Messias
Marie-Jose
Messias
Marie-Jose
No Thumbnail Available
Search Results
Now showing
1 - 6 of 6
-
ArticleEstimating a submesoscale diffusivity using a roughness measure applied to a tracer release experiment in the Southern Ocean(American Meteorological Society, 2015-06) Boland, Emma J. D. ; Shuckburgh, Emily ; Haynes, Peter H. ; Ledwell, James R. ; Messias, Marie-Jose ; Watson, Andrew J.The use of a measure to diagnose submesoscale isopycnal diffusivity by determining the best match between observations of a tracer and simulations with varying small-scale diffusivities is tested. Specifically, the robustness of a “roughness” measure to discriminate between tracer fields experiencing different submesoscale isopycnal diffusivities and advected by scaled altimetric velocity fields is investigated. This measure is used to compare numerical simulations of the tracer released at a depth of about 1.5 km in the Pacific sector of the Southern Ocean during the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) field campaign with observations of the tracer taken on DIMES cruises. The authors find that simulations with an isopycnal diffusivity of ~20 m2 s−1 best match observations in the Pacific sector of the Antarctic Circumpolar Current (ACC), rising to ~20–50 m2 s−1 through Drake Passage, representing submesoscale processes and any mesoscale processes unresolved by the advecting altimetry fields. The roughness measure is demonstrated to be a statistically robust way to estimate a small-scale diffusivity when measurements are relatively sparse in space and time, although it does not work if there are too few measurements overall. The planning of tracer measurements during a cruise in order to maximize the robustness of the roughness measure is also considered. It is found that the robustness is increased if the spatial resolution of tracer measurements is increased with the time since tracer release.
-
ArticleDiapycnal mixing in the Southern Ocean diagnosed using the DIMES tracer and realistic velocity fields(John Wiley & Sons, 2018-04-13) Mackay, Neill ; Ledwell, James R. ; Messias, Marie-Jose ; Naveira Garabato, Alberto C. ; Brearley, J. Alexander ; Meijers, Andrew J. S. ; Jones, Daniel C. ; Watson, Andrew J.In this work, we use realistic isopycnal velocities with a 3-D eddy diffusivity to advect and diffuse a tracer in the Antarctic Circumpolar Current, beginning in the Southeast Pacific and progressing through Drake Passage. We prescribe a diapycnal diffusivity which takes one value in the SE Pacific west of 678W and another value in Drake Passage east of that longitude, and optimize the diffusivities using a cost function to give a best fit to experimental data from the DIMES (Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean) tracer, released near the boundary between the Upper and Lower Circumpolar Deep Water. We find that diapycnal diffusivity is enhanced 20-fold in Drake Passage compared with the SE Pacific, consistent with previous estimates obtained using a simpler advection-diffusion model with constant, but different, zonal velocities east and west of 678W. Our result shows that diapycnal mixing in the ACC plays a significant role in transferring buoyancy within the Meridional Overturning Circulation.
-
ArticleDirect estimate of lateral eddy diffusivity upstream of Drake Passage(American Meteorological Society, 2014-10) Tulloch, Ross ; Ferrari, Raffaele ; Jahn, Oliver ; Klocker, Andreas ; LaCasce, Joseph H. ; Ledwell, James R. ; Marshall, John C. ; Messias, Marie-Jose ; Speer, Kevin G. ; Watson, Andrew J.The first direct estimate of the rate at which geostrophic turbulence mixes tracers across the Antarctic Circumpolar Current is presented. The estimate is computed from the spreading of a tracer released upstream of Drake Passage as part of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). The meridional eddy diffusivity, a measure of the rate at which the area of the tracer spreads along an isopycnal across the Antarctic Circumpolar Current, is 710 ± 260 m2 s−1 at 1500-m depth. The estimate is based on an extrapolation of the tracer-based diffusivity using output from numerical tracers released in a one-twentieth of a degree model simulation of the circulation and turbulence in the Drake Passage region. The model is shown to reproduce the observed spreading rate of the DIMES tracer and suggests that the meridional eddy diffusivity is weak in the upper kilometer of the water column with values below 500 m2 s−1 and peaks at the steering level, near 2 km, where the eddy phase speed is equal to the mean flow speed. These vertical variations are not captured by ocean models presently used for climate studies, but they significantly affect the ventilation of different water masses.
-
ArticleDiapycnal diffusivities from a tracer release experiment in the deep sea, integrated over 13 years(American Geophysical Union, 2012-02-21) Rye, Craig D. ; Messias, Marie-Jose ; Ledwell, James R. ; Watson, Andrew J. ; Brousseau, Andrew ; King, Brian A.A section across the Atlantic at 24°S recorded in March 2009, sampled a tracer plume released in the deep Brazil Basin 13 years earlier. The 1-D diffusion equation was used to model the vertical spread of the tracer, yielding a mean diapycnal diffusivity estimate of approximately 3 × 10−4 m2/s at 4 km depth. This estimate is similar to that found by surveys of the tracer plume made between 1996 and 2000, within four years of the tracer release and therefore provides strong evidence for the long-term stability of that result.
-
ArticleThe contribution of the Weddell Gyre to the lower limb of the Global Overturning Circulation(John Wiley & Sons, 2014-06-05) Jullion, Loic ; Naveira Garabato, Alberto C. ; Bacon, Sheldon ; Meredith, Michael P. ; Brown, Peter J. ; Torres-Valdes, Sinhue ; Speer, Kevin G. ; Holland, Paul R. ; Dong, Jun ; Bakker, Dorothee C. E. ; Hoppema, Mario ; Loose, Brice ; Venables, Hugh J. ; Jenkins, William J. ; Messias, Marie-Jose ; Fahrbach, EberhardThe horizontal and vertical circulation of the Weddell Gyre is diagnosed using a box inverse model constructed with recent hydrographic sections and including mobile sea ice and eddy transports. The gyre is found to convey 42 ± 8 Sv (1 Sv = 106 m3 s–1) across the central Weddell Sea and to intensify to 54 ± 15 Sv further offshore. This circulation injects 36 ± 13 TW of heat from the Antarctic Circumpolar Current to the gyre, and exports 51 ± 23 mSv of freshwater, including 13 ± 1 mSv as sea ice to the midlatitude Southern Ocean. The gyre's overturning circulation has an asymmetric double-cell structure, in which 13 ± 4 Sv of Circumpolar Deep Water (CDW) and relatively light Antarctic Bottom Water (AABW) are transformed into upper-ocean water masses by midgyre upwelling (at a rate of 2 ± 2 Sv) and into denser AABW by downwelling focussed at the western boundary (8 ± 2 Sv). The gyre circulation exhibits a substantial throughflow component, by which CDW and AABW enter the gyre from the Indian sector, undergo ventilation and densification within the gyre, and are exported to the South Atlantic across the gyre's northern rim. The relatively modest net production of AABW in the Weddell Gyre (6 ± 2 Sv) suggests that the gyre's prominence in the closure of the lower limb of global oceanic overturning stems largely from the recycling and equatorward export of Indian-sourced AABW.
-
ArticleObservations of diapycnal upwelling within a sloping submarine canyon(Nature Research, 2024-06-26) Wynne-Cattanach, Bethan L. ; Couto, Nicole ; Drake, Henri F. ; Ferrari, Raffaele ; Le Boyer, Arnaud ; Mercier, Herle ; Messias, Marie-Jose ; Ruan, Xiaozhou ; Spingys, Carl P. ; van Haren, Hans ; Voet, Gunnar ; Polzin, Kurt L. ; Naveira Garabato, Alberto C. ; Alford, Matthew H.Small-scale turbulent mixing drives the upwelling of deep water masses in the abyssal ocean as part of the global overturning circulation1. However, the processes leading to mixing and the pathways through which this upwelling occurs remain insufficiently understood. Recent observational and theoretical work2,3,4,5 has suggested that deep-water upwelling may occur along the ocean’s sloping seafloor; however, evidence has, so far, been indirect. Here we show vigorous near-bottom upwelling across isopycnals at a rate of the order of 100 metres per day, coupled with adiabatic exchange of near-boundary and interior fluid. These observations were made using a dye released close to the seafloor within a sloping submarine canyon, and they provide direct evidence of strong, bottom-focused diapycnal upwelling in the deep ocean. This supports previous suggestions that mixing at topographic features, such as canyons, leads to globally significant upwelling3,6,7,8. The upwelling rates observed were approximately 10,000 times higher than the global average value required for approximately 30 × 106 m3 s−1 of net upwelling globally9.