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F. Sévellec, A. C. Naveira Garabato, J. A. Brearley, and K. L. Sheen

system, so that a single snapshot is enough to determine the vertical flow. However, the equation requires a fine spatial discretization (i.e., spatial derivatives along the three spatial dimensions). It has been shown to be extremely useful in the atmosphere, where radiosondes capture accurately the three-dimensional system’s state on a global scale at a given instant (but repeated measurements at the same location are impossible, that is, time derivatives are not measurable). In the ocean, spatial

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J. H. LaCasce, R. Ferrari, J. Marshall, R. Tulloch, D. Balwada, and K. Speer

in the Southern Ocean is also strongly anisotropic because of the influence of the Antarctic Circumpolar Current (ACC). Diffusivity estimates in the direction parallel to the current typically exceed those in the perpendicular direction ( section 4a ). Determining along-stream diffusivities requires removing the mean contribution, usually by averaging drifter velocities in geographical bins; the diffusivities are then calculated from the residuals ( Davis 1991 ). However, a perfect mapping of the

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Louis-Philippe Nadeau and Raffaele Ferrari

transport. Fig . 14. Sketch of the barotropic streamfunction for the hypothesis of NS09 and the new hypothesis. In the hypothesis of NS09 , the total transport is decomposed in a basin and a channel contribution, defined by the geographic division between the Drake Passage latitudes and the basin region to the north. In the revised hypothesis, all the circumpolar transport is expressed in the circumpolar mode. The gyre mode is intrinsically linked with the bottom form stress exerted by the ridge and

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Byron F. Kilbourne and James B. Girton

Drake Passage. (a) Cruise track (dashed) and contours of AVISO absolute dynamic topography averaged from 2000 to 2013. Solid contours approximately correspond to the Subantarctic Front, Polar Front, and Southern Antarctic Circumpolar Current Front at 0, −30, and −100 cm respectively. (b) Closeup of the DIMES tracer release location, with the ship track from 2 to 19 Feb 2009. Shipboard data in this region are treated as a stationary time series in Fig. 5 . (c) Gridded wind speed and direction from a

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Ross Tulloch, Raffaele Ferrari, Oliver Jahn, Andreas Klocker, Joseph LaCasce, James R. Ledwell, John Marshall, Marie-Jose Messias, Kevin Speer, and Andrew Watson

strongly meander, and it is difficult to separate along- and across-jet dispersion. Furthermore, the tracer sampling downstream of the Drake Passage may not have been adequate to determine cross-stream isopycnal mixing as it was designed to estimate the diapycnal diffusivity; the tracer was sampled only along the individual transects shown in Fig. 1a with no attempt to map the whole tracer patch. Fig . 1. (a) Map of DIMES tracer patch region showing the injection location (US1) and the column

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Dhruv Balwada, Kevin G. Speer, Joseph H. LaCasce, W. Brechner Owens, John Marshall, and Raffaele Ferrari

were deployed as part of the DIMES experiment, primarily between the synoptically observed positions of the Subantarctic Front (SAF) and Polar Front (PF) at 105°W. Additional floats were deployed downstream of this deployment site to supplement the dataset. The total number of floats deployed was 210. However, after failures, 140 float tracks comprising 183 years of float data (66 795 float days) were retrieved. Figure 1 shows a summary of the experimental design and regional geography, together

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Sophia T. Merrifield, Louis St. Laurent, Breck Owens, Andreas M. Thurnherr, and John M. Toole

seafloor topography of the region. Fig . 1. Map showing the microstructure stations taken during the US2 (blue) and US5 (red) cruises. The gray contours show water depth from Smith and Sandwell (1997) . The white lines represent the climatological location of the Subantarctic and Polar Fronts, from Orsi et al. (1995) . The US2 expedition, on the R/V Thomas G. Thompson during January–March 2010, was the most expansive of the DIMES surveys, occupying a survey grid over a nearly 10 6 -km 2 region of

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Emma J. D. Boland, Emily Shuckburgh, Peter H. Haynes, James R. Ledwell, Marie-José Messias, and Andrew J. Watson

distribution of the tracer. In this paper, we will exploit the information available from the US2, UK2, and UK2.5 cruises, in particular concentrating on the horizontal variation of the column-integrated tracer. Fig . 2. Location of tracer measurements in the US2, UK2, and UK2.5 cruises as indicated. The triangle shows the location of the tracer release on the US1 cruise. The transects from the UK2 and UK2.5 cruises are labeled. The contours are mean streamlines, separation 2 × 10 4 m 2 s −1 . b

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