Pair Of Eddies Research Articles

Exact solutions for Stokes flow in and around a sphere and between concentric spheres

A general method is suggested for deriving exact solutions to the Stokes equations in spherical geometries. The method is applied to derive exact solutions for a class of flows in and around a sphere or between concentric spheres, which are generated by meridional driving on the spherical boundaries. The resulting flow fields consist of toroidal eddies or pairs of counter-rotating toroidal eddies. For the concentric sphere case the exact solution when the inner sphere is in instantaneous translation is also derived. Although these solutions are axisymmetric, they can be combined with swirl about a different axis to generate fully three-dimensional fields described exactly by simple formulae. Examples of such complex fields are given. The solutions given here should be useful for, among other things, studying the mixing properties of three-dimensional flows.

Measurements of turbulent premixed flame dynamics using cinema stereoscopic PIV

A new experimental method is described that provides high-speed movies of turbulent premixed flame wrinkling dynamics and the associated vorticity fields. This method employs cinema stereoscopic particle image velocimetry and has been applied to a turbulent slot Bunsen flame. Three-component velocity fields were measured with high temporal and spatial resolutions of 0.9 ms and 140 μm, respectively. The flame-front location was determined using a new multi-step method based on particle image gradients, which is described. Comparisons are made between flame fronts found with this method and simultaneous CH-PLIF images. These show that the flame contour determined corresponds well to the true location of maximum gas density gradient. Time histories of typical eddy–flame interactions are reported and several important phenomena identified. Outwardly rotating eddy pairs wrinkle the flame and are attenuated at they pass through the flamelet. Significant flame-generated vorticity is produced downstream of the wrinkled tip. Similar wrinkles are caused by larger groups of outwardly rotating eddies. Inwardly rotating pairs cause significant convex wrinkles that grow as the flame propagates. These wrinkles encounter other eddies that alter their behavior. The effects of the hydrodynamic and diffusive instabilities are observed and found to be significant contributors to the formation and propagation of wrinkles.

Geometry‐induced residual eddies in estuaries with curved channels: Observations and modeling studies

Observations using a vessel‐based acoustic Doppler current profiler conducted in two tidal channels with significant curvatures have shown persistent residual eddies. The residual eddies are clearly related to the bend of the channel. For a channel bending toward the north, the residual eddy on the east side of the channel tends to be clockwise, and the residual eddy on the west side of the channel tends to be counterclockwise. Water in these tidal channels is mostly well mixed or weakly stratified. Motivated by these observations, an analytic model is developed to study tidally induced mean circulation in a curved channel with arbitrary depth variations. A sinusoidal channel is used to simulate the effect of curved channels. A series of experiments with different parameters have shown consistently that a pair of residual eddies around the curvature exists. By analyzing the model results, it is found that advection is a major contributor to the residual eddies. Although the pressure gradient and the nonlinear wave propagation may not be negligible, especially for long channels, it is the advection that generates the vorticity of the residual eddies. The conclusion is true for both short and long channels. Here the division between a short and long channel is 0.6 of a quarter of the tidal wavelength. In addition, we also analyzed results from a finite volume numerical model for Lake Pontchartrain, which has curved tidal channels. The results demonstrate similar residual eddies as shown by the analytical model and the observations.

A numerical study of the eddying characteristics of the Leeuwin Current System

The Regional Ocean Modelling System (ROMS v2.1) was used to investigate the physical processes that sustained the mesoscale eddy fields. The model was able to mimic the spatial, temporal, and migratory scales of the Leeuwin Current (LC) System's mean flow and eddying characteristics. Importantly, eddy generation sites were consistent with those reported previously observed from satellite imagery, and two main regions were identified: site 1, adjacent to the Abrolhos Islands’ western edge (latitude 29°S); and site 2, adjacent to the shelf break (latitude 27.8°S). The description of the eddying dynamics suggested the eddy field evolution followed a similar pathway at both sites. An offshore flow impinged on the continental shelf and was redirected southward, intensifying the LC's southward alongshore flow on the shelf. This southward redirection formed a current loop, which closed and formed a distinct clockwise eddy. The eddy remained near stationary and attached to the bottom topography, but it redirected LC and continental shelf waters offshore to dominate the local dynamics. The resulting offshore meander grew laterally, shallowed, and closed to form an anticlockwise eddy to the original clockwise eddy's south, forming a characteristic LC eddy pair (dipole). The model demonstrated that the LC and Leeuwin Undercurrent coupling played an important role in the onset of eddies at both sites. An energy diagnostic scheme was used, and the dominant instability process linked to the developing anticlockwise eddy at site 1 was mixed mode barotropic and baroclinic instability. The baroclinic instability source was the available potential energy stored within the mean lateral density gradient between the eddy dipole. The LC's meandering southward flow interacting with the undercurrent's northward subsurface flow generated horizontal shear, which was the source of barotropic instability. The dominant instability process at site 2 was baroclinic in origin. At both sites, the baroclinic mechanism followed a series of phases coinciding with the physical description of the evolving eddies.

Characteristics of two counter-rotating eddies in the Leeuwin Current system off the Western Australian coast

Abstract A multi-disciplinary R.V. Southern Surveyor cruise was conducted in October 2003 to quantify upper ocean productivity within two adjacent, counter-rotating mesoscale eddies (an eddy pair off the coast of Western Australia (WA)) in the southeast Indian Ocean. In this study, a combination of satellite data (altimeter, sea surface temperature, and chlorophyll a) and shipboard measurements (acoustic Doppler current profiler (ADCP) and conductivity-temperature-depth (CTD)) were used to characterize the temporal evolution and spatial structures of the eddy pair. Satellite data show that the eddy pair evolved from meander structures of the poleward-flowing Leeuwin Current (LC) in May 2003 and fully detached from the current in late August–early September. Gaussian fits to the ADCP velocities show that the anticyclonic eddy had a maximum azimuthal velocity of 65 cm s−1 at 63 km from the eddy centre (the apparent radius), and the cyclonic eddy had a maximum azimuthal velocity of 60 cm s−1 at the apparent radius of 49 km. The mixed-layer temperature and salinity properties in the cores of the eddy pair were quite stable over the period of the cruise. Temperature–salinity diagrams indicate that the mixed-layer water in the core of the anticyclonic eddy was composed mainly of LC water, while in the core of the cyclonic eddy the mixed layer water was similar to surrounding open-ocean water. The total volume of the LC water retained in the anticyclonic eddy was 5.6×1012 m3, equivalent to 19 days’ annual mean transport of the LC. The chlorophyll a concentration in the surface layer of the anticyclonic eddy (LC water) was higher than in the cyclonic eddy (open-ocean water), which was likely a result of the entrainment of the productive water from the continental shelf during the eddy formation stage. The offshore transport of the chlorophyll a biomass may be important for the pelagic ecosystem off the coast of WA.

Microzooplankton community structure and grazing on phytoplankton, in an eddy pair in the Indian Ocean off Western Australia

Abstract We investigated the microzooplankton assemblage and the grazing impact of microzooplankton on phytoplankton in two contrasting eddies in an oligotrophic region of the eastern Indian Ocean during October 2003. Each eddy had a unique microzooplankton assemblage that did not conform to the expectations of downwelling and upwelling eddies. The warm-core (WC) eddy was ∼ 130 km in diameter and was well mixed to ∼ 250 m in the centre, while the cold-core (CC) eddy was smaller ( ∼ 100 km ) and had a warm cap and a well-developed deep chlorophyll a maximum (DCM). Both eddies had formed within the Leeuwin Current at the continental shelf break off Western Australia. Total microzooplankton biomass ranged from 0.5 to 3.8 μ g C L - 1 , and was significantly ( F ( 1 , 10 ) = 5.6 , p 0.05 ) greater in the WC eddy. The WC eddy contained a significant population of large diatoms, which were grazed by dinoflagellates; the CC eddy resembled a nutrient-limited, oceanic environment with small phytoplankton cells (90% 5 μ m ) and was dominated by ciliates. The growth rate of phytoplankton derived from dilution experiments using chlorophyll a analysis ranged between - 0.09 and 0.12 d - 1 , and phytoplankton mortality due to microzooplankton grazing was between 0 and 0.23 d - 1 . The growth rate of specific picoplankton genera, using significant flow-cytometry data, ranged between 0 and 1.3 d - 1 , and mortality due to microzooplankton grazing ranged between 0 and 0.42 d - 1 . In most cases there was a close balance between growth and mortality in both eddies. Although the eddies differed in their physico-chemical properties and community structures, microzooplankton grazing could account for > 100 % of primary production in experiments from both eddies. However, the structure of the phytoplankton and microzooplankton assemblages were not typical for either up- or downwelling eddies, and as such the fate of organic carbon will not reflect that of classic eddies.

Unicellular diazotrophy as a source of nitrogen to Leeuwin Current coastal eddies

Abstract N2-fixation (15N2 uptake) and C-fixation (H213CO3 uptake) rates of the natural phytoplankton assemblage were measured in a pair of counter-rotating mesoscale eddies in the Leeuwin Current, off the coast of Western Australia in October 2003. Day and night incubations of samples from both the surface and the deep chlorophyll maximum (DCM) were performed at nine stations across the warm core (WC) eddy and ten stations across the cold-core (CC) eddy with focal points in the center, the body, and periphery of each eddy. The mean daytime N2-fixation rate was 33±16 ρmole N L−1 h−1 (mean±SD, N=18) in the WC eddy and 21±12 ρmole N L−1 h−1 (mean±SD, N=20) in the CC eddy. We found significant time of day and depth effects, with higher N2-fixation during the day and at the surface in the WC and the CC eddy, respectively. The depth integrated mean a real N2-fixation rate in the WC eddy was 42.9±14.1 μmol N m−2 d−1(mean±SD, N=9), which was significantly greater than that of the CC eddy, 17.1±8.1 μmol N m−2 d−1 (mean±SD, N=10). Vertical profiles show measurable but highly variable volumetric rates of N2-fixation in the upper 100 m of the water column ranging from 0.13 to 9.6 ρmole N L−1 h−1, with a mean of 4.3±1.6 ρmole N L−1 h−1 (mean±SD, N=10), without any obvious vertical structure. Size-fractionated experiments generally showed no difference in N2-fixation rates in whole-water samples as compared to

Contrasting the vertical differences in the phytoplankton biology of a dipole pair of eddies in the south-eastern Indian Ocean

Abstract A dipole pair consisting of one warm-core (WC) and one cold-core (CC) eddy formed off Western Australia in the winter of 2003. Within these eddies the vertical distributions of phytoplankton, both in terms of community and physiology, were characterized in an effort to understand the impact of eddies on the phytoplankton ecology of the region. The WC eddy had a 275-m deep mixed layer in the centre and a vertically dispersed photoautotrophic community dominated by large diatoms, Pelagophyceae and coccolithophorids. The CC eddy had a shallower and more consistent mixed-layer depth with a deep chlorophyll a maximum (chlamax) well developed throughout the eddy. The euphotic zones and mixed layers of both eddies were very low in nitrate. Phytoplankton characteristics that are known to acclimate to variation in irradiance but require different amounts of time to reach steady state were measured at the surface and deep chlamax in both eddies. Differences between the surface and chlamax were found for characteristics that can diverge over a range of time scales from very short (seconds) to much longer (months) for physiological and ecological parameters such as: Ik and fluorescence quantum yield from PAM fluorometry, pigments (photoprotective and photosynthetic), pigment ratios, carbon to chlorophyll a ratios, PB and PBmax (measured over 24 and 4 h, respectively) community composition based on cell counts and pigments, and deep chlamax formation. Absolute time scales for change of these characteristics were obtained from the literature and used to quantify vertical mixing rates in the two eddies. Overall, vertical mixing is shown to be more pronounced, up to 20 times faster and deeper, in the WC eddy than in the CC eddy, especially near the WC eddy centre. This high rate of vertical mixing is hypothesized to allow the development and persistence of a vertically homogeneous community of large diatoms that spreads from the surface down to 275 m, well below the euphotic depth. As nitrate concentrations in the surface mixed layers of both eddies were very low there was no capacity for a significant increase in phytoplankton biomass within the eddies without additional nitrogen inputs. The longer-term biological trajectories of the eddies are discussed.

Eddy formation through the interaction between the Leeuwin Current, Leeuwin Undercurrent and topography

Abstract The surface and subsurface circulation off south-west Australia was simulated using the Regional Ocean Modelling System (ROMS), a primitive equation ocean model with terrain-following s-coordinates. The major currents in this region—the Leeuwin Current and Leeuwin Undercurrent—were reproduced by specifying only temperature and salinity distributions from climatology. The application of wind stress subsequent to the model reaching a quasi-steady state, resulted in the generation of the seasonal Capes Current, a northward-flowing current on the continental shelf associated with coastal upwelling. The simulated currents compared well with the observed current patterns including the location and maximum current speeds. Shelf topography variations and bottom shear, which generated vorticity, influenced eddy formation. Eddies separated from the Leeuwin Current and the Leeuwin Undercurrent, migrated westward, and exited the model domain through merging, dissipating or through an open boundary. In the model simulations, the Undercurrent produced mostly cyclonic eddies due to strong negative vorticity where the current flowed against the continental slope. The Leeuwin Current produced anticyclonic warm-core eddies initiated from the formation of meanders, which were strongest at the surface. The eddy field was dominated by anticyclonic eddies at the surface and cyclonic eddies at 500 m. The interaction between the Leeuwin Current and Leeuwin Undercurrent led to the formation of eddy pairs.

Dynamics of Kuroshio path variations in a high‐resolution general circulation model

A long‐lived (2‐year) typical large meander (LM) path of the Kuroshio is reproduced in a high‐resolution general circulation model (GCM) simulation with realistic topography, and the life cycle (formation, maintenance, and disappearance) of the LM is studied. The formation of the LM is triggered by a pair of upper layer anticyclonic and cyclonic eddies on the Kuroshio, with the cyclonic one on the downstream side. Upon reaching the western side of the Shikoku Basin south of Japan these anomalies induce a lower layer anticyclonic eddy. They enhance each other while they are carried eastward by the Kuroshio. Specifically, the southward flow on the eastern side of the lower anticyclonic eddy carries the upper cold cyclonic eddy offshore, resulting in the southward deflection of the Kuroshio path (large meander). These processes can be understood as a growth of baroclinic instability. A stationary LM is established when the westward tendency, caused by the beta effect owing to the large meridional displacement, counterbalances the eastward tendency, caused by the combination of advection and vortex induction owing to the curvature of the Kuroshio path. Large‐scale disturbances from the upstream tend to strengthen the LM through baroclinic energy conversion. In contrast, small‐scale disturbances from the upstream grow locally along the meandering Kuroshio front through baroclinic energy conversion. They extract cold waters from the cold cyclonic eddy of the LM as they flow away to the east. The horizontal extent of the cold cyclonic eddy is reduced, and the meander is carried eastward since the advection effect dominates over the beta effect that weakens as a result of the smaller meridional extent.

Circulation in the Gulf of Aqaba (Red Sea) during winter—spring

This study is devoted to oceanographic features of the semi-enclosed Gulf of Aqaba, Red Sea. The data were recorded in winter-spring 1999 on the R/V Meteor cruise leg 44/2. Temperature and salinity profiles were measured at six positions (I-VI). The shipboard NarrowBand Acoustic Doppler Current Profiler (NB ADCP) 150 kHz con- tinuously recorded current profiles down to 350 m en route. The research revealed that the current near the Strait of Tiran front (position VI) represents a semidiurnal signal of an internal tide wave (~12 h period; 0.2 ms -1 amplitude) that might be gen- erated by the barotropic tide at the sill of the Strait. A sequence of cyclonic and anti- cyclonic eddy pairs is found along the axis of the Gulf of Aqaba during winter-spring seasons. These sub-mesoscale signals are dominant above the main thermocline and might be caused by wind forcing and the narrowness of the Gulf; it might remain in other seasons with different dimensions in relation to the depth of thermocline. The total diameter of each pair was twice the baroclinic Rossby radius ( R ≈ 10 km). A single anti-cyclonic eddy was observed in the upper 300 m in the northern tip of the Gulf with a diameter of about 5-8 km.

The Tsushima Warm Current through Tsushima Straits Estimated from Ferryboat ADCP Data

Abstract Current structures across the Tsushima Straits are studied using results from long-term acoustic Doppler current profiler (ADCP) observations by a ferryboat between Hakata and Pusan conducted since February 1997. Two maxima of the northeastward current are observed in the central parts of the eastern and western channels, and the maximum velocity in the western channel is stronger than that of the eastern channel. Downstream of the Tsushima Islands, a southwestward countercurrent is observed associated with a pair of cyclonic and anticyclonic eddies. In the western channel, the deep countercurrent is observed pronouncedly on the bottom slope of the Korean side from summer to winter. The volume transport of the Tsushima Warm Current through the straits has strong seasonal variation with a minimum in January and two maxima from spring to autumn (double peaks). The spring peak of the volume transport through the eastern channel is more pronounced than the autumn peak, and the autumn peak of the western channel is more pronounced than the spring peak. The inflow volume transport into the Japan Sea through the western channel significantly increases in autumn because of an incrementation of the freshwater transport. The total volume transport averaged over the observation period (5.5 yr) is 2.64 Sv (Sv ≡ 106 m3 s−1). The average volume transports through the eastern and western channels are 1.10 and 1.54 Sv, respectively.

The temporal evolution of a geostrophic flow in a rotating stratified basin

A laboratory study in a rotating stratified basin examines the instability and long time evolution of the geostrophic double gyre introduced by the baroclinic adjustment to an initial basin-scale step height discontinuity Δ H in the density interface of a two-layer fluid. The dimensionless parameters that are important in determining the observed response are the Burger number S = R / R 0 (where R is the baroclinic Rossby radius of deformation and R 0 is the basin radius) and the initial forcing amplitude ɛ = Δ H / H 1 ( H 1 is the upper layer depth). Experimental observations and a numerical approach, using contour dynamics, are used to identify the mechanisms that result in the dominance of nonlinear behaviour in the long time evolution, τ > 2 ɛ − 1 (where τ is time scaled by the inertial period T I = 2 π / f ). When the influence of rotation is moderate ( 0.25 ≤ S ≤ 1 ), the instability mechanism is associated with the finite amplitude potential vorticity (PV) perturbation introduced when the double gyre is established. On the other hand, when the influence of rotation is strong ( S ≤ 0.1 ), baroclinic instability contributes to the nonlinear behaviour. Regardless of the mechanism, nonlinearity acts to transfer energy from the geostrophic double gyre to smaller scales associated with an eddy field. In the lower layer, Ekman damping is pronounced, resulting in the dissipation of the eddy field after only 40 T I . In the upper layer, where dissipative effects are weak, the eddy field evolves until it reaches a symmetric distribution of potential vorticity within the domain consisting of cyclonic and anticyclonic eddy pairs, after approximately 100 T I . The functional dependence of the characteristic eddy lengthscale L E on S is consistent with previous laboratory studies on continuously forced geostrophic turbulence. The cyclonic and anticyclonic eddy pairs are maintained until viscous effects eventually dissipate all motion in the upper layer after approximately 800 T I . The outcomes of this study are considered in terms of their contribution to the understanding of the energy pathways and transport processes associated with basin-scale motions in large stratified lakes.

Modeling Tidal Circulation and Exchange in Cobscook Bay, Maine

Cobscook and Passamaquoddy Bays and their connecting passages lie at the entrance to the Bay of Fundy, on the eastern boundary between the United States and Canada where the mean tidal range is about 6 m. Vigorous tidal currents maintain cold temperatures and efficient exchange with offshore waters year-round. Over the last several decades, a net-pen salmon aquaculture industry has developed in both bays. Recent outbreaks of fish diseases have led to heightened concerns about tidal coupling between net-pen sites and potential pathways for disease transmission. This paper summarizes a Cobscook Bay circulation study by Brooks et al. (1999) and presents some new results to improve understanding of the tidal circulation and potential exchange pathways linking the bays.A dipole pair of back-eddies forms in the central part of Cobscook Bay on each flood and plays an important role in the dispersion and retention of particles. Direct and indirect observations support the existence of the eddy pair and some associated flow details. Model flushing times are a day or two in the Outer Bay where most aquaculture lease sites are located but a week or longer in the inner arms of the Bay. The inner part of South Bay appears to be a repository for particulate matter. Model experiments including both bays and their connecting passages suggest that most water entering Cobscook Bay comes from Head Harbor Passage adjacent to Campobello Island, illustrating the importance of well-coordinated international plans for ecosystem management.

On axisymmetric creeping flows involving a hybrid droplet

The Stokes flow involving a hybrid droplet submerged in an immiscible liquid is considered. The droplet has a two-sphere geometry with the two spherical surfaces intersecting orthogonally. The types of hybrid droplet considered here include (i) vapor–solid; (ii) vapor–vapor; and (iii) vapor–liquid, with the first two being limiting cases of (iii). It is assumed that the surface tension is sufficiently large so that the interfaces have uniform curvature. In a constructive and simple manner, steady state solutions of Stokes equations involving such configurations are constructed. The theory leading to the general expressions for the flow fields under the above-mentioned limitations exploit the inverse transformation, and the reflection and translation properties of the axisymmetric biharmonic function associated with the Stokes flow. These are cast in the form of a theorem followed by a simple proof. The theorem is then applied to construct closed form singularity solutions for several axisymmetric flow fields disturbed by a hybrid droplet. The different primary flow fields considered here include paraboloidal flow, flow due to a, single stokeslet and a pair of stokelets, and flow due to a potential-dipole. The salient features of the image singularities are discussed in each case. In all cases, the drag force is found to vary significantly with respect to the two radii associated with the two-sphere geometry of the droplet, and the viscosity ratio of the two liquids in the continuous and dispersed phases. In the case of singularity driven flows, the drag is influenced by an additional parameter namely, the location of the singularity. The flow streamlines in some cases show interesting flow patterns. In the case of paraboloidal flows, either a single eddy or a pair of eddies is observed depending on the ratio of the two radii. The sizes and shapes of these eddies vary monotonically with viscosity ratio. For flows due to a single and a pair of stokeslets with the same strength, no eddy is noticed. However, a toroidal eddy appears in the case of a pair of stokelets with opposite strengths. The locations of the stokeslets and the viscosity ratio influence the eddy structure significantly.

Anticyclonic‐cyclonic eddies and their impact on near‐surface chlorophyll stocks and oxygen supersaturation over the Campeche Canyon, Gulf of Mexico

Physical and biological data were gathered over the Campeche Canyon area, Gulf of Mexico, in summer 1999. These include hydrographical and meteorological parameters, and oxygen content, as well as, at some oceanographic stations, suspended particulate matter (SPM), chlorophyll‐a, and in vivo natural fluorescence profiles. Dry air from the east set up a latent heat flux that ranged from 50 to 300 Wm−2. The wind mixed layer (ML) was isothermal and averaged 40 m thick, but with a noticeable increase of oxygen supersaturation toward the northwest. Temperature and dynamic height structure reveal an anticyclone‐cyclone eddy pair. At the southeastern part of the study area a noticeable thermal gradient was observed toward the shelf break of the Campeche Bank, which corresponded to a tilting up (doming) of low oxygen saturation midwater to near the base of the ML. This doming of isotherms along the margin of the Campeche escarpment is likely an upper‐ocean response to the sharp gradient in water depth between the canyon and outer continental shelf. It is suggested that the doming of midwater on to the Campeche escarpment and at the head of the Canyon, as denoted by locally low values of oxygen saturation, plays an important role in the biological patterns. Throughout the field area, the values of total SPM were higher than reported previously for subtropical open ocean environments. The impact of autotrophic‐heterotrophic biogeochemical processes on chlorophyll was reflected in the local deepening of the deep of chlorophyll maximum (DCM) and the depth of the euphotic zone within the anticyclonic eddy.

Eddies and dipoles around South Madagascar: formation, pathways and large-scale impact

During a hydrographic cruise in March 2001, we encountered a dipole-like vortex structure directly southwest of Madagascar. The cruise formed part of the Dutch-South African Agulhas Current Sources Experiment (ACSEX). Direct current observations with a lowered ADCP showed that the multipole involved a deep-reaching central jet, with over 20 cm/s speeds still at 2000 m depth, with two contra-rotating eddies on either side. Both eddies had a size of about 250 km. A subsequent analysis of the combined TOPEX-POSEIDON/ERS altimeter dataset revealed the regular formation of dipoles around south Madagascar, where the East Madagascar Current (EMC) separates from the shelf. Between April 1995 and June 2000, 16 such features could be identified, six of which were formed between August 1996 and May 1997 and six between May 1999 and June 2000. These periods of enhanced dipole formation coincided with the negative phases of the Indian Ocean Dipole and El Niño cycles, suggesting a connection between these climate modes and the interannual variability in the periods of dipole train formation. The temperature–salinity characteristics of the eddies from the ACSEX dipole were identical to those measured in the EMC during the same cruise. The cyclonic eddy appeared to have drawn its waters from the inshore side of the EMC, suggesting it was formed as a lee-eddy on the cyclonic flank of the separating jet. The data indicate that the eddies carry their water along. At the observed scales, one dipole per year would feed 8 Sv (1 Sv=10 6 m 3/s) of EMC-derived water into the Agulhas retroflection region. In the Mozambique Basin, the eddy pairs behaved quite irregularly, with many of them splitting and interacting with other eddies in the region. The anticyclones then propagated mostly westward, while the cyclone tracks diverged more between west and southwest and were influenced by the interaction with the Mozambique Plateau. A very regular train of dipoles started in December 1999 and continued through 2000. They were spawned roughly at 2-month intervals, stayed remarkably coherent and triggered an unusually early retroflection of the Agulhas Current in late 2000. Thus, interannual variability originating in the equatorial climate modes of the Indian Ocean seems to result in variability of the Agulhas retroflection and associated ring shedding.

Generation and unstable evolution of a density‐driven Eastern Poleward Current: The Iberian Poleward Current

The generation and evolution of a density‐driven Eastern Poleward Current is investigated using a high‐resolution primitive equation numerical model. The simulations focus on the Iberian Poleward Current (IPC) as a case study. The flow is generated by a meridional upper ocean density gradient balanced by an eastward surface‐intensified flow that adjusts at the coastal margin. The resulting current system has a baroclinic character with poleward flow at the surface layer, and equatorward flow underneath. A few weeks after initialization, the sheared along‐slope flow generates several vorticity structures downstream of the main topographic features. In the lee of the topography, persistent anticyclones are observed and deep cyclogenesis is induced in relation to the meandering of the upper layer jet. These structures evolve preferentially as cyclone/anticyclone eddy pairs, and after interaction some dipoles are ejected offslope. Within a period of a few months, the initial meridional gradient evolves into a complex system of fronts, eddies and slope flows. The dynamics of flow topography interaction is analyzed. A comparison with satellite imagery of the IPC is conducted and similarity in scales and patterns is noted.

On the structure and motions of cyclones in the northern Gulf of Mexico

The eddy field on the northern slope of the Gulf of Mexico has been investigated using high‐resolution AXBT and AXCP aircraft surveys and ARGOS tracked drifters. A number of vigorous cyclones were mapped. Their vertical structures showed solid body rotation with radii of 50–75 km with a subsurface maximum (∼40 cm s−1) at the outer edge at about 150 m. Flows had little shear in the upper 400 m and only weak currents below depths of 800 to 1000 m. Cyclones were observed to move on and off the slope under the influence of swirl velocities of larger warm‐core eddies, including Loop Current (LC) anticyclones. Rotational periods range from about 7 to 18 days. The mapped eddy fields contained both cyclones and anticyclones with a range of diameters from about 40 to 150 km. The presence of many relatively small‐scale features along the upper slope strongly affects features of the shelf break velocity field that were measured by a moored current meter array. The directions and vertical shears of the moored currents were consistent with flows caused by the northern edges of cyclones and anticyclones. These eddies can cause persistent events and strong cross‐isobath fluctuations of the currents. Counterrotating pairs of eddies on the upper slope seem to be a mechanism for the transport of material on and off the Louisiana and east Texas shelves. Coherences between moorings, 74 km apart, were generally not significant, as might be expected when the currents are dominated by small‐scale eddies. However, an EOF analysis of the shelf break array indicates that about 40% of the along‐slope current variances could be attributed to westward propagating continental shelf waves.

Ocean color observations of eddies during the summer in the Gulf of California

Using SeaWiFS ocean color satellite images collected during 1997–2001, a series of eddies have been observed in the Gulf of California. These eddies are not clearly seen in the summertime sea surface temperature images. The surface circulation between the midriff islands and the mouth of the gulf appears to be dominated by this series of eddies that have an alternating sense of rotation. An eddy pair is observed to extend from Cabo Lobos in each of the three years, suggesting that the eddies are topographically locked. The formation mechanism of the eddies remains unknown; however, we suspect the most likely mechanism includes baroclinic instabilities along the coast produced by interactions between coastally trapped waves, and local wind driven currents.