Core Of Eddies Research Articles

Mesoscale eddies in the southwestern tropical Atlantic

The southwestern tropical Atlantic is a region of complex ocean dynamic where originates the strong western boundary current system composed of North Brazil current and North Brazil undercurrent. The region includes a variety of features including the Atoll das Rocas (AR) and Fernando de Noronha (FN) ridge that may favour mesoscale eddy dynamics. However, origin, occurrence and characteristics of mesoscale eddies were still not described in the region. Using satellite altimetry data from 1993 to 2018 off Northeast Brazil (37-25°W; 13-1°S), we reconstruct eddy trajectories and analyse the main eddy surface characteristics (e.g., size, amplitude, polarity) and their spatiotemporal variations. The study reveals two distinct dynamic regions before quantifying mesoscale eddies characteristics. Approximately 2000 mesoscale eddies crossed the region during the study period, among which 76% were generated inside the region, with amplitudes and radii ranging between 1 and 2 cm and 25 and 205 km, respectively. Eddies are preferentially formed between August and September and propagate westward. In the region around the FN Archipelago (36-26°W; 6-1°S), the formation of cyclonic eddies is likely favoured by barotropic instabilities of surface currents and the wind stress curl. On the other hand, in the south of the region (36-26°W; 12-8°S), eddies formation is likely associated with the barotropic instabilities, wind stress curl and the meandering of surface currents. Based on vertical temperature and salinity profiles from Argo floats’ data, we determined that in average, the core of cyclonic eddies is centred at ~ 130 m (140 m) in the northern (southern) region while the core of anticyclonic eddies is centred at ~90 m (125 m) in the northern (southern) region. Moreover, mesoscale eddies formed in the tropical Atlantic do not connect the eastern tropical Atlantic and northeast Brazil.

Non-equilibrium three-dimensional boundary layers at moderate Reynolds numbers

Non-equilibrium wall turbulence with mean-flow three-dimensionality is ubiquitous in geophysical and engineering flows. Under these conditions, turbulence may experience a counter-intuitive depletion of the turbulent stresses, which has important implications for modelling and control. Yet, current turbulence theories have been established mainly for statistically two-dimensional equilibrium flows and are unable to predict the reduction in the Reynolds stress magnitude. In the present work, we propose a multiscale model which explains the response of non-equilibrium wall-bounded turbulence under the imposition of three-dimensional strain. The analysis is performed via direct numerical simulation of transient three-dimensional turbulent channels subjected to a sudden lateral pressure gradient at friction Reynolds numbers up to 1,000. We show that the flow regimes and scaling properties of the Reynolds stress are consistent with a model comprising momentum-carrying eddies with sizes and time scales proportional to their distance to the wall. We further demonstrate that the reduction in Reynolds stress follows a spatially and temporally self-similar evolution caused by the relative horizontal displacement between the core of the momentum-carrying eddies and the flow layer underneath. Inspection of the flow energetics reveals that this mechanism is associated with lower levels of pressure-strain correlation which ultimately inhibits the generation of Reynolds stress. Finally, we assess the ability of the state-of-the-art wall-modelled large-eddy simulation to predict non-equilibrium, three-dimensional flows.

The role of eddies on pathways, transports, and entrainment in dense water flows along a slope

The flow of dense water along slopes has been investigated in several numerical investigations based on the Dynamics of Overflow Mixing and Entrainment (DOME) setup. In the present study, we try to obtain further insight into the pathways, transports, dynamics, and entrainment of such flows by performing numerical model studies with horizontal grid sizes of 10 km and 2.5 km. It is found that the rates of descent of the plumes along the slope are robust to the horizontal resolution. With a high vertical resolution and a bottom boundary condition that facilitates the representation of Ekman drainage, the plumes will follow a deeper path than when using quadratic bottom drag with a constant drag coefficient. In the results from the studies with 2.5 km horizontal grid, ambient lighter water inside anticyclonic eddies is sucked downward. Due to Ekman drainage, this water flows outwards near the bottom and underneath denser plume water. The water column around the core of the anticyclonic eddies becomes unstable, and ambient water is entrained into the plume. Due to the increased mixing and entrainment in the eddy-permitting regime, there is a substantial increase in the along slope plume transports when we reduce the grid size from 10 km (the laminar case) to 2.5 km (the eddy-permitting case).

On the 3D structure of eddies in the Arabian Sea

In the Arabian Sea, mesoscale eddies are prominent features of the circulation. They influence biological activities, tracer transport, and physical and chemical properties of the seawater. In particular, the pathways of salty water masses outflowing from the marginal seas are strongly impacted by the mesoscale eddies that evolve in the western part of the basin. The impact of mesoscale eddies on the different salty water masses has been investigated in previous studies with altimetric data and in situ data for short durations. In this study, we extend this analysis to most of the mesoscale eddies detected from altimetry data for the period 2000–2015. We use Argo floats colocalized with eddies to describe the 3D impact of mesoscale eddies on the dynamical and thermohaline properties of water masses in the Arabian Sea. The ∼30,000 Argo stations used show a well characterized distribution of water masses in the different sub-basins of the Arabian Sea, with the Persian Gulf Water (PGW) flowing from the north and the Red Sea Outflow Water (RSOW) from the northwest. In addition, colocalizing Argo profiles in or out of these eddies allows for the extraction of temperature and salinity anomalies related to mesoscale eddies. Then, we present the vertical structure of density associated with these eddies. These results allow us to divide the Arabian Sea into areas depending on their hydrological and dynamical properties. In the Gulf of Oman and the Gulf of Aden, the water in the core of eddies is fresher than at their edge. Thus, most of eddies residing in these gulfs were generated in the Arabian Sea. In these eddies, the salinity anomalies corresponding to PGW and RSOW are on average stronger at their edge than in their core; this indicates that eddies in these gulfs impact outflow waters via advecting processes on their rim. Finally, we select the northern part of the Arabian Sea (including the Gulf of Oman), where most Argo profiles were collected. We calculate the composite 3D profiles of angular velocity and salinity anomaly for surface-intensified cyclonic eddies and subsurface anticyclonic eddies, with radii of about 100 km. The velocity profiles computed via the thermal wind equations are validated using surface velocities from altimetry and surface drifter measurements. On average, eddies in the northern Arabian Sea exhibit a clear pattern of PGW in their core. The comparison of these 3D composite structures with a particular cyclone near the Strait of Hormuz and a dipole sampled with a SeaSoar along the Omani Coast shows that they are representative of mesoscale eddies often observed in the area.

Seasonal and spatial variability of surface chlorophyll inside mesoscale eddies in the South China Sea

Mesoscale eddies can influence biogeochemical cycles through both vertical nutrient or plankton flux and horizontal advection of nutrient or plankton in eddy periphery. In this study, we analyzed the seasonal and spatial variability of near-sea-surface chlorophyll-a concentrations and their corresponding modulation mechanisms by collocating satellite observations of eddies tracked during 1998–2007 in the South China Sea. We found that in winter, cyclonic eddies show significantly high chlorophyll-a in the eddy core relative to the periphery, suggesting regulation by an eddy-pumping mechanism. While in summer, chlorophyll-a values in the core of cyclonic eddies are comparable with those in the periphery, and dipole features of chlorophyll-a around cyclonic eddies are conspicuous, which is largely induced by horizontal eddy advection. For anticyclonic eddies, however, low chlorophyll-a in eddy cores compared with the periphery are consistently present over most of the year. The impact of eddy advection on chlorophyll-a distribution in anticyclonic eddies is not clear from the composite images in different seasons. Variability of chlorophyll-a to the west of the Luzon Strait and northwest of the Luzon Island demonstrate that phytoplankton biomass in these two regions is mainly controlled by an eddy-pumping mechanism. In the regions off the Vietnam coast, chlorophyll-a distributions are generally associated with horizontal eddy advection. This research highlights different mesoscale mechanisms affecting biological structures in the upper ocean, which can potentially disturb ocean biogeochemical cycling processes in the South China Sea.

Submesoscale Dynamics of a Gulf Stream Frontal Eddy in the South Atlantic Bight

AbstractFrontal eddies are commonly observed and understood as the product of an instability of the Gulf Stream along the southeastern U.S. seaboard. Here, the authors study the dynamics of a simulated Gulf Stream frontal eddy in the South Atlantic Bight, including its structure, propagation, and emergent submesoscale interior and neighboring substructure, at very high resolution (dx = 150 m). A rich submesoscale structure is revealed inside the frontal eddy. Meander-induced frontogenesis sharpens the gradients and forms very sharp fronts between the eddy and the adjacent Gulf Stream. The strong straining increases the velocity shear and suppresses the development of barotropic instability on the upstream face of the meander trough. Barotropic instability of the sheared flow develops from small-amplitude perturbations when the straining weakens at the trough. Small-scale meandering perturbations evolve into rolled-up submesoscale vortices that are advected back into the interior of the frontal eddy. The deep fronts mix the tracer properties and enhance vertical exchanges of tracers between the mixed layer and the interior, as diagnosed by virtual Lagrangian particles. The frontal eddy also locally creates a strong southward flow against the shelf leading to topographic generation of submesoscale centrifugal instability and mixing. In eddy-resolving models that do not resolve these submesoscale processes, there is a significant weakening of the intensity of the upwelling in the core of the frontal eddies, and their decay is generally too fast.

The dynamical landscape of marine phytoplankton diversity.

Observations suggest that the landscape of marine phytoplankton assemblage might be strongly heterogeneous at the dynamical mesoscale and submesoscale (10-100 km, days to months), with potential consequences in terms of global diversity and carbon export. But these variations are not well documented as synoptic taxonomic data are difficult to acquire. Here, we examine how phytoplankton assemblage and diversity vary between mesoscale eddies and submesoscale fronts. We use a multi-phytoplankton numerical model embedded in a mesoscale flow representative of the North Atlantic. Our model results suggest that the mesoscale flow dynamically distorts the niches predefined by environmental contrasts at the basin scale and that the phytoplankton diversity landscape varies over temporal and spatial scales that are one order of magnitude smaller than those of the basin-scale environmental conditions. We find that any assemblage and any level of diversity can occur in eddies and fronts. However, on a statistical level, the results suggest a tendency for larger diversity and more fast-growing types at fronts, where nutrient supplies are larger and where populations of adjacent water masses are constantly brought into contact; and lower diversity in the core of eddies, where water masses are kept isolated long enough to enable competitive exclusion.

The influence of mesoscale eddies on a commercial fishery in the coastal waters of the Andaman and Nicobar Islands, India

Mesoscale eddies enhance the productivity in a stratified coastal environment by upwelling. The seas around the Andaman and Nicobar Islands have been found to have frequent mesoscale eddy activity. Commercial fishing grounds coincide with upwelling areas associated with cyclonic and anticyclonic eddies and also with areas between two adjacent eddies. There are different eddy zones supporting different types of fishing gears and fish. The current study aims at identifying the different zones of mesoscale eddies in the Andaman Sea and compares the productivity and fishing activity in each of them. Data collected from 454 commercial fishing trips in the Andaman Sea along with maps of sea level anomaly and Moderate Resolution Imaging Spectroradiometer (MODIS) global level 3 mapped thermal infrared (IR) daytime sea surface temperature (SST) from the Aqua and Terra satellites were used for the study. Known upwelling areas such as the periphery of anticyclonic and the core of cyclonic eddies showed higher catches in longlines, ring seines among the fishing gears, and among all the fish species groups. Downwelling areas such as the periphery of cyclonic and the core of anticyclonic eddies showed lower catches with ring seines and the fish species groups. Areas in between adjacent eddies were explored in this study and the fish captures in such areas were found to be different with types of fishing and the target fish group. The study shows results that link eddy activity with the performance of a fishery.

Foraging spots of streaked shearwaters in relation to ocean surface currents as identified using their drift movements

Ocean currents are in continuous motion and strongly influence oceanic ecosystems. In situ observation of currents is of primary importance for understanding how marine animals respond to ocean surface currents at various scales and for realizing effective ecosystem-based management and realistic oceanographic modelling. We developed a new method for obtaining in situ current measurements by using seabirds as Lagrangian current sensors akin to drifting buoys. We deployed high-resolution global positioning system (GPS) loggers on streaked shearwaters (Calonectris leucomelas) foraging in the Oyashio-Tsugaru Warm Current confluence in Japan, which is one of the most productive oceans in the world. The seabirds repeatedly performed foraging trips, including searching for prey and resting on the sea surface, over several hundred kilometres. The seabirds spent half of their time resting on the water surface and tended to be passive drifters. We inferred that the drift movements of C. leucomelas provided a direct and detailed description of the ocean surface currents, because currents deduced from their drift movements were in good agreement with ocean surface currents derived from in situ and satellite data. In addition, we extracted details of shearwaters’ intense searching flights associated with feeding (i.e. foraging spots) from GPS tracks. C. leucomelas did not forage at the core of anticyclonic eddies; rather, they used the boundary areas between eddies and the edge of eddies where primary productivity and prey density are thought to be high. Our study demonstrated that animal-borne GPS data can provide a detailed and cost-efficient tool for observing ocean surface currents and can reveal the ways in which marine animals respond to these currents at a fine scale.

Ecosystem functioning in the Mozambique Channel: Synthesis and future research

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.

Influence of mesoscale features on micronekton and large pelagic fish communities in the Mozambique Channel

We investigated the diversity and distribution of two communities, micronekton organisms and large predatory fishes, sampled in mesoscale features of the Mozambique Channel from 2003 to 2009, by combining mid-water trawls, stomach contents of fish predators and instrumented longline fishing surveys. The highest species richness for assemblages was found in divergences and fronts rather than in the core of eddies. Despite an unbalanced scheme, diversity indices did not differ significantly between cyclonic and anticyclonic eddies, divergences and fronts. We found that eddies and associated physical cues did not substantially affect the distribution of micronektonic species which are mainly driven by the diel vertical migration pattern. Top predators exhibited a more complex response. Swordfish (Xiphias gladius) associated better with mesoscale features than tunas, with a clear preference for divergences which is consistent with the diel vertical migrations and occurrence of its main prey, the flying squids Sthenoteuthis oualaniensis (Ommastrephidae). On the other hand, the probability of presence of yellowfin tuna was not tied to any specific eddy structure. However, the highest values of positive yellowfin CPUEs were associated with low horizontal gradients of sea-level anomalies. We also showed a non-linear response of positive yellowfin CPUEs with respect to the depth of the minimal oxygen content. The larger the distance between the hooks and the minimal oxygen layer, towards the surface or at greater depths, the higher the CPUE, highlighting that yellowfin congregated in well-oxygenated waters. Micronekton sampled by mid-water trawls and stomach contents exhibited different species composition. The highly mobile organisms were not caught by trawling whereas they remain accessible to predators. The combination of stomach contents and mid-water trawls undoubtedly improved our understanding of the micronekton assemblage distribution. Our results provide some evidence that mesoscale features in the Mozambique Channel do not strongly affect the distribution of the mid-trophic level organisms such as micronekton and most of the large predatory fishes, and hypotheses are proposed to support this result.

Subthermocline Eddies over the Washington Continental Slope as Observed by Seagliders, 2003–09

Abstract In the California Current System, subthermocline, lenslike anticyclonic eddies generated within the California Undercurrent (CU) are one mechanism for lateral transport of the warm, saline waters of the CU. Garfield et al. established the name “Cuddies” for eddies of this type and hypothesized that they account for a significant fraction of the offshore transport of CU water. This study presents observations of subthermocline eddies collected from a time series of Seaglider surveys in the northern California Current System. Gliders made 46 crossings of subthermocline anticyclones and 17 crossings of subthermocline cyclones over 5.5 yr. Close inspection grouped these into 20 distinct anticyclones and 10 distinct cyclones. Water properties at the core of anticyclonic eddies were similar to those in the core of the CU over the continental slope; these anticyclones are examples of Cuddies. Anticyclonic (cyclonic) eddies had average radii of 20.4 (20.6) km, peak azimuthal current speeds of 0.25 (0.23) m s−1, and average core anomalies of potential vorticity 65% below (125% above) ambient values. Anticyclones contained an order of magnitude greater available heat and salt anomaly relative to background conditions than cyclones on average. Circumstantial evidence of eddy decay through lateral intrusions was found although this was not observed consistently. Observed eddy properties and the geometry of flow over the continental slope were consistent with eddy formation due to frictional torque acting on the CU. Loss of heat and salt from the CU due to subthermocline eddies is estimated to account for 44% of the freshening and cooling of the CU as it flows poleward.

Pattern formation at the ocean surface: Sargassum distribution and the role of the eddy field

Lay AbstractOrganisms that float at the ocean surface such as the macroalga Sargassum and the cyanobacterium Trichodesmium often accumulate in lines visible in satellite imagery. These lines have lateral scale separations of a few kilometers and cannot be explained by Langmuir circulation. Here we discuss the physical process responsible for the accumulation of floating materials in the ocean in the presence of mesoscale (tens of kilometers) and submesoscale (few kilometers) turbulence. Using high‐resolution simulations in both an idealized domain (a 3‐dimensional box where turbulence is excited by small‐scale winds and eddies form) and a realistic domain (western Gulf of Mexico, where extensive concentrations of floating Sargassum have been recorded in satellite images), we show that the distribution of tracers at the ocean surface departs rapidly from that observed a few meters below the surface. The strong divergence and convergence zones generated at the ocean surface by ageostrophic processes (i.e., processes for which an exact balance between the Coriolis force and the pressure gradient force is not attained) at scales between hundreds of meters and a few kilometers are responsible for the creation of lines where the floating material accumulates. Floating particles in the model simulations are expelled from the core of eddies and concentrate in small filaments. The resulting patterns are similar to the distribution of Sargassum seen in satellite images. Such organization may be common in many surface‐dwelling organisms and could play a role in enhancing biological interactions.

Trace metal distributions within a Sitka eddy in the northern Gulf of Alaska

In the northern Gulf of Alaska, mesoscale, anticyclonic eddies have been implicated as a mechanism of the cross‐shelf exchange of iron (Fe)‐replete coastal waters with Fe‐deplete subarctic Alaskan gyre waters. Based on existing hydrography and macronutrient distributions, a Sitka eddy sampled during August 2007 is divided into a surface eddy core, a shallow subsurface eddy core, and a deeper subsurface eddy core. The distributions of aluminum (Al), manganese (Mn), and Fe in the eddy are examined and compared to distributions at shelf stations similar to where the eddy formed as well as basin stations representative of the Fe‐limited subarctic Alaskan gyre. Relative to basin stations, dissolved and particulate Al and dissolved Mn were elevated in eddy core waters. Reactive Fe concentrations within the shallow subsurface eddy core were nearly seven times greater than reactive Fe concentrations at similar densities at the basin stations. This shallow subsurface eddy core is likely mixed into the euphotic zone during storm‐induced mixing as well as mixing during isopycnal relaxation as an eddy dies out. The flux of reactive Fe to surface waters from the shallow subsurface core of eddies in the Gulf of Alaska is a significant source compared to both remote and local atmospheric dust deposition. It is hypothesized that Fe supply from eddies in the eastern Gulf of Alaska shifts the core of the high‐nutrient, lower‐than‐expected chlorophyll water farther west in conjunction with an “eddy desert” in that same region.

A tale of two eddies: Diagnosing coherent eddies through acoustic remote sensing

[1] A 38 kHz vessel-mounted acoustic Doppler current profiler is used to explore in detail the dynamics of an anticyclonic and a cyclonic eddy during two transits of the cruise vessel Explorer of the Seas from the Caribbean to New Jersey in July 2007. The radial scale of the two eddies is similar, but whereas the cyclone is strongly surface intensified, the anticyclone has its maximum expression with near–solid body rotation between 200 and 800 m depth. The anticyclone has a minimum in relative vorticity very close to −f at 800 m depth and the cyclone has a maximum of about +1.6 f close to the surface where f is the local Coriolis parameter. By integrating the momentum equation the geopotential anomaly field and hence the potential energy of the eddies can be determined quite accurately, which means that the kinetic and potential energy of the eddies can be determined purely through acoustic remote sensing. Given a density profile just outside the eddy one can integrate the gradient wind equation to obtain an estimate of the density and hence potential vorticity fields through the two eddies. The acoustic backscatter patterns in the eddies are quite distinct from the surrounding waters. The backscatter intensity of the main scattering layer at ∼600 m depth decreases by ∼10 dB in the core of both eddies. In the cyclonic eddy three identifiable scattering layers in the main thermocline show a strong tendency for the scattering layer to track the shoaling density structure toward the center of the eddy.

Vertical structure of mesoscale eddies in the eastern South Pacific Ocean: A composite analysis from altimetry and Argo profiling floats

The mean vertical structure of mesoscale eddies in the Peru‐Chile Current System is investigated by combining the historical records of Argo float profiles and satellite altimetry data. A composite average of 420 (526) profiles acquired by Argo floats that surfaced into cyclonic (anticyclonic) mesoscale eddies allowed constructing the mean three‐dimensional eddy structure of the eastern South Pacific Ocean. Key differences in their thermohaline vertical structure were revealed. The core of cyclonic eddies (CEs) is centered at ∼150 m depth within the 25.2–26.0 kg m−3 potential density layer corresponding to the thermocline. In contrast, the core of the anticyclonic eddies (AEs) is located below the thermocline at ∼400 m depth impacting the 26.0–26.8 kg m−3 density layer. This difference was attributed to the mechanisms involved in the eddy formation. While intrathermocline CEs would be formed by instabilities of the surface equatorward coastal currents, the subthermocline AEs are likely to be shed by the subsurface poleward Peru‐Chile Undercurrent. In the eddy core, maximum temperature and salinity anomalies are of ±1°C and ±0.1, with positive (negative) values for AEs (CEs). This study also provides new insight into the potential impact of mesoscale eddies for the cross‐shore transport of heat and salt in the eastern South Pacific. Considering only the fraction of the water column associated with the fluid trapped within the eddies, each CE and AE has a typical volume anomaly flux of ∼0.1 Sv and yields to a heat and salt transport anomaly of ±1–3 × 1011 W and ±3–8 × 103 kg s−1, respectively.

Southern elephant seal trajectories, fronts and eddies in the Brazil/Malvinas Confluence

This study describes the association between transient, mesoscale hydrographic features along the axis of the Brazil–Malvinas Confluence, in the SW Atlantic, and the foraging behavior of 2–3-year-old (focal) juvenile southern elephant seals, Mirounga leonina, from Península Valdés, Argentina. Departing from the dominant pattern of foraging on predictable bathymetric fronts on the Patagonian shelf and slope, three females out of 12 satellite-tracked juveniles remained at the edge of young warm-core eddies and near the outer core of cold-core eddies, coinciding with the most productive areas of these temperature fronts. Seal trajectories along high-temperature gradients were always consistent with the speed and direction of surface currents inferred from the temperature distribution and confirmed by surface drifters. Movements of foraging seals were compared with those of surface drifters, coinciding in time and space and yielding independent and consistent data on regional water circulation parameters. The diving pattern recorded for one focal seal yielded shallower dives and a loose diel pattern in the eddy, and a marked diurnal cycle compatible with foraging on vertically migrating prey in the cold waters of the Malvinas Current. Pre-reproductive females that use the mesoscale fronts of the Argentine Basin as an alternative foraging area would benefit from lower competition with more experienced seals and with other top predators that reproduce along the coast of Patagonia.

Features of 3‐dimensional barotropic and baroclinic circulation in the Hauraki Gulf, New Zealand

Predominant features of barotropic and baroclinic circulation and mixing in the Hauraki Gulf on New Zealand's north‐east coast are described using measurements and 3‐dimensional numerical model simulations. Circulation in the Hauraki Gulf is strongly 3‐dimensional with a primary dynamical balance between surface wind stress and the associated pressure gradients against the land. This leads to persistent up/downwelling and surface manifestations in sea surface temperature patterns which are shown to vary systematically and markedly with wind direction and stratification intensity. A high degree of correspondence between a baroclinic numerical model and measured temperature and nitrate concentrations indicated that many of the observed spatial patterns could be largely explained by the interaction of the wind and tidal circulation with the unique morphology of the Gulf. After strong southeasterly winds, local responses operated in conjunction with the regional “capping” mechanism described by Sharpies (1997) of downwind surface water intrusion from the shelf into the Gulf. However, the morphology acts to enhance local upwelling causing bottom waters to be injected into the surface layers which disrupts the “cap”. The headlands and islands play an additional vertical mixing role by presenting bathymetric variability leading to the formation of upwelling jets in the core of eddies forming during ebb and flood tides. By introducing bottom waters into the upper water column and acting to over‐turn the water body, up/downwelling is an important mechanism for mixing and biological productivity which could vary systematically within the Gulf in response to seasonal and interannual variability in the upwelling patterns. Persistent south‐east winds above a threshold of 10–12 m s‐1 were found to initiate breakdown of seasonal temperature stratification in the Gulf, with complete breakdown after 3 days during a cyclone with 8–23 m s‐l winds. Vertical eddy diffusivity increased from 0.0015 to 0.04 m2 s‐1 as the cyclone strengthened.