observations at 13 stations in deep area of Jiaozhou Bay (JZB), China, from December 6–18, 2015. Thereafter, travel time difference data observed via CAT were assimilated into a barotropic tidal model to realize an extrapolated study in tidal flats. The root mean square difference was 6.5 cm/s between assimilation and acoustic Doppler current profile data. Harmonic analyses indicated that the semidiurnal tide (M2) was predominant; however, relatively large nonlinear tidal currents (M4 and M6) were also found. Moreover, the spatial mean amplitudes of M2, M4, and M6 were 0.35 (excluding the tidal flat), 0.09, and 0.04 m/s, respectively. Dynamic analyses showed that continuity and advection terms were the main contributors to M4 in the bay mouth, whereas M4 in the tidal flats was generated by continuity and friction terms. Furthermore, bottom friction was the primary contributor to M6. In addition, residual currents with multi‐vortex structures appeared near the bay mouth, the maximum speed reached 0.60 m/s, and advection, continuity, and bottom friction contributed more to the generation of residual current eddies. Horizontal pressure gradient of residual elevation and advection of tidal and residual currents were the dominant factors maintaining the residual current near the bay mouth. The generation mechanisms of M4 and M6 were first discussed based on the assimilation of the current field data in JZB.

Tridimensional nonhydrostatic transient rip currents in a wave-resolving model

Flash rips and surf eddies are transient horizontal structures of the order of 10 to 100 m, which can be generated in the surfzone in the absence of bathymetric irregularities. They are traditionally evaluated in a depth averaged setting which involves intrinsic horizontal shear instabilities and the direct generation of vorticity by short-crested waves. In this article, we revisit the processes of surf eddy generation with a new three-dimensional wave resolution model (CROCO) and provide a plausible demonstration of new 3D non-hydrostatic instability and turbulent cascade. We first present a quick overview of a compressible free surface approach suitable for nearshore dynamics. Its ability to simulate the propagation of surface gravity waves and nearshore wave-driven circulation is validated by two laboratory experiments. Next, we present a real world application from Grand Popo Beach, Benin, forced by waves with frequency and directional spreading. The generation of surf eddies by the 3D model differs from depth-averaged models, due to the vertical shear associated with shallow breaking waves. In this case, the generation of eddies from both horizontal shear instability and the breaking of short-crested waves is hampered, the former by stretching the alongshore current and the latter by inhibiting the inverse energy cascade. Instead, the vertical shear flow is subjected to forced wave group variability and Kelvin–Helmholtz type instability at an inflection point. Primary and secondary instabilities generate spanwise and streamwise vorticity connecting small-scale eddies to larger horizontal surfzone structures. Streamwise filaments, appearing as 5 m wide ribs or mini-rips, can extend beyond the surfzone but with moderate energy. These results appear consistent with the velocity spectra and observed patterns of tracers and suspended sediments at Grand Popo Beach. The timescale associated with the mean shear-induced turbulence is several times the wave period and suggests an intermediate range between breaker-induced turbulence and large-scale surf eddies.

Wind speed and mesoscale features drive net autotrophy in the South Atlantic Ocean

A comprehensive in situ dataset of chlorophyll a (Chl a; N = 18,001), net primary production (NPP; N = 165) and net community production (NCP; N = 95), were used to evaluate the performance of Moderate Resolution Imaging Spectroradiometer on Aqua (MODIS-A) algorithms for these parameters, in the South Atlantic Ocean, to facilitate the accurate generation of satellite NCP time series. For Chl a, five algorithms were tested using MODIS-A data, and OC3-CI performed best, which was subsequently used to compute NPP. Of three NPP algorithms tested, a Wavelength Resolved Model (WRM) was the most accurate, and was therefore used to estimate NCP with an empirical relationship between NCP with NPP and sea surface temperature (SST). A perturbation analysis was deployed to quantify the range of uncertainties introduced in satellite NCP from input parameters. The largest reductions in the uncertainty of satellite NCP came from MODIS-A derived NPP using the WRM (40%) and MODIS-A Chl a using OC3-CI (22%).The most accurate NCP algorithm, was used to generate a 16 year time series (2002 to 2018) from MODIS-A to assess climate and environmental drivers of NCP across the South Atlantic basin. Positive correlations between wind speed anomalies and NCP anomalies were observed in the central South Atlantic Gyre (SATL), and the Benguela Upwelling (BENG), indicating that autotrophic conditions may be fuelled by local wind-induced nutrient inputs to the mixed layer. Sea Level Height Anomalies (SLHA), used as an indicator of mesoscale eddies, were negatively correlated with NCP anomalies offshore of the BENG upwelling fronts into the SATL, suggesting autotrophic conditions are driven by mesoscale features. The Agulhas bank and Brazil-Malvinas confluence regions also had a strong negative correlation between SLHA and NCP anomalies, similarly indicating that NCP is forced by mesoscale eddy generation in this region. Positive correlations between SST anomalies and the Multivariate ENSO Index (MEI) in the SATL, indicated the influence of El Niño events on the South Atlantic Ocean, however the plankton community response was less clear.

Eddy formation in the bays of Kamchatka and fluxes to the open ocean

The Eastern Kamchatka Current (EKC) is the western boundary current of the North Pacific subpolar gyre. Southeast of the Kamchatka Peninsula lies a large anticyclonic eddy, the Kamchatka Eddy (KE). This eddy is quasi-stationary. More generally, the oceanic region east of the EKC contains many eddies, several of them large and long lasting. Using surface currents derived from altimetry, particle tracking and a simple two-dimensional numerical model of fluid flow, we investigate the variability of this eddy field, the generation of eddies in the bays of Kamchatka by the EKC and fluxes of water to and from these bays. Firstly, we recover in our analysis of long-lasting eddies, the main eddies of the region. Among strong eddies, the parity bias favors anticyclones. Our numerical simulations give a possible explanation for the process of eddy creation in the bays of the peninsula and show that the northernmost bay produces most anticyclones. Then, we track forward the water particles from these bays and we determine their fate in the open ocean; southeastward and southwestward trajectories are the most frequent. We also track water particles backward from the KE site; they often drift near the Kamchatka coast, but others drift south of this site and remain there, a priori trapped in other eddies. This study confirms the complexity of mesoscale motions and water exchanges in this region.

A new linear forcing method for isotropic turbulence with controlled integral length scale

Turbulence is a common feature to all flows that surround us. Despite its ubiquity, particularly in industrial flows, it is very difficult to provide a mathematical framework for the generation of turbulent eddies. Several methods have been proposed which are able to reproduce realistic features for velocity fluctuations, exhibiting proper space- and time-correlations. Focusing on physical space forcing, these methods are usually first evaluated upon sustained homogeneous isotropic turbulence by introducing a body force to the Navier–Stokes equations. Since the pioneering work of Lundgren, these techniques usually experience difficulties in predicting the integral length scale. The present study provides a forcing through a reconstruction approach which consists in building velocity fluctuations with a prescribed energy spectrum model. The proposed approach is assessed by performing large-eddy simulations of a sustained homogeneous isotropic turbulence in a triply periodic box. Properties of this forcing technique are discussed, drawing on both spatial and time correlations and also on the shape of energy spectrum together with the level of resolved turbulent kinetic energy. A special attention is put on the control of resolved turbulent energy. In this framework, an efficient selective forcing technique is derived, making use of spectral space features. The results show that the proposed approach allows to drive efficiently the resolved kinetic energy toward its target value while preserving the integral length scale independent of the domain size. It is observed that the resulting longitudinal length scale is overestimated by 13%, while the two-time correlations are recovered when using stochastic frequencies.

Spatiotemporal Variability of Mesoscale Eddies in the Indonesian Seas

Mesoscale eddies are ubiquitous in the world ocean and well researched both globally and regionally, while their properties and distributions across the whole Indonesian Seas are not yet fully understood. This study investigates for the first time the spatiotemporal variations and generation mechanisms of mesoscale eddies across the whole Indonesian Seas. Eddies are detected from altimetry sea level anomalies by an automatic identification algorithm. The Sulu Sea, Sulawesi Sea, Maluku Sea and Banda Sea are the main eddy generation regions. More than 80% of eddies are short-lived with a lifetime below 30 days. The properties of eddies exhibit high spatial inhomogeneity, with the typical amplitudes and radiuses of 2–6 cm and 50–160 km, respectively. The most energetic eddies are observed in the Sulawesi Sea and Seram Sea. Eddies feature different seasonal cycles between anticyclonic and cyclonic eddies in each basin, especially given that the average latitude of the eddy centroid has inverse seasonal variations. About 48% of eddies in the Sulawesi Sea are highly nonlinear, which is the case for less than 30% in the Sulu Sea and Banda Sea. Instability analysis is performed using high-resolution model outputs from Bluelink Reanalysis to assess mechanisms of eddy generation. Barotropic instability of the mean flow dominates eddy generation in the Sulu Sea and Sulawesi Sea, while baroclinic instability is slightly more in the Maluku Sea and Banda Sea.

Dynamics of Eddy Generation in the Southeast Tropical Indian Ocean

AbstractSatellite observations indicate that the southeast tropical Indian Ocean (SETIO) is a region full of eddies, which are generated near 120°E and propagate westward/southwestward. The vertical structure of eddies shows that eddy‐induced temperature anomalies are mainly confined within the upper 200 m. The maximum anomaly is located at ∼100 m depth, near the depth of the thermocline, indicating that eddy generation is related to vertical motions of the thermocline. To further investigate the generation mechanisms of mesoscale eddies, we obtain a suite of solutions of the 1½‐layer reduced gravity model, which can capture the first baroclinic process quite well. The solutions suggest that wind forcing from the tropical Pacific Ocean and nonlinearity play important roles in eddy generation in the SETIO.

A dynamical systems characterization of atmospheric jet regimes

Abstract. Atmospheric jet streams are typically separated into primarily “eddy-driven” (or polar-front) jets and primarily “thermally driven” (or subtropical) jets. Some regions also display “merged” jets, resulting from the (quasi-)collocation of the regions of eddy generation with the subtropical jet. The different locations and driving mechanisms of these jets arise from very different underlying mechanisms and result in very different jet characteristics. Here, we link the current understanding of dynamical jet maintenance mechanisms, mostly arising from conceptual or idealized models, to the phenomena observed in reanalysis data. We specifically focus on developing a unitary analysis framework grounded in dynamical systems theory, which may be applied to both idealized models and reanalysis, as well as allowing for direct intercomparison. Our results illustrate the effectiveness of dynamical systems indicators to diagnose jet regimes.

Characteristics of mesoscale eddies of Arctic marginal seas: results of numerical modeling

A simple method is proposed for identifying mesoscale eddies in the results of numerical modeling. The method uses extrema in distributions of the sea level elevation as eddy markers. By using the method, we analyze statistics of mesoscale eddies resulting from SibPOM numerical simulations in the Eurasian sector of the Arctic marginal seas. The results of using this method show that the number of cyclonic eddies slightly exceeds the number of anticyclonic eddies, but the excess is only 2-3%. Also, we demonstrate that a significant number of eddies arise under the ice cover. The number of such eddies increases significantly in winter. This fact indicates that the convection caused by salt rejection during freezing plays an essential role in their formation. The numerical modeling results confirm the phenomenon of active eddy generation in the ice edge zone. Besides, the results show that in the near-edge zone, a more significant number of eddies are formed from the icy side adjacent to the edge, and not from the ice-free side. The periods of seawater freezing and ice melting, accompanied by corresponding displacements in the ice edge, produce eddies different in nature. The number of eddies in the marginal ice zone has two seasonal maxima corresponding to these two periods.

Variable Kuroshio Current intrusion into the northern South China Sea over the last 7.3 kyr

The branch intrusion of the Kuroshio Current (KC) into the northern South China Sea (SCS) has a significant effect on temperature, salinity, circulation, and eddy generation. However, the study of Holocene hydrological changes and the historical influence of the KC in this region is limited. Here, we report high-resolution sea surface temperature (SST) and δ18Osw (as a proxy for sea surface salinity, SSS) records based on paired Mg/Ca ratios and δ18O analyses of the planktonic foraminifera Globigerinoides ruber combined the Pulleniatina obliquiloculata and G. sacculifer/G. ruber ratios from core 05E306 (22°0.06′ N, 118°59.64′ E; water depth = 1440 m) in the northern SCS. By comparing these data with records from cores located within and outside of the main KC flow pathway, we evaluate the spatiotemporal evolution of the KC in this region over the last 7.3 kyr. The strongest influence was observed from 6.1 to 5.3 ka, recorded as an abrupt increase in SST and SSS with remarkably increased P. obliquiloculata abundance (37.4%) and G. sacculifer/G. ruber ratios (3.18). During the following period, 5–2.5 ka, SST decreased by 0.4–1.1 °C with an associated decrease in δ18Osw, indicating a weakened KC. After 2.5 ka, the influence of the KC on the northern SCS strengthened; however, the high SST record still exhibits a spatial constraint in the longitude distribution of the KC during this period of strongest intrusion. Moreover, the abundance of planktonic foraminifera in the study area more strongly correlates with SSS than SST. The δ18Osw records in core 05E306 show a freshening trend after 5.7 ka, which is inconsistent with the declining East Asian Summer Monsoon precipitation record but consistent with the Holocene freshening trend recorded in the Western Pacific. This suggests that hydrological variability of the northern SCS is mainly influenced by the Western Pacific Ocean.

Multi-Source Data Analysis of Mesoscale Eddies and Their Effects on Surface Chlorophyll in the Bay of Bengal

Mesoscale eddies are important to ocean circulation due to their roles in the transport of mass, energy, and heat. This study employs a combination of data sources to initiate a statistical analysis of eddy spatiotemporal characteristics in the Bay of Bengal (BOB) to elucidate the sea surface and vertical structures of the eddies and their impacts on sea surface chlorophyll (Chl) distributions. The results suggest that 1237 cyclonic eddies (CEs) and 1121 anticyclonic eddies (AEs) were detected in 26 years. The number of two eddy polarities was almost the same, and most of them spread to the west or southwest direction. The vertical change of temperature (T) and salinity (S) caused by the eddies is studied and the anomalous eddies, i.e., a CE (AE) eddy with warm (cold) water at the center, are mainly distributed on the northeast side of the Island of Sri Lanka. Furthermore, CEs are found to increase Chl concentration in the surrounding sea by approximately 11.15%, while AEs decrease concentrations also by approximately 11.25%. Changes in Chl concentrations occur most rapidly during the mature and intensification eddy phases. Observations also indicate that the strong local current and wind fields are the primary mechanisms in eddy generation.

Influence of the free surface on hydrodynamics in a bubble column

In this paper, the influence of the free surface on hydrodynamics is investigated numerically for a pilot-scale bubble column. The free surface is modelled by means of the Volume of Fluid method (VOF), whereas the gas entities are represented by means of the Direct Simulation Monte Carlo (DSMC) method. The VOF method is parallelized using MPI and incorporated in already parallel in-house numerical code for DSMC, FoxBerry. The results obtained with the modeled free surface are compared with the reference simulation, in which the free surface is mimicked using the outlet slits, placed at the top of the column. The results show a significant impact of the free surface on generation of the small-scale eddies. Moreover, the presence of the free surface reduces the gas-holdup in the domain.

Submesoscale Dynamics in the Gulf of Aden and the Gulf of Oman

We have investigated the surface and subsurface submesoscale dynamics in the Gulf of Aden and the Gulf of Oman. Our results are based on the analyses of regional numerical simulations performed with a primitive equation model (HYCOM) at submesoscale permitting horizontal resolution. A model zoom for each gulf was embedded in a regional mesoscale-resolving simulation. In the Gulf of Aden and the Gulf of Oman, the interactions of mesoscale structures and fronts instabilities form submesoscale eddies and filaments. Rotational energy spectra show that the Gulf of Aden has a higher ratio of submesoscale to mesocale energy than the Gulf of Oman. Fast waves (internal gravity waves, tidal waves, Kelvin waves) and slow waves (Rossby waves) were characterized via energy spectra of the divergent velocity. Local upwelling systems which shed cold filaments, coastal current instabilities at the surface, and baroclinic instability at capes in subsurface were identified as generators of submesocale structures. In particular, the Ras al Hamra and Ras al Hadd capes in the Gulf of Oman, and the Cape of Berbera in the Gulf of Aden, are loci of submesoscale eddy generation. To determine the instability mechanisms involved in these generations, we diagnosed the Ertel potential vorticity and the energy conversion terms: the horizontal and vertical Reynolds stresses and the vertical buoyancy flux. Finally, the impacts of the subsurface submesoscale eddy production at capes on the diffusion and fate of the Red Sea Water (in the Gulf of Aden) and the Persian Gulf Water (in the Gulf of Oman) are highlighted.

Performance improvement of mixed-flow centrifugal pumps with new impeller shrouds: Numerical and experimental investigations

A new impeller configuration with innovatively-designed shrouds is introduced, and its impact on the pressure head and efficiency of the semi-open centrifugal pump is explored using experimental and numerical approaches. The proposed design includes attaching specific plates called bladelets to a semi-open impeller to limit the secondary flows over the blades resulting in an increase in the pump head and efficiency. Hydraulic performance of a centrifugal pump with three different bladelets at angles of 30°, 60°, and 90° is investigated experimentally, and the results are compared to semi-open and closed impellers. The values of pressure head and efficiency of the proposed centrifugal pump are determined through numerical analysis and then compared to the experimental data in which an acceptable agreement between the results is observed. Both numerical and experimental results show the proposed impeller design promisingly improves the pressure head and efficiency of semi-open centrifugal pumps. Additionally, the flow analysis demonstrates that the impellers with 90° bladelets enhance the hydraulic performance of a semi-open centrifugal pump nearly to a closed impeller configuration. The loss reduction due to the secondary flow and the eddy generation leads to an improvement in the hydraulic performance of the proposed design.

Strengthening and Lengthening of the Hawaiian Lee Countercurrent Driven by the Pacific Trade Wind Acceleration

AbstractThe Hawaiian Lee Countercurrent (HLCC) is a zonal eastward flowing current driven by the orographic wind stress curl (WSC) dipole on the leeside of the Hawaiian Islands, which plays an important role in the regional air‐sea interactions. During the late 1990s and early 2010s, the Pacific trade winds exhibited an unprecedented acceleration with respect to the past century, thus implying a strengthening of the HLCC. Based on satellite observations, we found that the mean velocity and length of the HLCC increased by 53% and 97%, respectively, during the period of the Pacific trade wind acceleration (1999–2014) in comparison to the mean values during the weakened years (1993–1998 and 2015–2018). Correspondingly, the mean heat transport by the HLCC across the International Date Line during the strengthened years increased by 80% in comparison to that before 1999 and after 2014. The sea level anomaly (SLA) variation to the south of the HLCC was found to be the main factor controlling the changes in the HLCC. Local (i.e., near the Hawaiian Islands) and remote WSC both contributed to the SLA variation by exciting westward propagating oceanic Rossby waves, although the former was dominant. Our results also indicate that the dramatically changing HLCC plays an important role in eddy generation along the HLCC path.

A case study of Chlorophyll a response to tropical cyclone Wind Pump considering Kuroshio invasion and air-sea heat exchange

New evidences provided that the tropical cyclone (TC) Linfa in 2015 induced looping path of Kuroshio invasion into the northeastern South China Sea (NESCS) through the northwestern Luzon Strait (LS), based on the in-situ measurements, satellite data and model output data. This TC-enhanced Kuroshio invasion with low nutrients and denser waters suppressed the TC “Wind Pump” induced upwelling and nutrients uptake, and therefore inhibited the Chlorophyll a concentration (Chl-a) increase from surface to ~50 m in the open ocean of the NESCS. The TC-induced Kuroshio invasion promoted the generation of the strong cyclonic eddy to its left side where weak Ekman Pumping Velocity was observed. This enhancing cyclonic eddy then dominated the nutrients uplift and increased the surface and subsurface (0–50 m) Chl-a through eddy pumping rather than Ekman Pumping. The TC-declined anti-cyclonic eddy, which shoaled the Mixed Layer Depth (MLD), benefited to the nutrient uptake through TC-induced upwelling and thereby increased the surface Chl-a and raised the Chl-a Maximum Layer (CML) to ~20 m over the southwestern LS. The temporal Chl-a variations were also influenced by TC intensities and biochemical processes. The air-sea heat budget analysis indicated that, the air-sea heat exchange contributed to nearly 80% of the sea surface cooling (SST cooling) over the northwestern LS with Kuroshio invasion, while eddy-induced upwelling dominated the SST cooling over the western LS, and the wind-driven upwelling (and mixing) controlled the SST cooling over the southwestern LS. These different formations of SST cooling then played important role in Chl-a variations. This study is the first case of TC “Wind Pump” induced Chl-a variations considering air-sea heat exchange, Kuroshio invasion and mesoscale eddies over LS, which would help to evaluate the influence of TCs over the other major heat transport arteries of the world ocean: The Gulf Stream area.

Statistical analysis of long-lived mesoscale eddies in the Lofoten basin from satellite altimetry

The paper presents a statistical analysis of long-lived detected mesoscale eddies in the Lofoten Basin (LB). An automated eddy identification and tracking method is applied to the altimeter data during the time-period 1993–2017 to detect and track anticyclonic (ACEs) and cyclonic eddies (CEs) in the LB. Our analysis found that only one percent of the eddies detected are long-lived (eddy-life > 35 days). Even though only 1%, the detected 330 long-lived mesoscale eddies (CEs, 120; ACEs, 210) account for >11,550 daily individual eddy-observations. The lifetime, occurrence, generation sites, size, intensity, and drift of these long-lived eddies are quantified. The average drift speed of long-lived eddies is found to show a pronounced seasonal variation with a maximum from October to March. Long-lived eddies in the LB are further divided into four groups based on their region of generation and dissipation. Long-lived eddies generated and dissipated outside the region of the Lofoten Vortex (group 2 eddies) are found to be the predominant type (CEs, 73.3%; ACEs, 69.5%). The eddies generated and dissipated in the region of the permanent Lofoten Vortex form the second dominant type (CEs, 14.3%; ACEs, 26.2%). Based on their lifetime, properties of the two predominant groups of eddies are examined in detail. The difference found in the temporal variability of the eddy characteristics of the two groups reflects their different genesis. The analysis revealed that the mesoscale eddies of group 2 have a longer life than eddies of group 1, and ACEs are more long-lived in comparison to CEs. The analysis also found three main areas of eddy generation in the frontal zone of the NwASC from where mesoscale eddies propagate to the north-west, forming three main corridors of trajectories. The study further provides evidence of long-lived cyclonic CEs surrounding the large quasi-permanent Lofoten Vortex (LV) and forming a shield around it. Small CEs located in two areas with centers at 69.5° N, 4° E, and 70° N, 2.5° E survive nearby the LV resisting the vortex-vortex interaction with the large and strong LV.

Islands as eddy transformation and generation hotspots: Cabo Verde case study

Deep-ocean islands have long been associated with the generation of oceanic eddies in their wakes. However, their interaction with incoming eddies has seldom been considered. This study focuses on the characterization of background and locally generated mesoscale eddies in the Cabo Verde archipelago between 2003 and 2014. Special attention is given to the interaction of incoming eddies with the bathymetry of the islands, along with their impacts on the local generation of eddies. Island-induced wind-shear effects are also considered. In addition, some examples of the biological response to background and locally generated eddies are discussed. This is achieved by combining remote-sensing satellite observations for wind, sea surface height, and chlorophyll-a (Chla) surface concentrations. The results show that the interaction between incoming background eddies and the archipelago is a recurrent phenomenon, which results in eddy deflection, splitting, merging, intensification, and termination (sorted by highest to lowest number of occurrences). Local island-induced disturbances are also significant, mainly due to atmospheric effects. Such processes result in the generation of island-induced eddies and in wind-mediated eddy intensification and confinement, more often observed in the leeward group. Nonetheless, it is strongly suggested that many of the locally generated eddies are a direct product or by-product of the interaction of background eddies with the islands. With respect to the biological realm, a locally generated cyclonic eddy is observed to originate a pronounced phytoplankton bloom in the vicinity of the tallest island. Nonetheless, background eddies generated off the African coast are often associated with enhanced Chla concentrations when they intersect the archipelago. Such observations challenge the idea that local biological productivity in deep oceanic islands is exclusively driven by island-induced mechanisms.