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

. However, there are large meridional excursions in the regions where it goes over midocean ridges and approaches continents. On this broad, baroclinically unstable mean flow lays a convoluted structure of jets and eddies ( Sokolov and Rintoul 2009 ). The merging and splitting can at any instance be acting as a barrier to mixing and at another instance strongly mix fluid parcels ( Thompson 2010 ). This is in marked contrast to the Gulf Stream, for example, where a single primary jet exists. The ACC jets

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Alberto C. Naveira Garabato, Kurt L. Polzin, Raffaele Ferrari, Jan D. Zika, and Alexander Forryan

carried out in this study. 3. Data and analysis a. Data The data analyzed here consist of hydrographic, velocity, and microstructure measurements collected during three repeats of the SR1b section across the eastern Drake Passage ( Fig. 2 ), under the auspices of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean [DIMES; see Sheen et al. (2013) and references therein]. The SR1b section crosses the two major ACC jets in the region, associated with the Subantarctic Front (SAF) and

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Jesse M. Cusack, Alberto C. Naveira Garabato, David A. Smeed, and James B. Girton

could also be a result of spatiotemporal variability or advection. It was not possible to deduce the divergence of the momentum flux and therefore the implied drag force. However, the magnitude of the flux is more than two orders of magnitude greater than the time-mean wind stress on the ACC ( Wunsch 1998 ), suggesting that lee waves have the potential to be a significant term in the local momentum budget of ACC jets, as suggested by Naveira Garabato et al. (2013) . Further work will be needed to

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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

of less than 30 m relative to the target density surface and was thus confined to a very thin layer. Our analysis focuses on the first year of spreading when most of the tracer remained west of the Drake Passage; numerical simulations suggest that the leading edge of the tracer reached the Drake Passage after somewhat less than 2 yr. We focus on measurements collected in the sector upstream of the Drake Passage, because the ACC jets are mainly zonal there. Past the Drake Passage, the jets

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Ru Chen, Sarah T. Gille, Julie L. McClean, Glenn R. Flierl, and Alexa Griesel

. 2014 ). The terms k eddy , L , and γ are the eddy wavenumber, eddy length scale, and the reciprocal of eddy decorrelation time, respectively. Surface eddy diffusivities from the F–N theory agree with those from altimetry (e.g., Ferrari and Nikurashin 2010 ; Klocker and Abernathey 2014 ; Bates et al. 2014 ). The F–N theory also captures the three-dimensional structure of float-based eddy diffusivities in the intense Kuroshio Extension jet area ( Chen et al. 2014 ). On the other hand, the

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Michael Bates, Ross Tulloch, John Marshall, and Raffaele Ferrari

atmosphere. However, such effects are also likely to be significant in other regions where eddies and jets coexist, such as western boundary currents and equatorial regions. We now briefly review key elements of “mixing theory” in which waves moving zonally with phase speed c along a mean zonal flow induce fluid parcels to move transverse to the mean flow, thus transferring properties in the cross-stream direction. We then go on to assess whether these effects can account for some of the differences

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J. Alexander Brearley, Katy L. Sheen, Alberto C. Naveira Garabato, David A. Smeed, and Stephanie Waterman

deep ocean. Another mechanism that has gained attention recently is the generation of gravity waves by geostrophic eddy flows impinging on small-scale (1–100 km) sea floor topography. The ACC contains several equivalent barotropic jets, such as those associated with the Polar Front (PF) and Subantarctic Front (SAF), and is therefore likely to be a zone of concentrated internal wave generation. Observations of near-bottom velocities in Drake Passage ( Chereskin et al. 2009 ) have confirmed that

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

topographic roughness and convergence of the frontal jets in Drake Passage that support lee wave activity and enhance the internal wave field at middepth. The thermal variance dissipation rate is enhanced between 200 and 1000 m, with the largest values occurring in the US5 survey, which was primarily conducted in the Subantarctic Front and Polar Front. The large water mass variability in this region preconditions the water column for double-diffusive instabilities. c. Statistics of dissipation at middepth

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Louis-Philippe Nadeau and Raffaele Ferrari

is zero away from the boundaries, and the barotropic flow is approximately in Sverdrup balance. However, there is interesting vertical structure to the double-gyre circulation. The northern gyre is confined to the upper layer, while the southern gyre resides in the lower layer (first and second left panels in Fig. 4 ). The circulation observed in the lower layer must be driven by vortex stretching associated with transient eddies, including transient zonal jets, because winds act only on the

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

. Faure , V. , and K. Speer , 2013 : Deep circulation in the eastern South Pacific Ocean. J. Mar. Res., 70, 748–778 . Ferrari , R. , and M. Nikurashin , 2010 : Suppression of eddy diffusivity across jets in the Southern Ocean . J. Phys. Oceanogr. , 40 , 1501 – 1519 . Freeland , H. , P. Rhines , and T. Rossby , 1975 : Statistical observations of the trajectories of neutrally buoyant floats in the North Atlantic . J. Mar. Res. , 33 , 383 – 404 . Gille , S. , 2003

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