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Christopher C. Chapman and Rosemary Morrow

Abstract

The interaction of jets with topography in the Southern Ocean is investigated using 19 years of altimetry data. In particular, the “jet jumping” mode of variability, by which two or more jets passing close to the same topographic feature show strongly anticorrelated strengthening and weakening, is studied. Three regional case studies are described—the Southeast Indian Ridge south of Tasmania, the Macquarie Ridge south of New Zealand, and the Pacific–Antarctic Rise—where the jet jumping variability is found to occur. Using principal component analysis, the spatial patterns of variability show a vortex dipole forming on either side of a particular jet. For each regional study, it is found that the variability in strength of these vortices (as measured by the spatially averaged vorticity) is strongly correlated with time series of the principle component that describes the jet jumping variability. The observational analysis is complemented by a suite of idealized numerical experiments using a three-layer quasigeostrophic model with simple topography. The numerical results show similar spatial patterns of variability to those observed in the altimetric data. Internal variability is sufficient to generate jet jumping variability, as there is no time-varying external forcing applied in the model configuration. The simulations are used to investigate the effect of topographic scale and changing bottom friction. The authors find that both have a strong influence on the time scale of the variability, with larger topographic scales and higher bottom friction leading to faster time scales. This study shows that even in regions where the flow is strongly influenced by topography, Southern Ocean jet flow may exhibit low-frequency variability.

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Rosemary Morrow, Richard Coleman, John Church, and Dudley Chelton

Abstract

Satellite altimetry has previously been used to map the magnitude of the surface eddy variability of the global oceans, but the direction of the time-variable velocities have been more difficult to determine. Here, a technique is presented for resolving both magnitude and direction of residual surface geostrophic velocities at Geosat altimeter crossover points; providing a two-year time series with a temporal resolution of 17 days and horizontal resolution of around 100 km. The time series of residual velocity components are then used to determine surface eddy statistics in the Southern Ocean and to investigate the role of transient eddies in the Southern Ocean momentum balance. The surface eddy statistics from Geosat crossover points show a complex spatial distribution in the surface Reynolds stresses (u2, v2, uv). In contrast to the assumptions of isotropic variability in previous analyses of altimeter data, velocity variance ellipses are found to be distinctly anisotropic in many regions. The surface eddy statistics compare favorably with the available in situ current meter data and surface drifter data. The complex spatial distribution of surface eddy momentum flux is strongly influenced by bottom topography and the position of the mean current. On a zonal average, the horizontal divergence of eddy momentum flux from transient eddies is found to be around two orders of magnitude too small to directly balance the eastward momentum from the wind. In the Agulhas Return Current and near the Macquarie Ridge/Campbell Plateau, the Reynolds stress are significant and they act to concentrate the mean jet, in agreement with recent numerical models. However, circumpolar streamwise averages along paths parallel to the mean axis of the Antarctic Circumpolar Current show only a small net convergence.

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Maxime Ballarotta, Clément Ubelmann, Marine Rogé, Florent Fournier, Yannice Faugère, Gérald Dibarboure, Rosemary Morrow, and Nicolat Picot

Abstract

The dynamic optimal interpolation (DOI) method merges altimetric sea surface height (SSH) data into maps that are continuous in time and space. Unlike the traditional linear optimal interpolation (LOI) method, DOI has the advantage of considering a nonlinear temporal propagation of the SSH field. DOI has been successfully applied to along-track pseudo-observations in observing system simulation experiments (OSSEs), demonstrating a reduction in interpolation error in highly turbulent regions compared to LOI mapping. In the present study, we further extend the validation of the DOI method by an observing system experiment (OSE). We applied and validated the DOI approach with real nadir-altimetric observations in four regional configurations. Overall, the qualitative and quantitative assessments of these realistic SSH maps confirm the higher level of performance of the DOI approach in turbulent regions. It is more of a challenge to outperform the conventional LOI mapping in coastal and low-energy regions. Validations against LOI maps distributed by the Copernicus Marine Environment Monitoring Service indicate a 10%–15% increase in average performance and an improved resolution limit toward shorter wavelengths. The DOI method also shows improved mesoscale mapping of intense jets and fronts and reveals new eddies with smoother trajectories.

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Fabrice Ardhuin, Bertrand Chapron, Christophe Maes, Roland Romeiser, Christine Gommenginger, Sophie Cravatte, Rosemary Morrow, Craig Donlon, and Mark Bourassa
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