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• Enhanced Kuroshio looping path across the Luzon Strait is observed during 1993–2020. • Eddy Kinetic Energy shows long-term increasing in the Northeastern South China Sea. • Energy from baroclinic instability is dominant for the EKE long-term increasing. The seasonal to interannual variability of eddy kinetic energy (EKE) in the Northeastern South China Sea (NE-SCS) has been widely studied and it is recognized that they are strongly related to the state of the Kuroshio pathway in the Luzon Strait. While, due to the lack of long-term observations and high-resolution simulations, the decadal change of EKE in NE-SCS remains unexplored. In this study, we show the EKE trend in the past ∼ 30 years in the NE-SCS by using satellite observation and global HYbrid Coordinate Ocean Model reanalysis with the Navy Coupled Ocean Data Assimilation. It is found that due to the weakening of the Kuroshio in the Luzon Strait since 1990 s, the Kuroshio shows an enhanced looping path in the NE-SCS, inducing stronger EKE in this region. Further analysis confirms that the energy transfer by baroclinic instability is dominant for the increasing of EKE, when the Kuroshio intrudes into the NE-SCS and brings more potential energy inside the circulation. The Kuroshio state along the Luzon Strait is the key for modulating the EKE in the NE-SCS. Furthermore, the long-term weakening of Kuroshio current along the Luzon strait during 1993–2020 is determined by the decreasing of subtropical mode water, corresponding to the positive phase of the Atlantic Multidecadal Oscillation. This study provides insight into the interaction between marginal sea (i.e., the SCS) and the open ocean (i.e., the western Pacific Ocean), finally linking to the global climate change. [ABSTRACT FROM AUTHOR]

• The EKE exhibits vigorous interannual variations in western equatorial Pacific. • The EKE variations exhibit different characteristics in different flavors of El Niño. • Barotropic instability is the main mechanism for EKE interannual variations. • Variations of barotropic instability is closely related to the background currents. The interannual variations of eddy kinetic energy (EKE) in the western equatorial Pacific Ocean are investigated based on satellite observations and model outputs in this study. Results reveal that the EKE exhibits vigorous interannual variations, especially in the region of North Equatorial Countercurrent (NECC) and north of New Guinea, and the variations differ between the two types of El Niño events. The energy budget diagnosis indicates that the EKE variations are mainly attributed to the barotropic instability associated with the background currents. Specifically, the energetic NECC behaves northward shift and a stronger meander path, which favors the enhancement of EKE variations due to the enhanced barotropic instability. The interannual fluctuations of the strength of the New Guinea Coastal Current/Undercurrent (NGCC/NGCUC) and the eastward current along the equator contribute to the significant EKE interannual variations north of New Guinea. Further, the distinct features of EKE variations in two types of El Niño events are as follows: EKE typically weakens in the western equatorial Pacific during Eastern Pacific El Niño (EP-El Niño) events, whereas it intensifies north of New Guinea during Central Pacific El Niño (CP-El Niño) events. The opposite features north of New Guinea are attributed to the wind work and a stronger eastward current along the equator in CP-El Niño events. These results can contribute to a better understanding of the low-frequency eddy-mean flow interactions. [ABSTRACT FROM AUTHOR]

FAV in pump sump has been a matter of focus for years for the major effects on the efficiency of the pump, wasting a lot of energy. In order to explore the mechanism of the dynamic characteristics of FAV and provide the help to eliminate FAV,the flow field under the bell of an axial flow pump was measured by V3V. The velocity gradient in the vortex area, vortex intensity, and eddy kinetic energy during the evolution of FAV were analyzed. The experimental results show that a large velocity gradient is a key factor to the formation of a vortex. The process of FAV formation and development is the process of vortex shedding. The evolution process of FAV can be divided into five stages: inception, development, continuance, collapse, and disappearance. The velocity gradient, vortex intensity, and eddy kinetic energy of FAV increase with time, reaching maximum values, maintain these values for 0.4s, and then decrease rapidly. The increase rate was less than decrease rate, which is different from the calculation results of the vorticity transfer equation. The vortex intensity meets the vortex tube intensity conservation theorem in the continuance stage. The research results can provide reference value for the design of pump station and vortex elimination, and then realize the energy saving. • The 3D velocity fields of the floor-attached vortices were measured by using V3V. • The floor-attached vortices evolution has development, continuance, collapse stages. • The key factor of formation of floor-attached vortices was large velocity gradient. • Intensity of vortices meets vortex tube intensity conservation in continuance stage. • The intensity increase rate of floor-attached vortices is less than decrease rate. [ABSTRACT FROM AUTHOR]

The Slope Sea is the dynamic ocean region located between the United States and Canadian northeast continental shelves and the northeastward flowing Gulf Stream (GS) located farther offshore. Here we define it as located between the 200-m isobath and the monthly GS sea surface temperature (SST) front from −75° to −55° E. Monthly mean near-surface eddy kinetic energy (EKE) was computed for the Slope Sea using surface geostrophic current anomalies derived from gridded 1993–2016 Copernicus Marine Environment Monitoring Service (CMEMS) sea height anomalies. Long-term, monthly mean Slope Sea EKE anomalies show a robust seasonal cycle with a winter (February) minimum and summer (June) maximum. This agrees with both seasonally-varying density stratification and dissipation and also the seasonal variation in the formation of GS WCRs within the Slope Sea. The RMS of the Slope Sea EKE seasonal cycle generally increased after 2002 by a factor of up to ∼2 relative to prior years. The seasonal cycle of Slope Sea EKE displayed higher EKE in the vicinity of the New England Seamount Chain (NESC) that extends towards the shelf break front from approximately −67° E to −63° E. Interannual variability of annual mean near-surface EKE from individual digitized GS warm core ring (WCR) observations from a Bedford Institute of Oceanography (BIO) WCR database is highly correlated with Slope Sea EKE. However, interannual variability of annual mean near-surface EKE computed from a census of all newly formed WCRs displayed only a weak correlation. Many of the WCRs from both the BIO and WCR census displayed anomalously low EKE values and were observed within the northern Slope Sea away from the GS. Some were located inshore of the position of the climatological mean shelf break front. WCRs with higher EKE were located throughout the Slope Sea, with higher numbers in the vicinity of the NESC. The many observations of the less energetic features located close to or inshore of the mean shelf break front suggest they are important to cross-shelf fluxes of heat, salt, nutrients, shelf biota. They therefore likely impact the shelf ecosystem, similar to the more energetic and typical WCRs impacting the outer shelf as discussed by earlier workers. • Altimeter-derived currents describe eddy kinetic energy changes in the slope sea. • Slope sea eddy kinetic energy displays strong seasonal and inter-annual variation. • Slope sea eddies display large variation in strength and formation location. • Eddy kinetic energy changes do not support the warm-core ring regime shift of 2000. [ABSTRACT FROM AUTHOR]

The Gent and McWilliams (1990, GM) tracer transport scheme has long been a cornerstone of global-scale ocean modeling, and is well-known for its ability to mimic the effects of baroclinic turbulence by reducing the available potential energy of resolved flows. However, theory predicts that baroclinic turbulence also acts to transfer this potential energy to kinetic energy, which can then be fluxed toward larger scales by an inverse energy cascade. These dynamical processes represent a separate branch of the large-scale ocean energy cycle which GM presently does not account for. In this paper a framework is developed which allows the potential energy removed by the GM scheme to be transferred to resolved motions. In this framework, which is labeled GM+E (or "GM plus energetics"), the energy transfer occurs by employing GM alongside a parameterization of the barotropic Reynolds stresses in the momentum equations, which takes the form of a negative harmonic viscosity acting on the barotropic velocities. The Reynolds stress parameterization is considered as being complementary to GM, in that its energetics are explicitly tied to those of GM while being independent of the choice of transfer coefficient or form of the GM closure. A set of idealized baroclinic spindown simulations is used to compare GM+E against standard GM, and is shown to substantially improve the kinetic energy spectra, probability distributions of flow structures, and restratification rate within the front relative to an eddy-resolving truth simulation. • Mesoscale eddy closures should convert PE to KE, which is then cascaded upscale. • The energy extracted by the GM parameterization currently disappears from the model. • A closure for the barotropic Reynolds stresses is developed to complement GM. • "GM+E", the combined Reynolds stress and GM, closes the large scale energy cycle. • KE spectra and flow structures are improved by GM+E compared to standard GM. [ABSTRACT FROM AUTHOR]

Highlights • The maximum of PV locates at the onshore side of the path no matter which mode Kuroshio takes; and the distribution of anomalous relative vorticity is consistent with that of SSHA for each Kuroshio path. • The energy transferred from eddy fields to the mean flow possibly favors Kuroshio to keep its path at the downstream of the large meander. • Both oNLM and tLM paths have stronger baroclinic instability compared to nNLM path. Abstract Variability of the Kuroshio path to the south of Japan plays a central role in the local climate change and exerts tremendous influences on the local atmosphere and ocean. In this study, the response of ocean dynamics, in terms of the eddy kinetic energy (EKE), potential vorticity (PV), relative vorticity, and eddy-mean flow interaction, to the Kuroshio path change is discussed. Kuroshio path south of Japan includes the near-shore non-large meander (nNLM), the off-shore non-large meander (oNLM), and the typical large meander (tLM). Analyses reveal that the distribution of EKE, PV, relative vorticity, and energy exchange between the eddy field and the mean flow respectively varies with the Kuroshio path: (1) The tLM has the maximum EKE along the path; (2) The positive and negative PV are located at the onshore and offshore side of Kuroshio axis, respevetively; (3) The distributions of anomalous relative voritcity of nNLM, oNLM, and tLM are consistent with sea surface height anomalies (SSHAs); (4) The tLM has the largest energy exchange between the eddy field and the mean flow in terms of the rate of barotropic energy conversion. On the other hand, the stability analysis of ocean currents suggests that the three Kuroshio paths south of Japan have their own intrinsic properties of the instability. [ABSTRACT FROM AUTHOR]

• Probability distribution functions of mesoscale eddies of different lifetimes. • Maps showing hotspot regions of formations and annihilations of mesoscale eddies. • Seasonal and inter-annual oscillations of their occurrences. • Interannual oscillation of eddy number negatively correlated with prevailing currents. Mesoscale eddies are ubiquitous in the oceans and play a significant role in setting the turbulence, transporting momentum, heat, salinity, and nutrients from their formation site to the place of dissipation, and controlling biogeochemical processes and air-sea exchanges. Using satellite-measured daily sea level anomaly from 1993 to 2021, we studied statistical characteristics of mesoscale eddies in the north Indian Ocean. The procedure uses information on the vorticity vector, Okubo-Weiss velocity gradient tensor and its threshold, and Lagrange transport. The continental margin of the Arabian Sea (AS) with its western and northeastern flanks, the mouth of the Gulf of Aden, the Lakshadweep Sea, the western margin of the Bay of Bengal (BOB), and the Andaman Sea have been depicted as the hotspot regions. The occurrence of cyclonic eddies (CEs) and anticyclonic eddies (AEs) are comparable in numbers with similar probability distribution as a function of their lifetime in AS and BOB at the basin scale and differ significantly in the regional domains. CEs dominate over AEs during monsoon and post-monsoon periods in the AS and during January-August in the BOB. In contrast, AEs are prominent in the great whirl (GW) during the monsoon period, and the EICC exhibits their peaks in March, July and November. Among the total observed eddies during the study period, 87 % of which are short-lived (<30 days), 10 % are moderately lived (30–60 days), and 3 % are long-lived (>60 days). Eddies are smaller and less energetic, with longer life in the higher latitudes than the lower latitudes. The frequency of eddies exhibits distinct seasonal variability with preferred periods during May-August and November-March, respectively, for their occurrence in AS and BOB. They also show significant inter-annual oscillations and decreasing trends on the background of the weak boundary currents. [ABSTRACT FROM AUTHOR]

The eddy kinetic energy (EKE) variability associated with 26 major Loop Current eddies (LCEs) in the Gulf of Mexico from 1994 through 2019 was investigated. We employed 3D multivariate observation-based ARMOR3D monthly ocean analyses of salinity, temperature, and geostrophic velocity field data. In addition, we used ERA5 wind data, the fifth generation of the European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric global climate reanalysis, to analyze internal and external forcing processes affecting the evolution of these LCEs. The energy analysis was performed to understand the role of barotropic (BT) and baroclinic (BC) instabilities and their associated energy conversion mechanisms in EKE generation. Our results suggest that BT instabilities are the primary source of EKE variability in the upper water column of the LC system. Furthermore, BT was positively correlated with Yucatan Channel (YC) transport during these major LCE shedding events. YC transport plays a significant role in energy conversion from mean kinetic energy to EKE, Loop Current growth, and generation of LCEs. BC instability was inversely correlated with buoyancy frequency, and a decrease in stratification triggers the development of BC instability, which favors eddy shedding. An eddy shedding index (ESI) was developed to quantify EKE evolution. Major LCE shedding occurs when ESI ≥0.46. [ABSTRACT FROM AUTHOR]

The basin-scale wave propagation in the Southern Indian Ocean (SIO) is inferred to affect the low-frequency variability of eddy kinetic energy (EKE) within the Agulhas current system (ACS). Observations and numerical simulations indicate that westward-propagating waves, in the form of sea level variability from the east boundary, may cause "non-linear" oceanic responses (e.g., sea level and EKE) in the ACS. This is likely due to two factors: (i) the movement of wave crests and troughs continuously alters the stratification of the upper ocean, generating a positive baroclinic instability and prompting conversions of EKE from mean flow in the source regions, and (ii) wave-current interactions induce a rapid intension of an anticyclone gyre in the interior SIO and the corresponding EKE in the downstream regions. These basin-scale wave modulations may provide an additional perspective to the traditional wind driving view based on the barotropic Sverdrup relation. The local winds over the SIO play an important role in maintaining basin-scale Rossby waves in the southwest Indian Ocean and further modulating oceanic low-frequency variability in a "quasi-linear" way. • The basin-scale wave propagation in the Southern Indian Ocean (SIO) have an influence on the dynamics of the Agulhas current system. (ACS). • The waves can alter the stratification of the upper ocean, as well as strengthen the anticyclone gyre in the interior SIO and the corresponding EKE in the downstream regions. • These basin-scale wave modulations may provide an additional perspective to the traditional wind driving view based on the barotropic Sverdrup relation. [ABSTRACT FROM AUTHOR]

Sound Speed Profile (SSP) is the key factor affecting underwater acoustics and it is of great value to obtain SSP in near real-time. In this paper, the sea surface data were used to reconstruct the SSP with the single empirical orthogonal function regression (sEOF-r) method in a global scale. Argo floats data as well as the altimeter data and sea surface temperature (SST) in the year 2010–2013 were used to establish the regression dataset. Argo profiles worldwide were grouped into 2°×2° longitude/latitude grid cells. Then EOF vectors were obtained in each grid, and the regression coefficients for the vectors were obtained with the sea surface data. Analysis showed that SSP anomalies differ from place to place. An assumption was made that the difference was due to the dynamic eddy activity and the eddy kinetic energy (EKE) map was pictured. Results suggested that the two variables correlated with each other. The larger the EKE, the larger the SSP anomalies. However, compared to the absolute value of the SSP anomalies, the error estimation improvement ratio remained relatively stable in most places. [ABSTRACT FROM AUTHOR]

Global maps of the maximum bottom concentration, thickness, and integrated particle mass in benthic nepheloid layers are published here to support collaborations to understand deep ocean sediment dynamics, linkage with upper ocean dynamics, and assessing the potential for scavenging of adsorption-prone elements near the deep ocean seafloor. Mapping the intensity of benthic particle concentrations from natural oceanic processes also provides a baseline that will aid in quantifying the industrial impact of current and future deep-sea mining. Benthic nepheloid layers have been mapped using 6,392 full-depth profiles made during 64 cruises using our transmissometers mounted on CTDs in multiple national/international programs including WOCE, SAVE, JGOFS, CLIVAR-Repeat Hydrography, and GO-SHIP during the last four decades. Intense benthic nepheloid layers are found in areas where eddy kinetic energy in overlying waters, mean kinetic energy 50 m above bottom (mab), and energy dissipation in the bottom boundary layer are near the highest values in the ocean. Areas of intense benthic nepheloid layers include the Western North Atlantic, Argentine Basin in the South Atlantic, parts of the Southern Ocean and areas around South Africa. Benthic nepheloid layers are weak or absent in most of the Pacific, Indian, and Atlantic basins away from continental margins. High surface eddy kinetic energy is associated with the Kuroshio Current east of Japan. Data south of the Kuroshio show weak nepheloid layers, but no transmissometer data exist beneath the Kuroshio, a deficiency that should be remedied to increase understanding of eddy dynamics in un-sampled and under-sampled oceanic areas. [ABSTRACT FROM AUTHOR]

The influence of changing the mean wind stress felt by the ocean through alteration of the variability of the atmospheric wind, as opposed to the mean atmospheric wind, on Southern Ocean circulation is investigated using an idealised channel model. Strongly varying atmospheric wind is found to increase the (parameterised) near-surface viscous and diffusive mixing. Analysis of the kinetic energy budget indicates a change in the main energy dissipation mechanism. For constant wind stress, dissipation of the power input by surface wind work is always dominated by bottom kinetic energy dissipation. However, with time-varying atmospheric wind, near surface viscous dissipation of kinetic energy becomes increasingly important as mean wind stress increases. This increased vertical diffusivity leads to thicker mixed layers and higher sensitivity of the residual circulation to increasing wind stress, when compared to equivalent experiments with the same wind stress held constant in time. This may have implications for Southern Ocean circulation in different climate change scenarios should the variability of the atmospheric wind change rather than the mean atmospheric wind. [ABSTRACT FROM AUTHOR]

Benthic storms are episodic periods of strong abyssal currents and intense, benthic nepheloid (turbid) layer development. In order to interpret the driving forces that create and sustain these storms, we synthesize measurements of deep ocean currents, nephelometer-based particulate matter (PM) concentrations, and seafloor time-series photographs collected during several science programs that spanned two decades in the western North Atlantic. Benthic storms occurred in areas with high sea-surface eddy kinetic energy, and they most frequently occurred beneath the meandering Gulf Stream or its associated rings, which generate deep cyclones, anticyclones, and/or topographic waves; these create currents with sufficient bed-shear stress to erode and resuspend sediment, thus initiating or enhancing benthic storms. Occasionally, strong currents do not correspond with large increases in PM concentrations, suggesting that easily erodible sediment was previously swept away. Periods of moderate to low currents associated with high PM concentrations are also observed; these are interpreted as advection of PM delivered as storm tails from distal storm events. Outside of areas with high surface and deep eddy kinetic energy, benthic nepheloid layers are weak to non-existent, indicating that benthic storms are necessary to create and maintain strong nepheloid layers. Origins and intensities of benthic storms are best identified using a combination of time-series measurements of bottom currents, PM concentration, and bottom photographs, and these should be coupled with water-column and surface-circulation data to better interpret the specific relations between shallow and deep circulation patterns. Understanding the generation of benthic nepheloid layers is necessary in order to properly interpret PM distribution and its influence on global biogeochemistry. [ABSTRACT FROM AUTHOR]

Small-scale eddies play an important role in preconditioning and restratifying the water column before and after mixing events, thereby affecting deep water formation variability. Results from a realistic eddy-resolving (∼ 5 km local horizontal resolution) ocean model suggest that small-scale temperature fluxes due to turbulent potential to kinetic energy conversion are the main driver of mixed layer restratification during deep convection in the Labrador Sea interior and the West Greenland Current. This resupply of heat due to turbulent upward buoyancy fluxes exhibits a large interannual variability imposed by the atmospheric forcing. Eddy fluxes only become active in periods of strong buoyancy loss, while being quiescent otherwise. In a low-resolution (∼ 20 km) control simulation the modeled turbulence is strongly reduced and the associated modeled and parameterized heat fluxes are too weak to increase stratification. • High-resolution (5 km) temperature fluxes restratify Labrador Sea mixed layer. • Large interannual variability of baroclinic instabilities and eddy buoyancy fluxes. • Modeled and parameterized fluxes are too weak to increase stratification at 20 km. [ABSTRACT FROM AUTHOR]

Thirty-six years output of a 1/10° eddy-resolving Ocean General Circulation Model are used to analyze the energetics of eddy–mean flow interactions in the Deep Western Boundary Current (DWBC) region of the tropical South Atlantic between 5°S and 16°S. The DWBC flow has a coherent structure between 5°S and 8°S but breaks up into a train of eddies downstream of a region of strong bathymetric curvature at 8°S. In the train of eddies area, the seasonal cycle of eddy kinetic energy (EKE) exhibits poleward phase propagation from May to September. The connection between the seasonal cycle of mean kinetic energy and EKE indicates an intensification of the downstream eddy field associated with enhanced upstream mean flow. The magnitudes of the baroclinic conversion and vertical eddy density flux terms are small in the DWBC core layer depth but somewhat elevated 500 m above and below the core. Eddy processes, including eddy generation and propagation, are accompanied by high EKE and large barotropic conversion. While in the global ocean baroclinic conversion is thought to dominate the energy transfer to EKE, our results suggest that barotropic energy conversion is the primary source of EKE and modulates its variability in the DWBC region of the deeper ocean. • First eddy–mean flow interaction analysis in the Deep Western Boundary Current between 5°S and 16°S. • Eddy kinetic energy is mainly generated via barotropic instability. • Enhanced upstream mean flow induces intensification in the downstream eddy field. [ABSTRACT FROM AUTHOR]

The first measurements of bottom nepheloid layers in the central and southern deep waters of the Gulf of Mexico west of the Yucatan peninsula were made during the three summers of 2015–2017. Particulate matter concentrations (PM) were estimated from optical profiles of beam attenuation due to particles (c p). Near-bottom maps and vertical sections of c p and PM converted from c p show evidence of sediment resuspension, possibly linked with topographic Rossby waves, loop current eddies, or eddy-topography interactions. Additional c p profiles were made along cross-slope transects around the entire Gulf of Mexico, including across the Yucatan Channel and Straits of Florida in 2017. Near-bottom PM concentrations were barely elevated in the deep Yucatan Channel and Straits of Florida at that time, except in about the surface 200 m along the northern and western boundaries. Comparison was made between areas with benthic nepheloid layers and Eddy Kinetic Energy (EKE) patterns in the deep Gulf of Mexico. Regions of high EKE or strong bottom currents in the central and eastern Gulf were found over a large region of deeply eroded furrows in the seafloor previously imaged using 3-D seismic profiling and submersible observations. Few PM measurements were obtained in the high EKE areas during these expeditions, however, historical and recent sampling show very strong nepheloid layers at stations within and westward of the region of the actively eroding furrows. • Beam c p data reveal weak to very strong nepheloid layers in the deep Gulf of Mexico. • Loop Current eddies and other currents may create varying bottom nepheloid layers. • Bottom currents create mega-furrows and nepheloid layers at Sigsbee escarpment base. • Nepheloid layers were weak in the deep Yucatan Channel and Straits of Florida. [ABSTRACT FROM AUTHOR]

A three-layer model was used to investigate the impact of grid resolution, bottom topography, wind, throughflow (the Tsushima Warm Current) and deep water formation (DWF) forcing on deep circulation in the East/Japan Sea. High grid resolution (1/36°) and real bottom topography are essential to capture the observed features of surface and deep circulation. An increase of grid resolution from 1/12° to 1/36°, wind and DWF forcing dramatically increase the current velocity and eddy kinetic energy (EKE) of the lower-layer. Of the three forcings, the influence of wind forcing is the most powerful on the abyssal circulation, followed by the order of the throughflow and DWF forcing, respectively. DWF forcing does not largely change the horizontal circulation pattern of the lower-layer in the 1/36° resolution model, but it increases the current velocity by 33.9%. Model results indicate that surface EKE is larger in the southern region than in the northern region of the East/Japan Sea, while deep EKE is larger in the northern area. These results are in good agreement with EKE distributions obtained from the surface drifter and the ARGO float trajectories. The EKE maxima of the lower-layer is geographically concentrated on the bottom slope, indicating that the strong circulation in the lower-layer is due to the strong eddy activity on the slope. The flux calculation shows that the primary eddy fluxes of potential vorticity driving deep circulation are layer thickness flux (LAY) and relative vorticity flux (REL). [ABSTRACT FROM AUTHOR]

It has recently been proposed to formulate eddy diffusivities in ocean models based on a mesoscale eddy kinetic energy (EKE) budget. Given an appropriate length scale, the mesoscale EKE can be used to estimate an eddy diffusivity based on mixing length theory. This paper discusses some of the open questions associated with the formulation of an EKE budget and mixing length, and proposes an improved energy budget-based parameterization for the mesoscale eddy diffusivity. A series of numerical simulations is performed, using an idealized flat-bottomed β -plane channel configuration with quadratic bottom drag. The results stress the importance of the mixing length formulation, as well as the formulation for the bottom signature of the mesoscale EKE, which is important in determining the rate of EKE dissipation. In the limit of vanishing planetary vorticity gradient, the mixing length is ultimately controlled by bottom drag, though the frictional arrest scale predicted by barotropic turbulence theory needs to be modified to account for the effects of baroclinicity. Any significant planetary vorticity gradient, β , is shown to suppress mixing, and limit the effective mixing length to the Rhines scale. While the EKE remains moderated by bottom friction, the bottom signature of EKE is shown to decrease as the appropriately non-dimensionalized friction increases, which considerably weakens the impact of changes in the bottom friction compared to barotropic turbulence. For moderate changes in the bottom-friction, eddy fluxes are thus reasonably well approximated by the scaling relation proposed by Held and Larichev (1996), which ignores the effect of bottom friction. [ABSTRACT FROM AUTHOR]

The Lofoten Basin is the largest reservoir of ocean heat in the Nordic Seas. A particular feature of the basin is ‘the Lofoten Vortex’, a most anomalous mesoscale structure in the Nordic Seas. The vortex resides in one of the major winter convection sites in the Norwegian Sea, and is likely to influence the dense water formation of the region. Here, we document this quasi-permanent anticyclonic vortex using hydrographic and satellite observations. The vortex’ uniqueness in the Nordic Seas, its surface characteristics on seasonal, inter-annual, and climatological time-scales, are examined together with the main forcing mechanisms acting on it. The influence of the vortex on the hydrography of the Lofoten Basin is also shown. We show that the Atlantic Water in the Nordic Seas penetrate the deepest inside the Lofoten Vortex, and confirm the persistent existence of the vortex in the deepest part of the Lofoten Basin, its dominant cyclonic drift and reveal seasonality in its eddy intensity with maximum during late winter and minimum during late autumn. Eddy merging processes are studied in detail, and mergers by eddies from the slope current are found to provide anticyclonic vorticity to the Lofoten Vortex. [ABSTRACT FROM AUTHOR]

Highlights: [•] We quantify temporal resolution sensitivity up to 30-days of Lagrangian particles. [•] Divergence times are faster than intrinsic dispersion. [•] Divergence times are slower than interannual variability associated dispersion. [•] Connectivity transport increase and transit time decrease in high dispersion flows. [•] Up to nine-day averaging show no significant degradation in connectivity. [Copyright &y& Elsevier]

Abstract: Although mesoscale eddies are a ubiquitous feature of the world’s oceans, mechanisms determining their strength and variability remain poorly understood. Here we investigate the effects of surface buoyancy forcing on eddy kinetic energy (EKE), temperature variance, and lateral eddy heat fluxes. In keeping with previous investigations, sustained heat loss ultimately leads to an increase in the magnitude of EKE, whereas sustained heat gain only alters the spatial pattern of EKE, albeit enhancing the temperature variance. These changes to the eddy field and stratification are shown to result in changes in meridional eddy heat flux and residual meridional overturning circulation, which acts to bring the system to equilibrium with the applied surface buoyancy forcing. [Copyright &y& Elsevier]

Abstract: Many analyses of the interaction between ocean physics and biology in the Coastal Gulf of Alaska (CGOA) resolve chlorophyll variability separately from eddy-induced circulation, but eddy–chlorophyll covariability has not received much attention. The present research quantified eddy–chlorophyll interaction from the covariability of observed chlorophyll and eddy kinetic energy (EKE) in the CGOA for 1998–2002. Analyses with coupled empirical orthogonal functions (CoEOFs) showed that covariability between the two fields resulted in strongly coupled modes—a feature absent from standard-EOF analyses. Timescales of covariability were also incorporated into the analyses. The temporal evolution of each CoEOF mode was decomposed with the cross-wavelet power spectrum, and instances of covariability for synoptic timescales (2–6 months) were attributed to eddy–chlorophyll interaction. Further analyses in the present research included CoEOF decomposition for the output of a coupled physical–biological model in the CGOA. Model-observation comparisons with CoEOFs offer a new and important way to evaluate coupled models for eddy–chlorophyll interaction across multiple temporal and spatial scales. Implications for cross-shelf transport and spatiotemporal sampling for both observation and model data fields are also discussed. [Copyright &y& Elsevier]

Abstract: A high-resolution (1/16 degree) ocean general circulation model is used to estimate the mean dynamic topography (MDT) and surface eddy kinetic energy (EKE) in the Mediterranean Sea. Model outputs based on a 12-year run with a perpetual annual atmospheric forcing are compared with the MDT and EKE derived from satellite altimetry. The model MDT is consistent with previous estimates and reproduces most of the major Mediterranean currents and eddies. In contrast, the model MDT derived from temperature/salinity climatology misses many circulation features in the Central and Levantine basins. The model EKE reproduces well the magnitudes and spatial distributions of the altimeter derived EKE, and is a significant improvement over the previous 1/8 degree model studies. However, the use of perpetual atmospheric forcing may cause some eddy features to appear more stable than characterized with high interannual variability in the altimeter EKE. The study highlights the importance of large mesoscale variability in transport processes, especially along the southern areas of the Mediterranean. The generally good agreement suggests that model simulations can be used in combination with in situ and altimeter observations to better understand the dynamics and make forecasts of the mesoscale eddies in the Mediterranean Sea. [Copyright &y& Elsevier]

Abstract: Analysis of a quality-controlled database of Gulf Stream warm-core rings (WCRs) between 75° and 50°W during 1978–1999 demonstrates a significant correlation between WCR occurrences and variations in large-scale atmospheric forcing related to the state of the North Atlantic Oscillation (NAO). The mechanisms for linking the NAO with the rate of WCR occurrences are two-fold: (1) the influence of the NAO on Gulf Stream (GS) position, which could affect the interaction of the Gulf Stream with the New England Seamounts chain and thus allow for a higher/lower number of WCR occurrences; (2) the NAO-induced eddy kinetic energy (EKE) variability in the Gulf Stream region (GSR), which is indicative of the baroclinic instability processes necessary for WCR formation. Variability in GS movement is studied by analyzing annual mean positions of the Gulf Stream North Wall obtained from satellite-derived sea surface temperature (SST) frontal charts. Response of GSR EKE to fluctuations in the state of the NAO is examined with a numerical simulation of the North Atlantic basin from 1980–1999. The North Atlantic basin is simulated using a 1/6°-resolution eddy-resolving Regional Ocean Modeling System (ROMS) model that spins up with Southampton Oceanography Center (SOC) ocean-atmosphere atlas-derived atmospheric forcing fields. Model-derived EKE estimates are observed to be in good agreement with TOPEX/Poseidon altimeter-based EKE estimates as well as with results from other modeling studies for the North Atlantic basin. We suggest that lateral movement of the GS may not be the primary mechanism causing variation in the rate of WCR occurrences, because GS position is observed to respond at a lag of one year, whereas annual rates of WCR occurrences respond at 0-year lag to the NAO. Based on results from numerical simulations of the North Atlantic basin, adjustment to NAO-induced wind forcing is seen to impact the GSR EKE intensity and possibly the related baroclinic instability structure of the GS at 0-year lag. These results suggest that NAO-induced interannual variability in GSR EKE is the most likely mechanism affecting WCR occurrences. Numerical simulations show that high (low) phases in the state of the NAO exhibit higher (lower) EKE in the GSR, providing a greater (lesser) source of baroclinic instability to the GS front, possibly resulting in higher (lower) occurrences of WCRs. [Copyright &y& Elsevier]

Abstract: Twelve years (1993–2005) of altimetric data, combining different missions (ERS-1/2, TOPEX/Poseidon, Jason-1 and Envisat), are used to analyse sea level and Eddy Kinetic Energy variability in the Bay of Biscay at different time-scales. A specific processing of coastal data has been applied, to remove erroneous artefacts. Likewise, an optimal interpolation has been used, to create a series of regional Sea Level Anomaly maps, merging data sets from two satellites. The sea level presents a trend of about 2.7 mm/year, which is within the averaged values of sea level rise in the global ocean. Frequency spectra show that the seasonal cycle is the main time-scale affecting the sea level and Eddy Kinetic Energy variability. The maximum sea level occurs in October, whilst the minimum is observed in April. The steric effect is the cause of this annual cycle. The Northern French shelf/slope presents intense variability which is likely due to internal tides. Some areas of the ocean basin are also characterised by intense variability, due to the presence of eddies. The Eddy Kinetic Energy, in turn, is higher from December to May, than during the rest of the year and presents a weak positive trend from April 1995 to April 2005. Several documented mesoscale events, occurring at the end of 1997 and during 1998, are analysed. Altimetry maps prove to be a useful tool to monitor swoddy-like eddies from their birth to their decay, as well as the inflow of seasonal slope water current into the southeastern corner of the Bay of Biscay. [Copyright &y& Elsevier]

Abstract: Four altimeter missions [Jason-1, ERS-2, TOPEX/POSEIDON interleaved with Jason-1 (T/P) and Geosat Follow-On (GFO)] are intercalibrated and merged in an objective analysis scheme with the aim of improving the estimation of mesoscale surface ocean circulation in the Mediterranean Sea. A validation with independent altimetric data shows that, with the combination of three altimeters, in regions of large mesoscale variability, the sea level and velocity can be mapped with a relative accuracy of about 6% and 23%, respectively, which is a factor of 2.2 less than the results derived from Jason-1 alone, and a factor of about 1.5 less than the results obtained from Jason-1+ERS-2. Mean eddy kinetic energy (EKE) is computed from the different altimeter configurations. It shows that the combination of Jason-1+ERS-2 fails to reproduce some intense signals. On the contrary, when T/P is added, these features are well recovered and the EKE does not show significant discontinuities due to sampling effects. The impact of the fourth mission (GFO) is less critical but it also improves the representation of energetic structures. In average, the merged Jason-1+ERS-2+T/P+GFO maps yield EKE levels 15% higher than Jason-1+ERS-2. Finally, we show that the consistency between altimetry and Sea Surface Temperature, drifting buoys and tide gauges, is significantly improved when four satellites are merged compared to the results derived from the two-satellite configuration. This study demonstrates that, at least three, but preferably four, altimeter missions are needed for monitoring the Mediterranean mesoscale circulation. [Copyright &y& Elsevier]

Abstract: We employ a non-decimated wavelet decomposition to analyse inter-annual variations of the North Atlantic Oscillations (NAO) indices and relationship of these variations with both the Eddy kinetic energy contents (K E) in the atmosphere of Northern mid-latitudes and the precipitation in the different regions of Ukraine during July 1960–February 2003. Major advantage of using this tool is to isolate short- and long-term components of fluctuations. Such analysis allows revealing basic periodic behaviours for the NAO indices such as the 4–8-year and the natural change of dominant phase. The main results can be posed as follows. The most essential relationship between the NAO index and K E content is found for the low frequency variations with the periods of 4–8 and 8–16 years, at that if the NAO phase tends to abrupt changes then the impact of these changes on the K E content is more significant than for the durational dominance of certain phase. The largest coefficients of correlation for the NAO index and Ukrainian precipitation are stated for the periodicities of 2–4 and 4–8 years; the former is caused by the synoptic-scale activity forced by the NAO while the former is conditioned by the air-mass processes in the warm season. [Copyright &y& Elsevier]

Abstract: What most clearly distinguishes near-shore and off-shore currents is their dominant spatial scale, O (1–30)km near-shore and O (30–1000)km off-shore. In practice, these phenomena are usually both measured and modeled with separate methods. In particular, it is infeasible for any regular computational grid to be large enough to simultaneously resolve well both types of currents. In order to obtain local solutions at high resolution while preserving the regional-scale circulation at an affordable computational cost, a 1-way grid embedding capability has been integrated into the Regional Oceanic Modeling System (ROMS). It takes advantage of the AGRIF (Adaptive Grid Refinement in Fortran) Fortran 90 package based on the use of pointers. After a first evaluation in a baroclinic vortex test case, the embedding procedure has been applied to a domain that covers the central upwelling region off California, around Monterey Bay, embedded in a domain that spans the continental U.S. Pacific Coast. Long-term simulations (10years) have been conducted to obtain mean-seasonal statistical equilibria. The final solution shows few discontinuities at the parent–child domain boundary and a valid representation of the local upwelling structure, at a CPU cost only slightly greater than for the inner region alone. The solution is assessed by comparison with solutions for the whole US Pacific Coast at both low and high resolutions and to solutions for only the inner region at high resolution with mean-seasonal boundary conditions. [Copyright &y& Elsevier]

Abstract: The characterisation of the geostrophic surface flow field around the Cape Verde Archipelago in the northeast Atlantic Ocean with satellite altimeter data is presented. The aim is to analyse the main current systems present in the region 3°–30°N, 40°–10°W and their seasonal and interannual variability. A merged data set of Topex/Poseidon (T/P) and ERS-2 altimeter data for an 8-year period, beginning in June 1995, has been used and corrected sea surface heights were computed by applying a homogeneous set of relevant geophysical corrections. ERS-2 data were crossover adjusted to T/P. Monthly maps of sea level anomalies were created for the whole period and were used in the computation of monthly maps of absolute dynamic topography, geostrophic currents and eddy kinetic energy (EKE). The seasonal signal of the northeast Tropical Atlantic large-scale surface circulation appears as the prevailing cause of the variability in the region, particularly in the southernmost portion of the region being studied. This signal is also present in the flow field along the African coast and in the Guinea Dome. Regions of highest EKE values are clearly associated with the North Equatorial Counter-Current and with the currents along the African coast. The significant interannual variability found for 1998 seems to be associated with the 1997–1998 ENSO Pacific event, but other anomalous periods (1996–1997 and 2001–2002) uncorrelated with ENSO are also evident. [Copyright &y& Elsevier]

Nine years of Topex/Poseidon and ERS satellite altimetry and XBT data from the SURVOSTRAL program were used to analyze the seasonal and interannual variations of the eddy energetics in terms of its spatial distribution and relation with the upper ocean heat content. Eddy kinetic energy is calculated in two frequency bands one associated with transient and the other with low-frequency variability. The two eddy components have distinct geographical distribution. At the SURVOSTRAL line, the transient eddy energy is twice the low-frequency energy, with maximum transient energy occurring during the austral summer period and maximum low-frequency energy in winter. The site is one of growing eddy energy. Eddy momentum flux is northward over the SURVOSTRAL line, and the summertime eddy heat flux is poleward across the Subantarctic and Subtropical Fronts, and equatorward either side of the fronts. Eddy fluxes are strongly influenced by their position relative to the bathymetry and the mean current. [Copyright &y& Elsevier]

Recent observations report a poleward shift in several of the world's western boundary current extensions in response to a warming climate. However, the nature and variability of the Agulhas Return Current (ARC) is still relatively unknown, leading to uncertainty in its response to gyre intensification. The objective of this work is to investigate meridional migration and eddy kinetic energy (EKE) trends in the ARC over the past 27 years using satellite-derived altimetry products. A difference is seen between the western and eastern regions of the ARC, the former exhibiting a predominantly equatorward migration and the latter moving predominantly poleward. The ARC is shown to be a region highly influenced by its bathymetry. Additionally, we show that the regional EKE peaks during austral summer months and that there has been a significant rise in summertime EKE levels between 1994–2011. • Opposing migrational trends are seen between the western and eastern regions of the ARC. • The ARC is shown to be a region highly influenced by its bathymetry. • A significant rise in summertime EKE levels between 1994–2011. [ABSTRACT FROM AUTHOR]

Using 39 years of ECMWF renalysis data, an established energetics framework and simple composite analysis this study has shown that South African cut-off low (COL) pressure systems are preceded by downstream development of a baroclinic wave. Downstream development is characterised by the evolution of two energy centres, one located upstream in the midlatitudes and another downstream in the subtropics. The upstream eddy kinetic energy, which is associated with a midlatitude jet streak, develops and reaches its maximum before the formation of the closed COL cyclonic circulation. The downstream eddy kinetic energy centre maximises at the point where the closed circulation forms. The upstream eddy kinetic energy centre grows from baroclinic conversion from eddy available potential energy to eddy kinetic energy, whilst the downstream centre grows by receiving energy by means of ageostrophic geopotential fluxes that transport eddy kinetic energy in a north-eastward direction from the upstream centre. These ageostrophic geopotential fluxes are induced, increased in magnitude and directed by processes associated with Rossby wave breaking (RWB) on the midlatitude dynamical tropopause and so the downstream energy transfer connects South African COLs to midlatitude processes. The study has further shown that the baroclinic kinetic energy configuration previously associated with wet seasons over South Africa is consistent with times when COLs forms over the country. Unlabelled Image • The study has shown that South African cut-off low (COL) pressure systems are preceded by downstream development. • The eddy kinetic energy associated with COLs is transferred from the midlatitudes, is not converted from eddy available potential energy. • Rossby wave breaking induce the ageostrophic geopotential fluxes that transfer energy from the midlatitudes to the subtropics. [ABSTRACT FROM AUTHOR]

This work is part of the MADRidge Project special issue which aims to describe pelagic ecosystems in the vicinity of three prominent shallow seamounts in the South West Indian Ocean: one here named MAD-Ridge (240 m below the surface) plus Walters Shoal (18 m) on the Madagascar Ridge, and La Pérouse (60 m) on the abyssal plain east of Madagascar. The three span latitudes 20°S and 33°S, some 1500 km. The study provides the background oceanography for the once-off, multidisciplinary snapshot cruise studies around the seamounts. As life on seamounts is determined by factors such as summit depth, proximity to the light layers of the ocean, and the ambient circulation, a first description of regional spatial-field climatologies (16–22 years) and monthly along-ridge gradients of surface wind (driving force), water column properties of sea surface temperature, mixed layer depth, chlorophyll- a and eddy kinetic energy, plus ocean currents is provided. Being relevant to many applications in the study domain, these properties in particular reveal contrasting environments along the Madagascar Ridge and between the three seamounts that should drive biological differences. Relative to the other two seamounts, MAD-Ridge is in the more extreme situation, being at the end of the East Madagascar Current, where it experiences sturdy, albeit variable, currents and the frequent passing of mesoscale eddies. [ABSTRACT FROM AUTHOR]