Speer Kevin G.

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Speer
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Kevin G.
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  • Article
    Direct 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.
  • Article
    Pacific anthropogenic carbon between 1991 and 2017
    (American Geophysical Union, 2019-04-29) Carter, Brendan ; Feely, Richard A. ; Wanninkhof, Rik ; Kouketsu, Shinya ; Sonnerup, Rolf E. ; Pardo, Paula Conde ; Sabine, Christopher L. ; Johnson, Gregory C. ; Sloyan, Bernadette M. ; Murata, Akihiko ; Mecking, Sabine ; Tilbrook, Bronte ; Speer, Kevin G. ; Talley, Lynne D. ; Millero, Frank J. ; Wijffels, Susan E. ; Macdonald, Alison M. ; Gruber, Nicolas ; Bullister, John L.
    We estimate anthropogenic carbon (Canth) accumulation rates in the Pacific Ocean between 1991 and 2017 from 14 hydrographic sections that have been occupied two to four times over the past few decades, with most sections having been recently measured as part of the Global Ocean Ship‐based Hydrographic Investigations Program. The rate of change of Canth is estimated using a new method that combines the extended multiple linear regression method with improvements to address the challenges of analyzing multiple occupations of sections spaced irregularly in time. The Canth accumulation rate over the top 1,500 m of the Pacific increased from 8.8 (±1.1, 1σ) Pg of carbon per decade between 1995 and 2005 to 11.7 (±1.1) PgC per decade between 2005 and 2015. For the entire Pacific, about half of this decadal increase in the accumulation rate is attributable to the increase in atmospheric CO2, while in the South Pacific subtropical gyre this fraction is closer to one fifth. This suggests a substantial enhancement of the accumulation of Canth in the South Pacific by circulation variability and implies that a meaningful portion of the reinvigoration of the global CO2 sink that occurred between ~2000 and ~2010 could be driven by enhanced ocean Canth uptake and advection into this gyre. Our assessment suggests that the accuracy of Canth accumulation rate reconstructions along survey lines is limited by the accuracy of the full suite of hydrographic data and that a continuation of repeated surveys is a critical component of future carbon cycle monitoring.
  • Thesis
    The influence of geothermal sources on deep ocean temperature, salinity, and flow fields
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1988-05) Speer, Kevin G.
    This thesis is a study of the effect of geothermal sources on the deep circulation, temperature and salinity fields. In Chapter 1 background material is given on the strength and distribution of geothermal heating. In Chapter 2 evidence for the influence of a hydrothermal system in the rift valley of the Mid-Atlantic Ridge on nearby property fields and a model of the flow around such a heat source are presented, with an analysis of a larger-scale effect. Results of an analytical model for a heat source on a β-plane in Chapter 3 show how the response far from the source can have a structure different from the forcing because of its dependence on two parameters: a Peclet number (the ratio of horizontal advection and vertical diffusion), and a Froude-number-like parameter (the ratio of long wave phase speed to background flow speed) which control the relative amount of damping and advection of different vertical scales. The solutions emphasize the different behavior of a dynamical field like temperature compared to tracers introduced at the source. These ideas are useful for interpreting more complicated solutions from a numerical model presented in the final chapter.
  • Article
    The 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, Eberhard
    The 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.