Geostrophic and ageostrophic circulation of a shallow anticyclonic eddy off Cape Bojador
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Abstract:
A shallow mesoscale anticyclonic eddy, observed south of the Canary Islands with satellite altimetry, has been intensively studied with multiparametric sampling. Hydrographic data from a CTD installed on an undulating Nu-shuttle platform reveal the presence of a mesoscale anticyclonic eddy of ∼125 km diameter. The difference in sea level anomaly (SLA) between the interior and the edge of the eddy, as determined from altimetry, is ∼15 cm, which compares well with the maximum dynamic height differences as inferred using a very shallow reference level (130 m). Further, the associated surface geostrophic velocities, of about 35 cm s−1 in the northeast and southwest edges of the eddy, are in good agreement with direct velocity measurements from the ship. Deep rosette-CTD casts confirm that the structure is a shallow eddy extending no deeper than 250 m before the fusion with another anticyclone. The SLA-tendency (temporal rate of change of sea surface height) indicates a clear northwestward migration during the two first weeks of November 2008. Applying an eddy SSH-based tracker, the eddy's velocity propagation is estimated as 4 km d−1. Use of the QG-Omega equation diagnoses maximum downward/upward velocities of about ±2 m d−1. The instability of the Canary coastal jet appears to be the mechanism responsible for the generation of the shallow anticyclonic eddy.Keywords:
Anticyclone
Sea-surface height
Geostrophic current
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Eddy
Anticyclone
Sea-surface height
Mooring
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Eddy
Anticyclone
Drifter
Eddy diffusion
Boundary current
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Direct microstructure observations and fine-scale measurements of an anticyclonic mesoscale eddy were conducted in the northern South China Sea in July 2020. An important finding was that suppressed turbulent mixing in the thermocline existed at the center of the eddy, with an averaged diapycnal diffusivity at least threefold smaller than the peripheral diffusivity. Despite the strong background shear and significant wave–mean flow interactions, the results indicated that the lack of internal wave energy in the corresponding neap tide period during measurement of the eddy’s center was the main reason for the suppressed turbulent mixing in the thermocline. The applicability of the fine-scale parameterization method in the presence of significant wave–mean flow interactions in a mesoscale eddy was evaluated. Overprediction via fine-scale parameterization occurred in the center of the eddy, where the internal waves were inactive; however, the parameterization results were consistent with microstructure observations along the eddy’s periphery, where active internal waves existed. This indicates that the strong background shear and wave–mean flow interactions affected by the mesoscale eddy were not the main contributing factors that affected the applicability of fine-scale parameterization in the northern South China Sea. Instead, our results showed that the activity of internal waves is the most important consideration.
Eddy diffusion
Anticyclone
Eddy
Mean flow
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Eddy
China sea
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An intense, anticyclonic, warm core winter eddy off the east coast of Australia was surveyed with an airborne radiation thermometer, expendable bathythermographs and a continuously recording surface thermosalinograph in September 1974. The eddy had a diameter of 250 km, a dynamic relief of 0.7 dyn-m and a mixed layer depth extending to over 300 m in the core. A strong current ring was present halfway from the center to the edge of the eddy with surface speeds ranging from 0.6 to 1.78 m s−1. The dynamics of the eddy are related to previous knowledge of mesoscales in the East Australian Current region. The simplicity of the eddy structure defines a dominant and lone horizontal wavenumber whose inverse is the Rossby radius of deformation predicted by complex baroclinic instability theory. This simplicity allows a very simple eddy model to be proposed with first vertical baroclinic mode structure with a vertical depth scale of 430 m. The interior deep mixed layer was completely enclosed by a shell of isothermal water; this double layering indicates that large-scale entrainment of surface water may be an important feature of eddy generation off East Australia.
Anticyclone
Eddy
Eddy diffusion
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Bathythermograph
Argo
Geostrophic current
Current meter
Temperature salinity diagrams
Drifter
Buoy
Eddy
Dynamic height
Boundary current
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Mesoscale eddies are common in the ocean and their surface characteristics have been well revealed based on altimetric observations. Comparatively, the knowledge of the three-dimensional (3D) structure of mesoscale eddies is scarce, especially in the open ocean. In the present study, high-resolution field observations of a cyclonic eddy in the Kuroshio Extension have been carried out and the anatomy of the observed eddy is conducted. The temperature anomaly exhibits a vertical monopole cone structure with a maximum of −7.3 °C located in the main thermocline. The salinity anomaly shows a vertical dipole structure with a fresh anomaly in the main thermocline and a saline anomaly in the North Pacific Intermediate Water (NPIW). The cyclonic flow displays an equivalent barotropic structure. The mixed layer is deep in the center of the eddy and thin in the periphery. The seasonal thermocline is intensified and the permanent thermocline is upward domed by 350 m. The subtropical mode water (STMW) straddled between the seasonal and permanent thermoclines weakens and dissipates in the eddy center. The salinity of NPIW distributed along the isopycnals shows no significant difference inside and outside the eddy. The geostrophic relation is approximately set up in the eddy. The nonlinearity—defined as the ratio between the rotational speed to the translational speed—is 12.5 and decreases with depth. The eddy-wind interaction is examined by high resolution satellite observations. The results show that the cold eddy induces wind stress aloft with positive divergence and negative curl. The wind induced upwelling process is responsible for the formation of the horizontal monopole pattern of salinity, while the horizontal transport results in the horizontal dipole structure of temperature in the mixed layer.
Eddy
Sverdrup
Argo
Mode water
Temperature salinity diagrams
Mixed layer
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Eddy
Temperature Gradient
Ekman layer
Pressure gradient
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Relative dynamic heights and geostrophic fields were derived from TOPEX/Poseidon altimetry data and then used to track mesoscale eddies over the Subtropical Countercurrent (STCC). The radii, centers, vorticities, shearing deformation rates, stretching deformation rates, divergences, and center velocities of all identified eddies over the STCC were determined using a model that assumes constant velocity gradients. Most eddies are concentrated in a zonal band near 22°N, and there is an interannual variation in the number of eddies. A case study was made for a cyclonic eddy and an anticyclonic eddy, with time series of eddy kinematic parameters computed. Both eddies survive for ∼220 days and propagate westward along over 22°N–24°N to reach the Kuroshio Current east coast of Taiwan, where the eddies were dissipated and in turn affected the Kuroshio Current in many ways. Sea surface temperature data and drifter data confirm the existence of these two eddies. The radii of both eddies vary and their shapes are mostly elliptical during propagation. The anticyclonic eddy propagated almost westward with oscillating north‐south components, and the mean speed is 8.3 km/day. The cyclonic eddy moved southwestward before reaching 130°E and then moved northwestward, with a mean speed of 7.6 km/day. The propagations of these two eddies are basically consistent with the standard theory of eddy propagation but with larger speeds. The propagating direction could be altered while passing steep bottom topography or merging with the other eddies.
Eddy
Anticyclone
Drifter
Boundary current
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Citations (86)