Search

The tropical Indian Ocean receives the world's most significant tropical ocean rain and river runoff per unit area. The 3-dimensional spreading of Rain and River Water (RRW) entering the tropical Indian Ocean and associated salinity and circulation anomalies are explored for 60 years using ocean reanalysis data tailored to a tracer transport model. Over 60 years, the cumulative impact of RRW entering the tropical Indian Ocean is to freshen the Indian Ocean basin as large as 2.0–0.1 p.s.u from the surface to 500 m via subduction and transport. The RRW propagates to a vast extent of the Indian Ocean and a limited extent of the Atlantic and Pacific Oceans via general circulation pathways. A quasi-equilibrium model of accumulation of RRW over the tropical Indian Ocean suggests that it induces clockwise geostrophic currents from the Bay of Bengal to the Arabian Sea over 0–500 m depths with a net transport tendency of 0.80 ± 0.14 Sv year−1. The study implies that coupled climate models with precipitation biases may miscalculate such salinity and circulation anomalies due to RRW and aggravating biases in simulated climate dynamics. • The spreading of rain and river water entering the tropical Indian Ocean for 60 yr is identified from ocean reanalysis data. • RRW mixes in the surface, subducts and spreads, offering a dilution in salinity and expansion in dynamic heights. • RRW offers a net transport tendency of water from the Bay of Bengal to the Arabian Sea at a rate of 0.80 ± 0.14 Sv year−1. [ABSTRACT FROM AUTHOR]

Wind-driven geostrophic currents have been considered as one of the major energy sources for stimulating abyssal mixing that shapes the ocean overturning circulation. However, the mechanism governing the energy transfer from surface geostrophic currents to small dissipation scales available for abyssal mixing remains elusive. In this study, we identify a potential energy transfer pathway from surface geostrophic eddies to abyssal small-scale turbulence based on observations and numerical simulations of flows over seamounts in the northwestern Pacific Ocean. Our analyses reveal the generation of strong near-bottom anticyclonic circulations following the passage of surface geostrophic eddies across seamounts. These circulations drive downslope Ekman transports and convective instabilities, leading to enhanced turbulent mixing and thickened bottom mixed layers. Altimetry observations suggest that the energy transfer from surface eddies to deep currents and subsequent turbulence may be a dominant mechanism that accounts for 25–40% energy decay of mesoscale eddies passing seamounts. Strong near-bottom anticyclonic circulations associated with the surface geostrophic eddies are likely common for many seamounts and other topographic slopes in the global ocean, and may therefore play a significant role in the oceanic energy balance and water mass transformation in the abyssal ocean. [ABSTRACT FROM AUTHOR]

We study the production and dissipation of the eddy kinetic energy (EKE) in a submesoscale eddy field forced with downfront winds using the Process Study Ocean Model (PSOM) with a horizontal grid resolution of 0.5 km. We simulate an idealized 100 m deep mixed-layer front initially in geostrophic balance with a jet in a domain that permits eddies within a range of O(1 km–100 km). The vertical eddy viscosities and the dissipation are parameterized using four different subgrid vertical mixing parameterizations: the k − ϵ , the KPP, and two different constant eddy viscosity and diffusivity profiles with a magnitude of O ( 10 − 2 m 2 s − 1 ) in the mixed layer. Our study shows that strong vertical eddy viscosities near the surface reduce the parameterized dissipation, whereas strong vertical eddy diffusivities reduce the lateral buoyancy gradients and consequently the rate of restratification by mixed-layer instabilities (MLI). Our simulations show that near the surface, the spatial variability of the dissipation along the periphery of the eddies depends on the relative alignment of the ageostrophic and geostrophic shear. Analysis of the resolved EKE budgets in the frontal region from the simulations show important similarities between the vertical structure of the EKE budget produced by the k − ϵ and KPP parameterizations, and earlier LES studies. Such an agreement is absent in the simulations using constant eddy-viscosity parameterizations. [ABSTRACT FROM AUTHOR]

The seasonal variability of the hydrology and the poleward subsurface Peru-Chile Undercurrent (PCUC) off the central Chilean coast (29–41°S) were examined using a high-resolution regional model. The model realistically reproduced observed sea level variability, such as intense anticyclonic eddies, the offshore intensification of the poleward flow and the reduced nearshore equatorward flow during autumn, as well as the equatorward intensification of nearshore meandering flow during spring. Values for geostrophic eddy kinetic energy were high along the coast between 30° and 37°S, and lower south of this area. The modelled poleward undercurrent showed latitudinal variability in velocity and transport. The maximum average transport reported was 0.8 Sv near 30°S, consistent with previous modelling studies and estimations derived from in situ observations. The poleward reduction in undercurrent strength was shown to be partly generated by the poleward decrease in wind stress curl and by the formation of a westward jet near 35°S associated with westward-propagating eddies. A Lagrangian analysis of the modelled water parcels transported by the undercurrent shows that only 14–20% of the subsurface floats transported by the undercurrent upwelled into the surface layer within the subsequent six months after their release. The floats remaining within the subsurface layer were likely transported further south by the current, offshore by westward-propagating eddies or equatorward by the deeper part of the surface coastal current. [ABSTRACT FROM AUTHOR]

An anomalously large Harmful Algal Bloom (HAB) was observed in the southwest coast of the East/Japan Sea (hereafter the East Sea) during the summer of 2013. During this time period, the presence of Cochlodinium polykrikoides ( C. polykrikoides ) was detected by the Geostationary Ocean Color Imager (GOCI) and validated by in-situ observations. GOCI observations have been available since 2011, thus allowingto examine various stages of the physical condition of the developing C. polykrikoides bloom, thereby other multi-satellite and buoy measurements obtained between 2011 and 2013. Research results indicate that this HAB is related to four processes: the transport of C. polykrikoides from the south coast of Korea to the HAB area; a relatively high insolation; continuous coastal upwelling; and a favorable Sea Surface Temperature (SST) for C.polykrikoide growth. In examination of the main transport mechanisms, geostrophic current measurements were used to estimate the flow trajectories, showing water from the south coast to the HAB area off the southeast coast of Korea. Result shows that ninety percent of the water from the south coast reached the HAB area in 2013. Furthermore, to examine the insolation mechanism, the Photosynthetically available radiation (PAR) value was derived from the Moderate Resolution Imaging Spectoradiometer (MODIS), showing that PAR values were relatively high in the HAB area during HAB period (47 Ein m −1 day −1 ). Moreover, Upwelling age (UA) was calculated in order to investigate the strength of coastal upwelling events, which were found to support relatively high UA values during the HAB period. The mean UA value during the HAB period was 1.01, higher than those in 2011 and 2012 which were 0.61 and 0.76, respectively. Finally, SST in the HAB area was also analyzed to examine which conditions were most favorable for HAB growth. Therefore, the results of this study suggest that the four mechanisms can explain the relative contributions of the anomalously HAB development observed off the southeast coast of Korea. [ABSTRACT FROM AUTHOR]

The wind-driven circulation and wind-induced coastal upwelling in a large shelf sea with a zonally oriented coast are examined. The Yucatan shelf is located to the north of the Yucatan peninsula in the eastern Gulf of Mexico. This area is a tropical shallow body of water with a smooth sloping bottom and is one of the largest shelves in the world. This study describes the wind-driven circulation and wind-induced coastal upwelling in the Yucatan shelf, which is forced by easterly winds throughout the year. Data obtained from hydrographic surveys, acoustic current profilers and environmental satellites are used in the analysis. Hydrographic data was analyzed and geostrophic currents were calculated in each survey. In addition an analytical model was applied to reproduce the currents. The results of a general circulation model were used with an empirical orthogonal function analysis to study the variability of the currents. The study area is divided in two regions: from the 40 m to the 200 m isobaths (outer shelf) and from the coast to the 40 m isobath (inner shelf). At the outer shelf, observations revealed upwelling events throughout the year, and a westward current with velocities of approximately 0.2 m s −1 was calculated from the numerical model output and hydrographic data. In addition, the theory developed by Pedlosky (2007) for a stratified fluid along a sloping bottom adequately explains the current’s primary characteristics. The momentum of the current comes from the wind, and the stratification is an important factor in its dynamics. At the inner shelf, observations and numerical model output show a wind-driven westward current with maximum velocities of 0.20 m s −1 . The momentum balance in this region is between local acceleration and friction. A cold-water band is developed during the period of maximum upwelling. [ABSTRACT FROM AUTHOR]

We analysed the 1997 summer mucilage event in the northern Adriatic and showed that macroaggregates were formed in the upper layers of the water column in central parts of large gyres and were dispersed by currents which seem to be of geostrophic origin. Beside Cylindrotheca closterium , an already established regular opportunistic species of the mucilage phenomenon, originating from the sediment, we detected another benthic species, Thalassionema nitzschioides , of sporadic character. Both were present in scarce amounts in the water column at the time of sampling first macroaggregates, but reached high abundances after/close to the mucilage event ending. The first, and the most intense surface heat loss episode, on 18 July (heat loss of around 240 W m −2 , during episode of NW wind), in time of first macroaggregates formation, introduced C. closterium from sediment into the water column and enabled dispersion of T. nitzschioides, present in central region both in water and in first macroaggregate samples, eastwards and westwards. The later, less intense surface heat loss episodes, between end of July and mid of August (30–160 W m −2 , induced mostly by NE winds) were efficient (1) in tearing already formed macroaggregates, enabling in that way release of opportunistic species from a macroaggregate into the surrounding water column, and (2) in transporting lower salinity waters of specific species populations with macroaggregates form west to east, extending the event to large area. We hypothesise, first, that summer surface heat loss episodes play a role in sustaining mucilage events by introduction of C. closterium from the sediment and/or decaying macroaggregates in the water column. Second, that geostrophic currents are important in spreading of various types of aggregates, including marine snow, over the wider marine areas. [ABSTRACT FROM AUTHOR]

Mesoscale eddies can elevate turbulent mixing, but the mechanisms have not been fully understood. Based on a field experiment in two mesoscale eddies near the critical latitude of the M 2 internal tides in the northwestern Pacific Ocean, enhanced turbulent mixing was found both in and below the mixed layer. Microstructure observations indicate that enhanced turbulent mixing mainly appeared outside the eddy center in the mixed layer (approximately 117 m), which was speculated to be related to geostrophic current shear. By analyzing the CTD and Argo measurements through a fine-scale parameterization method, enhanced turbulent mixing was found to be concentrated at the southern flank of the anticyclonic eddy below 300 m, which showed little correlation with topographic roughness and local winds. Based on a ray-tracing model, the wind-generated near-inertial waves at higher latitudes were also excluded as the cause of the enhanced turbulent mixing. The enhanced turbulent mixing below 300 m was generally consistent with the features of subthermocline eddies and eddy-modulated parametric subharmonic instability (PSI) of the M 2 internal tides. However, due to the limitation of observations, the eddy-modulated PSI mechanism was not fully validated. • Enhanced turbulent mixing was found in and below mixed layer in mesoscale eddies. • Geostrophic current shear may elevate turbulent mixing in the mixed layer. • Enhanced mixing below 300 m was consistent with features of eddy-modulated PSI. • Subthermocline eddy may also contribute to the enhanced mixing below 300 m. [ABSTRACT FROM AUTHOR]

We present a B-spline based conforming finite-element method of the streamfunction formulation of the stationary one-layer quasi-geostrophic equations for the study of the large scale wind-driven ocean circulation. The method encompasses standard simplifications of these equations, in particular the linear Stommel and Stommel–Munk models. A variational form of the method is developed and its consistency is established. In this formulation Dirichlet boundary conditions are enforced only weakly and stabilization is achieved via Nitsche’s method. Stability parameters are evaluated by solving a local generalized eigenvalue problem on the Dirichlet boundary and by monitoring the condition number of the resulting linear systems. Comparisons of the results from our simulations with previously published results and convergence studies lead us to conclude that our finite-element discretization is accurate. [ABSTRACT FROM AUTHOR]

The paper documents dynamics of the middle Adriatic region, and is based on computation of relative geostrophic currents from temperature and salinity data taken along the Palagruža Sill transect between 1957 and 2010. The strongest westward current is found in the central part of the sill, corresponding to the offshore deflected Eastern Adriatic Current (EAC). The Western Adriatic Current (WAC) is, as expected, found in the southern part of the measured transect. The WAC is most significant during the summer season and exhibits stronger variability than the EAC due to the frequent growth of eddies within it. Long-term trends in both the central and southern parts of the sill oppose mean currents, indicating the weakening of the EAC and WAC and therefore of the entire Adriatic thermohaline cell. These opposing trends have not been reproduced by a climate model. [ABSTRACT FROM AUTHOR]

Stochastic superparameterization, a stochastic parameterization framework based on a multiscale formalism, is developed for mesoscale eddy parameterization in coarse-resolution ocean modeling. The framework of stochastic superparameterization is reviewed and several configurations are implemented and tested in a quasigeostrophic channel model – an idealized representation of the Antarctic Circumpolar Current. Five versions of the Gent–McWilliams (GM) parameterization are also implemented and tested for comparison. Skill is measured using the time-mean and temporal variability separately, and in combination using the relative entropy in the single-point statistics. Among all the models, those with the more accurate mean state have the less accurate variability, and vice versa. Stochastic superparameterization results in improved climate fidelity in comparison with GM parameterizations as measured by the relative entropy. In particular, configurations of stochastic superparameterization that include stochastic Reynolds stress terms in the coarse model equations, corresponding to kinetic energy backscatter, perform better than models that only include isopycnal height smoothing. [ABSTRACT FROM AUTHOR]

We investigate an inequality constraining the energy and potential enstrophy flux spectra in two-layer and multi-layer quasi-geostrophic models. Its physical significance is that it can diagnose whether any given multi-layer model that allows co-existing downscale cascades of energy and potential enstrophy can allow the downscale energy flux to become large enough to yield a mixed energy spectrum where the dominant k-3 scaling is overtaken by a subdominant k-5/3 contribution beyond a transition wavenumber kt situated in the inertial range. The validity of the flux inequality implies that this scaling transition cannot occur within the inertial range, whereas a violation of the flux inequality beyond some wavenumber kt implies the existence of a scaling transition near that wavenumber. This flux inequality holds unconditionally in two-dimensional Navier-Stokes turbulence, however, it is far from obvious that it continues to hold in multi-layer quasi-geostrophic models, because the dissipation rate spectra for energy and potential enstrophy no longer relate in a trivial way, as in two-dimensional Navier-Stokes. We derive the general form of the energy and potential enstrophy dissipation rate spectra for a generalized symmetrically coupled multi-layer model. From this result, we prove that in a symmetrically coupled multi-layer quasi-geostrophic model, where the dissipation terms for each layer consist of the same Fourier-diagonal linear operator applied on the streamfunction field of only the same layer, the flux inequality continues to hold. It follows that a necessary condition to violate the flux inequality is the use of asymmetric dissipation where different operators are used on different layers. We explore dissipation asymmetry further in the context of a two-layer quasi-geostrophic model and derive upper bounds on the asymmetry that will allow the flux inequality to continue to hold. Asymmetry is introduced both via an extrapolated Ekman term, based on a 1980 model by Salmon, and via differential small-scale dissipation. The results given are mathematically rigorous and require no phenomenological assumptions about the inertial range. Sufficient conditions for violating the flux inequality, on the other hand, require phenomenological hypotheses, and will be explored in future work. [ABSTRACT FROM AUTHOR]

In the near future we expect the resolution of many IPCC-class ocean models to enter the "eddy-permitting" regime. At this resolution models can produce reasonable eddy-like disturbances, but can still not properly resolve geostrophic eddies at all relevant scales. Adequate parameterizations representing sub-grid eddy effects are thus necessary. Most eddy-permitting models presently employ some kind of hyper-viscosity, which is shown to cause a significant amount of energy dissipation. However, comparison to higher resolution simulations shows that only enstrophy, but almost no energy, should be dissipated below the grid-scale. As a result of the artificial energy sink associated with viscous parameterizations, the eddy fields in eddy permitting models are generally not energetic enough. To overcome this problem, we propose a class of sub-grid parameterizations which dissipate enstrophy but little or no energy. The idea is to combine a standard hyperviscous closure with some mechanism to return dissipated energy to the resolved flow. Enstrophy dissipation remains ensured because the energy is returned at larger scales. Two simple ways to return the energy are proposed: one using a stochastic excitation and one using a negative Laplacian viscosity. Both approaches are tested in an idealized two-layer quasi-geostrophic model. Either approach is shown to greatly improve the solutions in simulations with typical eddy-permitting resolutions. The adaptation of the proposed parameterization for use in realistic ocean models is discussed. [ABSTRACT FROM AUTHOR]

Abstract: This article studies the pullback asymptotic behavior of solutions for a non-autonomous multi-layer quasi-geostrophic model in a two-dimensional domain. We prove the existence of pullback attractors in (the velocity has the -regularity) and in (the velocity has the -regularity). Then we verify the regularity of the pullback attractors by proving that , which implies the pullback asymptotic smoothing effect of the model in the sense that the solutions eventually become more regular than the initial data. The method used in this article is similar to the one used in Zhao and Zhou (2007) in the case of the non-autonomous incompressible non-Newtonian fluid in a two-dimensional domain. Let us mention that the non-homogeneous boundary conditions (and the non-local constraint) present in the multi-layer quasi-geostrophic model makes the estimates more complicated, see Bernier (1994). These difficulties are overcome using the new formulation presented in Tachim Medjo (2009). [Copyright &y& Elsevier]

Highlights: [•] We present quasi-geostrophic eigenmodes for cores of terrestrial-type planets. [•] Dependence of mode properties on Lehnert and Lundquist numbers is documented. [•] Magnetic diffusion causes slow modes to rapidly decay, especially in Mercury’s core. [•] Fast modes will play important roles in the cores of Earth, Ganymede and Mercury. [Copyright &y& Elsevier]

Abstract: The MESOBIO programme investigated mesoscale dynamics using an integrated ecosystem approach, linking physical and biogeochemical processes with different trophic levels. Observation and modeling were used in combination to explain the main processes occurring in the mesoscale eddy field. The particular shape of the Mozambique Channel, composed of two basins interconnected through a narrow zone, favours the generation of mesoscale eddies and increases the opportunity for eddy-shelf interactions. Phytoplankton abundance peaked in areas of nutrient enrichment that are often found in the core of cyclonic eddies, as well as on the continental shelf. Grazers in zooplankton communities exhibited high biovolume in cyclonic eddies, but their abundance was lower in fronts and divergence zones, with lowest biovolume in anticyclones. Biovolume was highest at shelf stations, but very variable and similar to phytoplankton. Age of eddies, their subsequent maturation stage and the dynamics of the eddy field played a major role effecting zooplankton abundance. Micronekton presented abundance patterns coherent with zooplankton distribution, however this was only demonstrated by acoustic methods, whereas mid-water trawl collection and predators stomach contents (predators being used as biological samplers) did not reveal significant relationships with mesoscale features. For upper trophic levels, the average density of foraging seabirds was lowest in anticyclones, highest in cyclones and at intermediate levels in divergence, shelf and frontal zones. However, multifaceted behavioral responses were observed in such a highly variable environment. Swordfish was clearly associated with divergence zones, and to a lesser extent with fronts, suggesting that the higher density in divergences was related to the presence of its main prey, essentially large squids. Although tunas tended to be more abundant in areas with weak geostrophic currents, their relationship to mesoscale features was not straightforward as adult tunas caught by longline have the ability to explore different foraging habitats over a broad range of depths. Several suggestions for advancing eddy-related research from the current state of knowledge are proposed in the second part of the paper. [Copyright &y& Elsevier]

Abstract: Circulation and the related biological production have been studied during five cruises conducted in the Mozambique Channel (MZC) between 2005 and 2010. The circulation in the MZC is known to be highly turbulent, favouring enhanced primary production as a result of mesoscale eddy dynamics, and connectivity throughout the Channel due to the variable currents associated with migrating eddies. This paper presents the results of in situ measurements that characterize the horizontal and vertical currents in the surface and subsurface layers (0–500m). The in situ data were analysed together with the geostrophic eddy field observed from satellite altimeter measurements. Different circulation regimes were investigated, including the “classical” anticyclonic eddy generated at the Channel narrows (16°S), the enhancement of southward migrating eddies by merging with structures (both cyclonic and anticyclonic) formed in the east of the Channel, and the presence of a fully developed cyclonic eddy at the Channel narrows. Comparison between in situ measurements (S-ADCP and velocities derived from surface drifters) and the geostrophic current derived from sea surface height measurements indicated that the latter can provide a reliable, quantitative description of eddy driven circulation in the MZC, with the exception that these currents are weaker by as much 30%. It is also suggested from in situ observation (drifters) that the departure from geostrophy of the surface circulation might be linked to strong wind conditions. Finally, our observations highlight that a-geostrophic currents need to be considered in future research to facilitate a more comprehensive description of the circulation in this area. [Copyright &y& Elsevier]

Spatial and temporal variability of the island mass effect (IME; defined as local increases of phytoplankton associated with the presence of islands) at the Juan Fernández Archipelago (JFA) is analyzed using chlorophyll-a (Chl-a) satellite data, altimetry, sea surface temperature, wind, geostrophic currents and net heat flux over a ten year period (2002–2012). The the JFA islands (Robinson Crusoe-Santa Clara (RC-SC) and Alejandro Selkirk (AS)) present wakes with significant Chl-a increases, mainly during spring time. These wakes can reach Chl-a values of one order of magnitude higher (~1mgm−3) than the surrounding oligotrophic waters (<0.1mgm−3). The wakes are similar to von Kármán vortex streets which have been used to explain the impact of IME on Chl-a increases in numerical models. The wakes are formed from a high productivity area in the lee of the island, extending to the oceanic region as high Chl-a patches associated with submesoscale eddies that are detached from the islands and connected by less-productive zones. This pattern coincides with previous models that predict the effects of island-generated flow perturbations on biological production variability. The IME is a recurrent feature of islands that has even been observed in decadal average fields. In such average fields, the Chl-a values in RC-SC and AS islands can exceed values found in a Control Zone (a zone without islands) by ~50% and 30%, respectively. Seasonal and interannual variability reveals that, as a consequence of the IME, the winter Chl-a maximum associated with the development of winter convection and mesoscale eddies that propagate from the continental zone, promote that the Chl-a maximum extends towards spring. The IME has an impact on the island on both a local as well as a more regional scale that affects an area of ~40,000km2 (1°Latitude×4°Longitude) centered on the islands. The transport of high productivity patches associated with submesoscale eddies may be responsible for IME propagation at a regional scale. Around the islands, the presence of a weak oceanic incident flow and strong and recurrent wind-wakes, suggest that the generation of Chl-a wakes result from a combined effect between both forcings. [ABSTRACT FROM AUTHOR]

Abstract: We calculate scenarios for the mean sea level on the Finnish coast by combining the land uplift, wind-induced changes in the local sea level, and large-scale sea level rise due to changes in ocean density and circulation and melting of land-based ice. The wind-induced changes were estimated by utilising their correlation with the zonal geostrophic wind, which explains 84–89% of the observed interannual variability of sea level on the Finnish coast. Future scenarios were based on the geostrophic wind projections from nine global circulation models. Land uplift rates are 4.1–9.9mm/yr, determined from the observations after filtering out the wind-induced effect. A 26–155cm range for the global mean sea level rise up to 2100 was obtained by combining several recently published scenarios. This rise is geographically unevenly distributed, and on the Finnish coast it is estimated to be only 24–126cm. Relative sea level change in the Gulf of Finland in 2000–2100 is projected to be +29cm (−22 to +92cm). A change of −5cm (−66 to +65cm) is projected for the Bothnian Sea, and −27cm (−72 to +28cm) for the Bothnian Bay, where the land uplift is stronger. [Copyright &y& Elsevier]

Highlights: [•] Predictability of small-scale ocean variability from large-scales is demonstrated. [•] Spectral nudging of large-scales can recover small-scales of a quasi-geostrophic model. [•] A practical scheme for assimilating large scales and point observations is proposed. [•] Limited point observations are more valuable if large-scales are spectrally nudged. [Copyright &y& Elsevier]

Abstract: Rings are marine vortices with a scale of hundreds of kilometers that can last for months, whose associated transport and mixing play an important role in the ocean dynamics. Such features are traditionally treated as a geostrophic flow, but since the centrifugal acceleration is not negligible in the inner core, the cyclo-geostrophic balance is a better approximation for the rings. In the present work, we describe a novel objective technique to identify the ring center, which is used as the origin of a convenient framework to handle rings under the cyclo-geostrophic balance. Furthermore, we correct the velocity field by the translation to isolate the swirl movement, a procedure ignored by other methodologies. We show that the lack of such correction would lead to an error of 30km on the center definition of a ship surveyed North Brazil Current Ring with 160km of radius. Another distinct characteristic of our approach is the flexibility in the spatio-temporal structure of the data, because it allows for ungridded data, an important ability for in situ observations. That also enables the use of a hybrid dataset composed from different instruments. The error on the Monte Carlo experiments to identify the center of the propagating ring is less than 10km, and depends on the level of noise, sampling strategy, and strength of the ring, among other factors. This technique was fully implemented in PyRings, an open Python library with a collection of procedures to handle oceanic rings and mesoscale eddies in general. [Copyright &y& Elsevier]

Abstract: Integrating the observations gathered by different platforms into a unique physical picture of the environment is a fundamental aspect of networked ocean observing systems. These are constituted by a spatially distributed set of sensors and platforms that simultaneously monitor a given ocean region. Remote sensing from satellites is an integral part of present ocean observing systems. Due to their autonomy, mobility and controllability, underwater gliders are envisioned to play a significant role in the development of networked ocean observatories. Exploiting synergism between remote sensing and underwater gliders is expected to result on a better characterization of the marine environment than using these observational sources individually. This study investigates a methodology to estimate the three dimensional distribution of geostrophic currents resulting from merging satellite altimetry and in situ samples gathered by a fleet of Slocum gliders. Specifically, the approach computes the volumetric or three dimensional distribution of absolute dynamic height (ADH) that minimizes the total energy of the system while being close to in situ observations and matching the absolute dynamic topography (ADT) observed from satellite at the sea surface. A three dimensional finite element technique is employed to solve the minimization problem. The methodology is validated making use of the dataset collected during the field experiment called Rapid Environmental Picture-2010 (REP-10) carried out by the NATO Undersea Research Center-NURC during August 2010. A marine region off-shore La Spezia (northwest coast of Italy) was sampled by a fleet of three coastal Slocum gliders. Results indicate that the geostrophic current field estimated from gliders and altimetry significantly improves the estimates obtained using only the data gathered by the glider fleet. [Copyright &y& Elsevier]

Abstract: Two episodic phytoplankton bloom events were identified in the southeastern and southwestern Bay of Bengal in January 1998 and December 2005, respectively, from SeaWiFS-derived chlorophyll-a concentration data collected between September 1997 and December 2010. Possible causes were examined using time series datasets of satellite-derived sea surface height anomalies, sea surface temperature, wind stress and Ekman pumping velocity data. It was found that strong cyclonic eddies with long residence time transported cold, nutrient-rich water from the subsurface layer to the euphotic zone, and were responsible for sustaining the continuous growth of phytoplankton that led to the two aforementioned phytoplankton blooms. Moreover, regarding the event in December 2005, three tropical cyclones passed through the region of the phytoplankton bloom and could have been additional factors that enhanced the bloom. Regarding the event in January 1998, transportation from the adjacent Andaman Sea by the geostrophic current and an anomalous zonal easterly wind contributed to the phytoplankton bloom in the southeastern Bay of Bengal. In addition, both blooms occurred during Indian Ocean Dipole (IOD) years. The absence of the second downwelling Kelvin waves in positive IOD year of 1997–1998 favouring the long residence of the strong cyclonic eddy in the eastern bay, and the strong wind-induced mixing in negative IOD year of 2005 might contribute to enhancing productivity. [Copyright &y& Elsevier]

Abstract: This paper quantitatively assesses the mesoscale spatial variability in vertical velocity associated with an open ocean eddy dipole. High-resolution, in situ data were collected during a research cruise aboard the NERC research ship RRS Discovery to the Iceland Basin in July/August 2007. A quasi-synoptic SeaSoar spatial survey revealed a southeastward flowing jet with counter-rotating eddies on either side. The anti-cyclonic component was identified as a mode water eddy, characterised by a homogenous core (∼35.5psu and 12°C) centred at a depth of ∼600m. Vertical velocities were calculated by inverting the quasi-geostrophic (QG) Omega equation at each point in a three-dimensional grid encompassing the dipole. The strongest vertical velocities (up to 5mday−1) were found primarily in the central jet between the eddies, as fast flowing water was forced over raised isopycnals associated with the large potential vorticity anomaly of the mode water eddy. Weaker upward (downward) vertical velocity was diagnosed ahead of the cyclonic (mode water) eddy in the direction of propagation, reaching 0.5mday−1 (2.5mday−1) at the depth of maximum potential vorticity (PV) anomaly. The results demonstrate that the mesoscale velocity field cannot be accurately reconstructed from analysis of individual isolated eddy features and that detailed three-dimensional maps of potential vorticity are required to quantify the cumulative effects of their interactions. An examination of potential sources of error associated with the vertical velocity diagnosis is presented, including sampling strategy, quasi-synopticity, sensitivity to interpolation length scale and the unquantified effect of lower boundary conditions. The first three of these errors are quantified as potentially reaching 50%, ∼20% and ∼25% of the calculated vertical velocity, respectively, indicating a potential margin of error in the vertical velocity diagnosis of order one. [Copyright &y& Elsevier]

Abstract: Nearly every spring since 1990, hydrographic data have been collected along a section in the Labrador Sea known as AR7W. Since 1995, lowered acoustic doppler current profiler (LADCP) data have also been collected. In this work we use data from six of these sections, spanning the time period 1995 through 2008, to determine absolute velocity across AR7W and analyze the main features of the general circulation in the area. We find that absolute velocity fields are characterized by strong, nearly barotropic flows all along the section, meaning there is no “level of no motion” for geostrophic velocity calculations. There is strong variability from year to year, especially in the strength of the boundary currents at each end; nevertheless, combining data from.all 6 sections yields a well-organized velocity field resembling that presented by Pickart and Spall (2007), except that our velocities tend to be stronger: there is a cyclonic boundary current system with offshore recirculations at both ends of the line; the interior is filled with virtually uniform, top-to-bottom bands of velocity with alternating signs. At the southwestern end of the section, the LADCP data reveal a dual core of the Labrador Current at times when horizontal resolution is adequate. At the northeastern end, the location of the recirculation offshore of the boundary current is bimodal, and hence the apparent width of the boundary current is bimodal as well. In the middle of the section, we have found a bottom current carrying overflow waters along the Northwest Atlantic Mid-Ocean Channel, suggesting one of various possible fast routes for those waters to reach the central Labrador Sea. We have used the hydrographic data to compute geostrophic velocities, referenced to the LADCP profiles, as well as to compute ocean heat transport across AR7W for four of our sections. For all but one year, these fluxes are comparable to the mean air–sea heat flux that occurs between AR7W and Davis Strait from December to May (O(50–80TW)), and much larger than the annual average values (O(10–20TW)). [Copyright &y& Elsevier]

Abstract: Horizontal density structure in the surface Chukchi Sea (in ice-free conditions) is investigated through analysis of high-resolution CTD data from two glider surveys. Temperature and salinity fields in the summer/fall surface layer indicate that horizontal temperature, salinity and density variability extends down to submesoscales. Horizontal temperature and salinity gradients in the surface layer are universally non-compensating, with salinity dominating the density variability. Spectral slopes of (k is horizontal wavenumber) of horizontal potential density variance in the surface layer were found over 0.5 to 20km wavelengths, consistent with quasi-geostrophic turbulence scaling. The character of horizontal density structure in the surface Arctic Ocean differs from spectra commonly observed in the lower latitudes. The observed submesoscale fronts play a role in setting surface-layer properties by restratifying the surface layer, in opposition to forcing such as winds that vertically mix the surface ocean. [Copyright &y& Elsevier]

Abstract: Hydrographic and velocity data collected during 11 surveys at the Chesapeake Bay entrance were used to assess fortnightly variations in the lateral structure of the subtidal water exchange and density stratification. The bay entrance extends from Cape Henry to Fishermans Island and features two channels, Chesapeake Channel and North Channel. The channels are separated by Middle Ground (<10m deep) and Six-Meters shoal. Water density data were available in 8 of the 11 surveys, but current velocity data were available for all surveys. The structure of density and velocity were analyzed by comparing river discharge and wind forcing. Seasonal river input modified the eddy viscosity coefficient, through stratification changes, and became the main factor determining the velocity structure. Bathymetry, wind forcing and fortnightly tidal variations were the other modifiers. Scenarios with similar river input and wind forcing conditions showed consistent structures and they were clearly influenced by neap and spring tidal conditions. Stronger vertical stratification occurred during springs rather than during neaps in the Chesapeake Channel, in contrast to what is observed in the North Channel and to what typically would be expected elsewhere from fortnightly variability. Consistent with the vertical stratification pattern, increased transverse density gradients and two-layer circulation developed over Middle Ground and Six-Meters Shoal during spring tides. Confined to the Chesapeake Channel longitudinal velocity seems to result in larger and closer to the channel transverse density gradient during springs than during neaps and consequently larger transverse velocities. [Copyright &y& Elsevier]

Abstract: A new estimate of the Global Ocean 3D geostrophic circulation from the surface down to 1500m depth (Surcouf3D) has been computed for the 1993–2008 period using an observation-based approach that combines altimetry with temperature and salinity through the thermal wind equation. The validity of this simple approach was tested using a consistent dataset from a model reanalysis. Away from the boundary layers, errors are less than 10% in most places, which indicate that the thermal wind equation is a robust approximation to reconstruct the 3D oceanic circulation in the ocean interior. The Surcouf3D current field was validated in the Atlantic Ocean against in-situ observations. We considered the ANDRO current velocities deduced at 1000m depth from Argo float displacements as well as velocity measurements at 26.5°N from the RAPID-MOCHA current meter array. The Surcouf3D currents show similar skill to the 3D velocities from the GLORYS Mercator Ocean reanalysis in reproducing the amplitude and variability of the ANDRO currents. In the upper 1000m, high correlations are also found with in-situ velocities measured by the RAPID-MOCHA current meters. The Surcouf3D current field was then used to compute estimates of the Atlantic Meridional Overturning Circulation (AMOC) through the 25°N section, showing good comparisons with hydrographic sections from 1998 and 2004. Monthly averaged AMOC time series are also consistent with the RAPID-MOCHA array and with the GLORYS Mercator Ocean reanalysis over the April 2004–September 2007 period. Finally a 15 years long time series of monthly estimates of the AMOC was computed. The AMOC strength has a mean value of 16Sv with an annual (resp. monthly) standard deviation of 2.4Sv (resp. 7.1Sv) over the 1993–2008 period. The time series, characterized by a strong variability, shows no significant trend. [Copyright &y& Elsevier]

Abstract: The study of mesoscale and submesoscale [hereafter (sub)mesoscale] hydrodynamic features is essential for understanding thermal and biogeochemical exchanges between coastal areas and the open ocean. In this context, a glider mission was conducted in August 2008, closely co-located and almost simultaneously launched with a JASON 2 altimetric pass, to fully characterize the currents associated with regional (sub)mesoscale processes regularly observed to the north of Mallorca (Mediterranean Sea). A synoptic view from satellite remote-sensing fields, before and during the glider mission, provided a descriptive picture of the main surface dynamics at the Balearic Basin scale. To quantify the absolute surface geostrophic currents, the coastal altimetry-derived current computation was improved and cross-compared with its equivalent derived from glider measurements. Model simulations were then validated both qualitatively and statistically with the multi-sensor observations. The combined use of modeling and multi-sensor observational data reveals the baroclinic structure of the Balearic Current and the Northern Current and a small-scale anticyclonic eddy observed northeast of the Mallorca coast (current∼15cm/s, <30km in extent and >180m deep). This mesoscale structure, partially intercepted by the glider and along-track altimetric measurements, is marked by relatively strong salinity gradients and not, as is more typical, temperature gradients. Finally, the use of the validated model simulation also shows that the geostrophic component of this small-scale eddy is controlled by sub-surface salinity gradients. We hypothesize that this structure contains recently modified Atlantic water arriving from the strait of Ibiza due to a northerly wind, which strengthens the northward geostrophic circulation. [Copyright &y& Elsevier]

Abstract: A linear internal-tide generation model is applied to the Faeroe–Shetland Channel, using observed profiles of stratification. Several degrees of simplification are considered: 1) uniform, i.e. constant N; 2) vertically varying N (z); 3) the full N(x, z) and associated geostrophic background flows. It is shown that clearly identifiable internal tidal beams and internal-wave attractors occur in the first case, but not in the other cases where the pattern is patchy due to internal reflections from the strong inhomogeneities in the medium. Given the general occurrence of pycnoclines and geostrophic background flows in the ocean, it is argued that this internal scattering can be expected to be a widespread phenomenon. [Copyright &y& Elsevier]

Abstract: Multiscale analysis is used to derive two sets of coupled models, each based on the same distinguished limit, to represent the interaction of the midlatitude oceanic synoptic scale—where coherent features such as jets and rings form—and the mesoscale, defined by the internal deformation scale. The synoptic scale and mesoscale overlap at low and mid latitudes, and are hence synonymous in much of the oceanographic literature; at higher latitudes the synoptic scale can be an order of magnitude larger than the deformation scale, which motivates our asymptotic approach and our nonstandard terminology. In the first model the synoptic dynamics are described by ‘Large Amplitude Geostrophic’ (LAG) equations while the eddy dynamics are quasigeostrophic. This model has order one isopycnal variation on the synoptic scale; the synoptic dynamics respond to an eddy momentum flux while the eddy dynamics respond to the baroclinically unstable synoptic density gradient. The second model assumes small isopycnal variation on the synoptic scale, but allows for a planetary scale background density gradient that may be fixed or evolved on a slower time scale. Here the large-scale equations are just the barotropic quasigeostrophic equations, and the mesoscale is modeled by the baroclinic quasigeostrophic equations. The synoptic dynamics now respond to both eddy momentum and buoyancy fluxes, but the small-scale eddy dynamics are simply advected by the synoptic-scale flow—there is no baroclinic production term in the eddy equations. The energy budget is closed by deriving an equation for the slow evolution of the eddy energy, which ensures that energy gained or lost by the synoptic-scale flow is reflected in a corresponding loss or gain by the eddies. This latter model, aided by the eddy energy equation—a key result of this paper—provides a conceptual basis through which to understand the classic baroclinic turbulence cycle. [Copyright &y& Elsevier]

Abstract: Based on current measurements at two mooring stations M2 from 25 April to 12 June, 2008 and M1 from 24 April to 11 June, CTD and wind data obtained in the Luzon Strait (LS), a three-dimensional diagnostic model with modified inverse method was used to study the regional circulation from April 23 to 26, 2008 (a La Nina year). The main results with the aid of diagnostic computation, T-S curve analysis and satellite geostrophic currents revealed the following: (1) Part of the Kuroshio flowed northwestward through the upper layer above 400m of mooring stations M2 and M1 in the LS, respectively. (2) In the layer above 400m the Kuroshio intrusion flowed northwestward through the longitudinal section at 120°53′E in the LS, and most continued to flow northwestward across the longitudinal section at 120°30′E. (3) In the northeastern region the circulation in the layer from 400m to 1200m was mainly dominated by the southeastward current, which came from the northern boundary at 21°17′N, instead of the Kuroshio. Finally, the southeastward current flowed eastward across the longitudinal section at 120°53′E in the LS into the Pacific in the layer from 400m to 1200m. (4) In the southern region the northward current with the South China Sea water flowed through the southern boundary from 20°00′N to 20°10′N, then it turned cyclonically and flowed westward across the southern part of longitudinal section at 120°30′E. (5) In comparison with the observed currents, the speeds of modeled currents at the 150, 200, 300 and 400m of the computed point (120°30.54′E, 21°2′N) were 35.75, 25.50, 15.27 and 12.67cm/s, respectively; the mean value of modeled currents at above four observed levels was 22.30cm/s, while the mean of observed currents values at the 150, 200, 300 and 400m levels of mooring station M2 (120°30.332′E, 20°59.961′N) were 33.08, 27.64, 13.18 and 11.67cm/s, respectively; the mean value of observed currents at the above four observed levels was 21.39cm/s. This signifies that the mean value of modeled currents in the upper layer above the 400m levels was overestimated by 4.3%. (6) After comparison of the observations, the adjusted net westward modeled volume transports (VT) across the longitudinal section at 120°53′E were 4.66 and 2.66×106 m3 s−1, respectively, in the entire depth and the upper layer above 500m levels of the LS during spring 2008. This indicates that the modeled VT of the westward Kuroshio intrusion across the upper layer of LS in during spring 2008 was much less than that in spring 1992 (an El Nino year) and normal years. That is to say, the westward intruding VT of Kuroshio across the upper layer of the LT may be closely related to the El Nino (or La Nina) phenomenon, being higher during El Nino and lower during La Nina, and it may be also related to the mesoscale variability in the LS. [Copyright &y& Elsevier]

Abstract: Short-term flow and water temperature fluctuations in Sagami Bay were examined using mooring and hydrographic data observed during the non-large-meander path (NLM) of the Kuroshio. In the surface layer (<150m), the flow fluctuation is predominant and is excited by the sudden strong eastward flow with speeds greater than 20cms−1 in the central part of the bay. This flow is a part of an intrusion of Kuroshio water via the Oshima West Channel into the bay and/or a cyclonic circulation in the northern part of the bay. The cyclonic circulation, which is approximately balanced under the geostrophic flow relationship, is mainly generated or enhanced by the intrusion, which is accompanied by eastward propagation of the small meander of the Kuroshio south of Japan during the transition from the nearshore non-large-meander path (nNLM) to the offshore non-large-meander path (oNLM), regardless of season. The water temperature fluctuation in the subsurface layer (∼150m) is caused by upwelling of cold deeper water and is closely related to the eastward flow fluctuations in the surface layer in the central part of the bay. Our analysis leads us the conclusion that the upwelling occurs as the response of the subsurface density field to the surface flow field under the Earth’s rotation. Additionally, the intrusion of Kuroshio water via the Oshima West Channel tends to cause, not only cyclonic circulation and upwelling in the northern part of the bay, but also the Kyucho, which is the coastal density current under the Earth’s rotation, in the bay’s coastal area during the non-large-meander path (NLM). [Copyright &y& Elsevier]

Abstract: This paper is concerned with the weak–strong uniqueness of the Cauchy problem for the 2D dissipative quasi-geostrophic equation. We proved the weak–strong uniqueness when one of the two weak solutions lies in the regular class . [Copyright &y& Elsevier]

Abstract: There are two zonal bands of eminently high eddy kinetic energy (EKE) in the North Pacific Ocean. The highest one is located in the Kuroshio Extension and the second one is in the subtropical area. This paper focuses on the latter. An eddy detection scheme based on velocity vector geometry is applied to the SSHA-derived geostrophic currents to identify and track eddies, and to generate an eddy dataset which includes spatial and temporal information on eddy generation, evolution and termination. Analysis of this dataset allowed the investigation of a broad range of eddy parameters. From 1993 to 2010, about six thousand eddies with lifetime longer than eight weeks were generated within the band. All eddies moved westward, and both cyclonic and anticyclonic eddies deflected northward south of 21°N and southward north of 21°N, respectively. Statistically, three different stages of an eddy''s lifetime can be identified: the first one-fifth of its lifetime corresponds to the growing period; the successive three-fifths after that to its stable stage; the last one-fifth to its decaying period. Observed Argo vertical profiles collected within the detected eddy areas are used to investigate the eddy-induced vertical displacement of the thermocline and the halocline. Frontal intensity derived from the SST data is used to explain the mechanism modulating temporal and spatial eddy variations within the zonal band. [Copyright &y& Elsevier]

Abstract: An intense oceanic front was detected at the west of Mallorca Island (Balearic sub-basin of the North Western Mediterranean Sea) during an oceanographic survey in December 2009. This contribution analyses the hydrography and geostrophic motions observed at the front, together with the ageostrophic motion derived from the omega equation. The front separated resident Atlantic water (to the north) from more recent Atlantic water (to the south). Maximum upward vertical velocities of 6m/day were found at the northern side of the front, related with relative maxima of dissolved oxygen and fluorescence. The vertical velocities in this study are mainly due to relative vorticity advection. AVISO altimetry data is used to discuss the advection of the front over a nearby mooring equipped with temperature and salinity sensors at 300m below the sea surface. The biological implications of the front are discussed by means of acoustic backscatter data. There is an observed increase in mesopelagic fish biomass at the frontal area. Our hypothesis states that the vertical velocities associated to the front would lead to the observed increase in mesopelagic biomass near the front, which is in agreement with previous studies in other frontal systems. [Copyright &y& Elsevier]

Abstract: We obtain regularity criteria for a quasi-geostrophic equation that depends more on one direction than the others. In particular, we show that in the critical case, the global regularity depends only on a partial derivative rather than a gradient of the solution. [Copyright &y& Elsevier]

Abstract: We present arguments supporting the hypothesis that the flow in the Earth''s core, for the time scales of the historical secular variation, is well described by a quasi-geostrophic (QG) model, almost invariant along the rotation axis. A previous study showed that for axisymmetric motions, the dimensionless number appropriate to compare magnetic and rotation forces is independent of magnetic diffusivity, increases with decreasing length scales, and takes value much smaller than unity for lengths about 106 m. Here, we extend this result to non-axisymmetric motions, and give a criterion for QG to hold based on length scale, rotation rate and magnetic field intensity. The numerical simulations exhibit a columnar behaviour at parameters representative of the Earth''s core, supporting the quasi-geostrophic hypothesis for fast, large length-scale motions. In addition, we present the results of several inversions of the core flow from geomagnetic field models, showing that (a) the energy of the motions symmetrical with respect to the equatorial plane represents about 80% of the total energy when no symmetry is assumed a priori; (b) for the same number of parameters, an equatorially symmetric (QG) flow model explains more of the secular variation than a flow without specified geometry. [Copyright &y& Elsevier]

Abstract: The surface currents of the Levantine sub-basin (Mediterranean Sea) are described using 18 years (1992–2010) of drifter data and satellite-derived sea level anomalies. The combination of drifter and satellite data allowed to estimate maps of surface geostrophic circulation and to obtain more accurate pseudo-Eulerian velocity statistics for different time periods. Seasonal and interannual variability of surface currents are investigated with particular focus on the main sub-basin eddies of the eastern Levantine. The mean velocity field depicts the typical patterns of the along-slope and offshore currents and outlines the sub-regions where eddies are generated recurrently (west Egyptian coast, Ierapetra, Mersa-Matruh, south-west of Cyprus, Israel–Lebanon coast, Latakia) or persist steadily (Rhodes Gyre). Highly variable and energetic currents are observed between the Ierapetra and Mersa-Matruh regions, as the result of the interaction of the Mid-Mediterranean Jet meandering in between, and interacting with, the eddies generated by the instability of the coastal current. Seasonal pseudo-Eulerian maps show the current field stronger in summer and weaker in winter, mainly in the western Levantine and in the Cyprus–Syria Passage. The Shikmona Eddy displays a periodic nature with higher intensities during the cold months and an enhanced activity in the period 1998–2005. The Cyprus Eddy has a less periodic nature, characterised by events of high activity and periods in which it dominates as a single enlarged eddy in the southeast Levantine, eventually including the Shikmona Eddy. The Latakia Eddy is mainly cyclonic with higher intensities in summer and fall; occasional weekly or monthly inversions of circulation from cyclonic to anticyclonic are triggered by the interaction between the MMJ and the northward coastal meandering current. [Copyright &y& Elsevier]

Abstract: The impact of assimilation of sea surface velocity fields observed by a set of high-frequency (HF) radars is studied using a three-dimensional ocean circulation model configured along the Oregon coast. The study period is June–July 2008 featuring upwelling and separation of the coastal currents into the adjacent interior ocean. The nonlinear model is based on the Regional Ocean Modeling System (ROMS) and the data assimilation (DA) component on the AVRORA system utilizing the representer-based variational algorithm. Assimilation proceeds in a series of 3-day windows, providing an analysis solution in each window and a 3-day forecast into the next window. Experiments with two different initial condition error covariances are compared (one is dynamically balanced, based on the linearized equation of state, temperature-salinity relation, and geostrophic and thermal wind balance relations and the other is multivariate unbalanced). While the assimilation impact is statistically better in the case of the balanced covariance, the case with the unbalanced covariance also provides sensible improvement in terms of surface velocity and sea surface temperature (SST) model-data forecast statistics. The analysis of representer functions shows that even if the initial condition error covariance is unbalanced, the correction fields at the model initial time are partially balanced after each dynamical field is smoothed independently, due to inherent dynamical properties of the adjoint model. Assimilation of the HF radar surface currents improves not only surface velocity forecasts, but also geometry of the upwelling SST front and the sea surface height (SSH) slope near the coast, as verified against unassimilated satellite SSH and SST data. The assimilation also alters the latitudinal distribution of the time-averaged offshore transport. Combined HF radar velocity and other observations, e.g., altimetry, is needed to better constrain surface geostrophic currents in the entire model domain, including the area not covered by the HF radar data. [Copyright &y& Elsevier]

Abstract: Variable properties of the Mediterranean outflow and the variability of the Western Alboran Gyre are analyzed by means of 5-year long time series of near bottom potential temperature at Espartel sill in the Strait of Gibraltar and altimetry data in the Alboran Sea. Geostrophic velocity at the southern edge of the gyre and potential temperature at Espartel sill are significantly correlated (correlation coefficient 0.67), suggesting that the intensification of the Alboran Gyre favors the ventilation of Western Mediterranean Deep Water. The analysis of historical temperature profiles shows that Western Mediterranean Deep Water in the Alboran Sea can be suctioned from a layer between 500 and 700m depth for typical changes of the gyre intensification. [Copyright &y& Elsevier]

Abstract: Four repeat sections across the East Greenland shelf and slope south of Denmark Strait are analysed to investigate the components of the boundary current system. The sections were occupied in summer 2001, 2003, 2004 and 2007, and included use of a vessel-mounted acoustic Doppler current profiler, enabling the computation of absolute geostrophic velocities. The components of the boundary current are the East Greenland/Irminger Current (EGIC) in the upper layer, the Deep Western Boundary Current (DWBC) at the base of the continental slope, and the East Greenland spill jet which resides inshore and beneath the EGIC. Special emphasis is placed on the spill jet, a recently discovered feature about which relatively little is known. The spill jet was observed in each occupation, transporting 5.0±2.2Sv equatorward in the mean, which is similar to the DWBC at this latitude (4.9±1.4Sv). The spill jet displayed considerable variability between sections, which appears to be linked to the geographical location of the upper-layer hydrographic front associated with the EGIC. When the front is located near the shelfbreak, the spill jet is confined to the outer shelf/upper slope and its transport is smaller. During these times there is less mixing and the water advected by the jet is generally lighter than that transported by the DWBC. In contrast, when the front is located seaward of the shelfbreak, the spill jet extends farther down the continental slope and its volume flux is larger. At these times, there is stronger mixing and the spill jet can transport water as dense as the Denmark Strait Overflow Water. A vorticity analysis indicates that the jet is susceptible to a variety of instability processes including baroclinic, barotropic and symmetric instability. In addition, it is subject to double diffusive mixing that may influence its downstream evolution. It appears that the spill jet is a permanent feature of the summertime circulation in this region and contributes significantly to the intermediate, and at times deep, limb of the Atlantic Meridional Overturning Circulation. [Copyright &y& Elsevier]

Abstract: A perceived limitation of z-coordinate models associated with spurious diapycnal mixing in eddying, frontal flow, can be readily addressed through appropriate attention to the tracer advection schemes employed. It is demonstrated that tracer advection schemes developed by Prather and collaborators for application in the stratosphere, greatly improve the fidelity of eddying flows, reducing levels of spurious diapycnal mixing to below those directly measured in field experiments, ∼1×10−5 m2 s−1. This approach yields a model in which geostrophic eddies are quasi-adiabatic in the ocean interior, so that the residual-mean overturning circulation aligns almost perfectly with density contours. A reentrant channel configuration of the MIT General Circulation Model, that approximates the Antarctic Circumpolar Current, is used to examine these issues. Virtual analogs of ocean deliberate tracer release field experiments reinforce our conclusion, producing passive tracer solutions that parallel field experiments remarkably well. [Copyright &y& Elsevier]

Abstract: This paper concerns with the asymptotic stability for the weak solution of the two-dimensional dissipative subcritical quasi-geostrophic equation in the classIt is proved that although the initial data and external force perturbations from θ are large, every perturbed weak solution converges asymptotically to θ in L 2(R 2) as t →∞. As a byproduct, we also obtain the weak–strong uniqueness criterion of subcritical quasi-geostrophic equation which improves the earlier results. [Copyright &y& Elsevier]

Abstract: Hydrographic observations based on XBT, XCTD, CTD, and ADCP were performed around the eastern mouth of the Tsugaru Strait in June 2003. This study aims to describe the hydrographic structure of the eastern mouth focusing on the subarctic Oyashio water occupying the northern part, and to discuss the intrusion pathway of this water and its modification after the intrusion. Residual and absolute geostrophic velocities show a nearly barotropic inflow (0–25cm/s) and outflow (0–80cm/s) structures in the northern and southern parts of the strait mouth, respectively. In the north, the Oyashio water, classified as the near-bottom and modified Oyashio waters in this study, inflows into the strait. It is inferred that the near-bottom water is changed to the modified Oyashio water after the intrusion by diapycnal mixing and outflow as the modified Oyashio water from underneath of the Tsugaru Warm Current (TWC) water in the deepest part of the strait mouth, together with the TWC water. Horizontal salinity distribution suggests that the near-bottom water is supplied along the isopycnal surface from the subsurface of Hidaka Bay deeper than the strait. [Copyright &y& Elsevier]

Abstract: The California Current System is described in its regional setting using two modern datasets. Argo provides a broadscale view of the entire eastern North Pacific Ocean for the period 2004–2010, and the High Resolution XBT Network includes transects from Honolulu to San Francisco (1991–2010) and to Los Angeles (2008–2010). Together these datasets describe a California Current of 500–800km width extending along the coast from 43°N to 23°N. The mean southward transport of the California Current is about 5Sv off Central and Southern California, with about 2.5Sv of northward flow on its inshore side. Interannual variations are 50% or more of the mean transports. The salinity minimum in the core of the California Current is supplied by the North Pacific Current and by freshwater from the northern continental shelf and modified by alongshore geostrophic and across-shore Ekman advection as well as eddy fluxes and air–sea exchange. The heat and freshwater content of the California Current vary in response to the fluctuating strength of the alongshore geostrophic flow. On its offshore side, the California Current is influenced by North Pacific Intermediate Waters at its deepest levels and by Eastern Subtropical Mode Waters on shallower density surfaces. In total, the sources of the California Current, its alongshore advection, and its strong interactions with the inshore upwelling region and the offshore gyre interior combine to make this a rich and diverse ecosystem. The present work reviews previous contributions to the regional oceanography, and uses the new datasets to paint a spatially and temporally more comprehensive description than was possible previously. [Copyright &y& Elsevier]

Abstract: High-resolution data collected southeast of the Canary Islands during late winter 2006 are analyzed to describe the hydrography and three-dimensional circulation in the coastal transition zone off NW Africa. The data are optimally interpolated over a regular grid, the geostrophic velocity field is calculated and the Q-vector formulation of the omega equation is used to compute the quasi-geostrophic (QG) mesoscale vertical velocity. The coastal transition zone is divided into upwelling, frontal and offshore regions with distinct physical and dynamic characteristics. The upwelling region is characterized by cold and weakly stratified waters flowing towards the equator, with a poleward undercurrent of approximately 0.05ms−1 over the continental slope. The frontal region exhibits a southwestward baroclinic jet associated with cross-shore raising isopycnals; the jet transport is close to 1Sv, with maximum velocities of 0.18ms−1 at surface decreasing to 0.05ms−1 at 300db. Vertical sections across the frontal region show the presence of deep eddies probably generated by the topographic blocking of the islands to the southward current, as well as much shallower eddies that likely have arisen as instabilities of the baroclinic upwelling jet. The QG mesoscale vertical velocity field is patchy, estimated to range from −18 to 12mday−1, with the largest absolute values corresponding to an anticyclonic eddy located south of Fuerteventura Island. These values are significantly larger than estimates for other vertical velocities: diapycnal vertical velocities associated with mixing in the frontal region (a few meters per day), and wind-induced vertical velocities (non-linear Ekman pumping arising from the interaction between the wind stress and the background vorticity, maximum values of a few meters per day; linear Ekman pumping due to the divergence of Ekman transport, a fraction of a meter per day; or the coastal constraint in the upwelling region, about 0.7mday−1). However, the patchiness in both the QG mesoscale vertical velocity and the non-linear Ekman pumping velocity cause their integrated vertical transports to be one order of magnitude smaller than either coastal Ekman transport (0.08Sv), integrated linear Ekman pumping (−0.05Sv) or diapycnal transfer (about 0.1–0.2Sv). The pattern of the near-surface fluorescence field is a good indicator of these different contributions, with large homogeneous values in the coastal upwelling region and a patchy structure associated with the offshore mesoscale structures. [Copyright &y& Elsevier]

Abstract: Results are presented from a series of laboratory modelling experiments on dense oceanic outflows, in which the behaviour of dense bottom gravity currents in an inclined, submerged, rotating and convergent vee-shaped channel has been investigated. High-resolution density and velocity profile measurements are presented to demonstrate that the currents are in geostrophic balance along the converging channel section for the full range of parametric conditions investigated. Comparisons with theoretical model predictions based on inviscid rotating hydraulics and Ekman boundary layer dynamics modelling approaches are shown to demonstrate well many qualitative aspects of the hydraulic behaviour of dense bottom gravity currents within such rotating systems. Quantitative discrepancies are ascribed primarily to the omission, in the theoretical analyses, of shear-induced turbulent entrainment and mixing processes. The results demonstrate that the essential dynamics and physical description of the dense water gravity current along the converging, up-sloping channel are well represented by the combined application of the two theoretical modelling approaches adopted. [Copyright &y& Elsevier]

Abstract: Morphogroups offer a way of overcoming taxonomic differences and thereby making comparisons between assemblages of different geological ages. Relating morphogroups to environments has proved useful in palaeoecological interpretations. The model proposed by Jones and Charnock in 1985 has subsequently been adapted and still serves as the main reference for such palaeoecological interpretations. New data from >400 samples from marsh to deep sea off NW Europe are used here to refine the model by providing the range of abundance of each morphogroup in each environment. It is shown that this range of variation is great so that there is considerable overlap between environments. Nevertheless, the mean values show patterns of distribution. Tubular forms (morphogroup A) are characteristic of slope environments, especially those under the influence of geostrophic currents. Morphogroup B is not common in marginal marine environments but is an indicator of shallow fjord to deep sea. The elongate forms of morphogroup C are widely distributed but C1 is especially common in marginal marine and shelf to upper slope settings. The quinqueloculine coiled C2 subgroup is characteristic of marsh and marginal marine environments in the temperate zone but around Antarctica this form is common down to 2000m. The plano-convex, low trochospiral trochamminids of morphogroup D are typical of marshes. Therefore, these trends have application to palaeoecology of similar well-oxygenated temperate environments. [Copyright &y& Elsevier]

Abstract: Laboratory realizations and numerical simulations of buoyant, gravity-driven coastal plumes are summarized and compared to the inviscid geostrophic theory of . The lengths, widths and velocities of the buoyant currents, as well as their internal structure and dynamics, are studied. Agreement between the laboratory and numerical experiments and the geostrophic theory is found to depend on two non-dimensional parameters which characterize, respectively, the steepness of the plumes isopycnal interface (I) and the strength of horizontal viscous forces (Ek H , the horizontal Ekman number). In general, the numerical and laboratory experiments are in good agreement when conducted at comparable values of I and Ek H . The best agreement between experiments (both laboratory and numerical) and the geostrophic theory are found for the least viscous flows, though important departures from the theoretical predictions are nonetheless found, particularly in the early development of the plume system. At elevated values of the horizontal Ekman number, laboratory and numerical experiments depart more significantly from theory, e.g., in the rate of plume movement along the coast. A simple extension to the geostrophic theory suggests that the discrepancy between the theoretical and experimental propagation speed should be proportional to the square root of the horizontal Ekman number. The numerical simulations confirm this relationship. For some combinations of the non-dimensional parameters, instabilities develop in the seaward edge of the buoyant plumes. The laboratory and numerical experiments are used together to infer the region within parameter space within which the instabilities occur. Mixing of ambient and buoyant fluids by the plume-edge instabilities is explored using the numerical results. [Copyright &y& Elsevier]