• Bacon, S., 1998: Decadal variability in the outflow from the Nordic seas to the Deep Atlantic Ocean. Nature, 394 , 871874.

  • Baehr, J., , J. Hirschi, , J. O. Beismann, , and J. Marotzke, 2004: Monitoring the meridional overturning circulation in the North Atlantic: A model-based array design study. J. Mar. Res., 62 , 283312.

    • Search Google Scholar
    • Export Citation
  • Baringer, M. O., , and J. C. Larsen, 2001: Sixteen years of Florida Current transport at 27°N. Geophys. Res. Lett., 28 , 31793182.

  • Böning, C. W., , C. Dieterich, , B. Barnier, , and Y. L. Jia, 2001: Seasonal cycle of meridional heat transport in the subtropical North Atlantic: A model intercomparison in relation to observations near 25°N. Progress in Oceanography, Vol. 48, Pergamon, 231–253.

  • Bryan, K., 1962: Measurements of meridional heat transport by ocean currents. J. Geophys. Res., 67 , 34033414.

  • Bryan, K., 1982: Seasonal variation in meridional overturning and poleward heat transport in the Atlantic and Pacific Oceans: A model study. J. Mar. Res., 40 , 3953.

    • Search Google Scholar
    • Export Citation
  • Bryden, H. L., , and S. Imawaki, 2001: Ocean heat transport. Ocean Circulation and Climate: Observing and Modelling the Global Ocean, G. Siedler, J. Church, and J. Gould, Eds., Academic Press, 455–474.

    • Search Google Scholar
    • Export Citation
  • Bryden, H. L., , H. R. Longworth, , and S. A. Cunningham, 2005: Slowing of the Atlantic meridional overturning circulation at 25 degrees N. Nature, 438 , 655657.

    • Search Google Scholar
    • Export Citation
  • Collins, M., , S. F. B. Tett, , and C. Cooper, 2001: The internal climate variability of HadCM3, a version of the Hadley Centre coupled model without flux adjustments. Climate Dyn., 17 , 6181.

    • Search Google Scholar
    • Export Citation
  • Cooper, C., , and C. Gordon, 2002: North Atlantic oceanic decadal variability in the Hadley Centre coupled model. J. Climate, 15 , 4572.

    • Search Google Scholar
    • Export Citation
  • Cox, M. D., 1984: A primitive equation 3-dimensional model of the ocean. GFDL Ocean Group Tech. Rep. 1, 143 pp.

  • Dickson, R. R., , and J. Brown, 1994: The production of North Atlantic deep water: Sources, rates, and pathways. J. Geophys. Res., 99 , C6. 1231912341.

    • Search Google Scholar
    • Export Citation
  • Dong, B. W., , and R. T. Sutton, 2001: The dominant mechanisms of variability in Atlantic Ocean heat transport in a coupled ocean-atmosphere GCM. Geophys. Res. Lett., 28 , 24452448.

    • Search Google Scholar
    • Export Citation
  • Dong, B. W., , and R. T. Sutton, 2002a: Adjustment of the coupled ocean–atmosphere system to a sudden change in the Thermohaline Circulation. Geophys. Res. Lett., 29 .1728, doi:10.1029/2002GL015229.

    • Search Google Scholar
    • Export Citation
  • Dong, B. W., , and R. T. Sutton, 2002b: Variability in North Atlantic heat content and heat transport in a coupled ocean–atmosphere GCM. Climate Dyn., 19 , 485497.

    • Search Google Scholar
    • Export Citation
  • Ganachaud, A. S., 2003a: Error budget of inverse box models: The North Atlantic. J. Atmos. Oceanic Technol., 20 , 16411655.

  • Ganachaud, A. S., 2003b: Large-scale mass transports, water mass formation, and diffusivities estimated from World Ocean Circulation Experiment (WOCE) hydrographic data. J. Geophys. Res., 108 .3213, doi:10.1029/2002JC001565.

    • Search Google Scholar
    • Export Citation
  • Gill, A. E., , and P. P. Niiler, 1973: The theory of the seasonal variability in the ocean. Deep-Sea Res., 20 , 141178.

  • Gordon, C., , C. Cooper, , C. A. Senior, , H. Banks, , J. M. Gregory, , T. C. Johns, , J. F. B. Mitchell, , and R. A. Wood, 2000: The simulation of SST, sea ice extents and ocean heat transports in a version of the Hadley Centre coupled model without flux adjustments. Climate Dyn., 16 , 2–3. 147168.

    • Search Google Scholar
    • Export Citation
  • Haines, K., , and C. Old, 2005: Diagnosing natural variability of North Atlantic water masses in HadCM3. J. Climate, 18 , 19251941.

  • Hall, M. M., , and H. L. Bryden, 1982: Direct estimates and mechanisms of ocean heat transport. Deep-Sea Res., 29 , 339359.

  • Hirschi, J., , J. Baehr, , J. Marotzke, , J. Stark, , S. Cunningham, , and J. O. Beismann, 2003: A monitoring design for the Atlantic meridional overturning circulation. Geophys. Res. Lett., 30 .1413, doi:10.1029/2002GL016776.

    • Search Google Scholar
    • Export Citation
  • Koltermann, K. P., , A. V. Sokov, , V. Tereschenkov, , S. A. Dobroliubov, , K. Loracher, , and A. Sy, 1999: Decadal changes in the thermohaline circulation of the North Atlantic. Deep-Sea Res. II, 46 , 1–2. 109138.

    • Search Google Scholar
    • Export Citation
  • Lee, T., , and J. Marotzke, 1998: Seasonal cycles of meridional overturning and heat transport of the Indian Ocean. J. Phys. Oceanogr., 28 , 923943.

    • Search Google Scholar
    • Export Citation
  • Lee, T. N., , W. E. Jones, , R. J. Zantopp, , and E. R. Fillenbaum, 1996: Moored observations of western boundary current variability and thermohaline circulation at 26.5°N in the subtropical North Atlantic. J. Phys. Oceanogr., 26 , 962983.

    • Search Google Scholar
    • Export Citation
  • Lumpkin, R., , and K. Speer, 2003: Large-scale vertical and horizontal circulation in the North Atlantic Ocean. J. Phys. Oceanogr., 33 , 19021920.

    • Search Google Scholar
    • Export Citation
  • McDonagh, E. L., , and B. A. King, 2005: Oceanic fluxes in the South Atlantic. J. Phys. Oceanogr., 35 , 109122.

  • Meinen, C. S., 2001: Structure of the North Atlantic current in stream-coordinates and the circulation in the Newfoundland basin. Deep-Sea Res., 48 , 15531580.

    • Search Google Scholar
    • Export Citation
  • Pardaens, A. K., , H. T. Banks, , J. M. Gregory, , and P. R. Rowntree, 2003: Freshwater transports in HadCM3. Climate Dyn., 21 , 177195.

  • Pope, V. D., , M. L. Gallani, , P. R. Rowntree, , and R. A. Stratton, 2000: The impact of new physical parametrizations in the Hadley Centre climate model: HadAM3. Climate Dyn., 16 , 2–3. 123146.

    • Search Google Scholar
    • Export Citation
  • Roberts, M. J., , and R. A. Wood, 1997: Topographic sensitivity studies with a Bryan–Cox-type ocean model. J. Phys. Oceanogr., 27 , 823836.

    • Search Google Scholar
    • Export Citation
  • Saunders, P. M., , and B. A. King, 1995: Oceanic fluxes on the WOCE A11 section. J. Phys. Oceanogr., 25 , 19421958.

  • Talley, L. D., 2003: Shallow, intermediate, and deep overturning component of the global heat budget. J. Phys. Oceanogr., 33 , 530560.

    • Search Google Scholar
    • Export Citation
  • Thorpe, R. B., , J. M. Gregory, , T. C. Johns, , R. A. Wood, , and J. F. B. Mitchell, 2001: Mechanisms determining Atlantic thermohaline circulation response to greenhouse gas forcing in a non-flux-adjusted coupled climate model. J. Climate, 14 , 31023116.

    • Search Google Scholar
    • Export Citation
  • Thorpe, R. B., , R. A. Wood, , and J. F. B. Mitchell, 2004: Sensitivity of the modelled thermohaline circulation to the parameterisation of mixing across the Greenland–Scotland ridge. Ocean Modell., 7 , 3–4. 259268.

    • Search Google Scholar
    • Export Citation
  • Thurnherr, A. M., , and K. G. Speer, 2004: Representativeness of meridional hydrographic sections in the western South Atlantic. J. Mar. Res., 62 , 3765.

    • Search Google Scholar
    • Export Citation
  • Vellinga, M., , and R. A. Wood, 2002: Global climate impacts of a collapse of Atlantic thermohaline circulation. Climate Change, 54 , 3. 251267.

    • Search Google Scholar
    • Export Citation
  • Vellinga, M., , and R. A. Wood, 2004: Timely detection of anthropogenic change in the Atlantic meridional overturning circulation. Geophys. Res. Lett., 31 .L14203, doi:10.1029/2004GL020306.

    • Search Google Scholar
    • Export Citation
  • Webb, D. J., , and N. Suginohara, 2001: The interior circulation of the ocean. Ocean Circulation and Climate: Observing and Modelling the Global Ocean, G. Siedler, J. Church, and J. Gould, Eds., Academic Press, 205–214.

    • Search Google Scholar
    • Export Citation
  • Willebrand, J., and Coauthors, 2001: Circulation characteristics in three eddy-permitting models of the North Atlantic. Progress in Oceanography, Vol. 48, Pergamon, 123–161.

  • Wunsch, C., 1996: The Ocean Circulation Inverse Problem. Cambridge University Press, 442 pp.

All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 65 65 4
PDF Downloads 64 64 2

A Decomposition of the Atlantic Meridional Overturning

View More View Less
  • 1 School of Environmental Sciences, University of East Anglia, Norwich, United Kingdom
  • | 2 School of Mathematics, University of East Anglia, Norwich, United Kingdom
  • | 3 School of Environmental Sciences, University of East Anglia, Norwich, United Kingdom
© Get Permissions
Restricted access

Abstract

A decomposition of meridional overturning circulation (MOC) cells into geostrophic vertical shears, Ekman, and bottom pressure–dependent (or external mode) circulation components is presented. The decomposition requires the following information: 1) a density profile wherever bathymetry changes to construct the vertical shears component, 2) the zonal-mean zonal wind stress for the Ekman component, and 3) the mean depth-independent velocity information over each isobath to construct the external mode. The decomposition is applied to the third-generation Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3) to determine the meridional variability of these individual components within the Atlantic Ocean. The external mode component is shown to be extremely important where western boundary currents impinge on topography, and also in the area of the overflows. The Sverdrup balance explains the shape of the external mode MOC component to first order, but the time variability of the external mode exhibits only a very weak dependence on the wind stress curl. Thus, the Sverdrup balance cannot be used to determine the external mode changes when examining temporal change in the MOC. The vertical shears component allows the time-mean and the time-variable upper North Atlantic MOC cell to be deduced at 25°S and 50°N. A stronger dependency on the external mode and Ekman components between 8° and 35°N and in the regions of the overflows means that hydrographic sections need to be supplemented by bottom pressure and wind stress information at these latitudes. At the decadal time scale, variability in Ekman transport is less important than that in geostrophic shears. In the Southern Hemisphere the vertical shears component is dominant at all time scales, suggesting that hydrographic sections alone may be suitable for deducing change in the MOC at these latitudes.

* Current affiliation: Physical Sciences Division, British Antarctic Survey, Cambridge, United Kingdom

Corresponding author address: Louise C. Sime, Physical Sciences Division, British Antarctic Survey, Cambridge CB3 0ET, United Kingdom. Email: lsim@bas.ac.uk

Abstract

A decomposition of meridional overturning circulation (MOC) cells into geostrophic vertical shears, Ekman, and bottom pressure–dependent (or external mode) circulation components is presented. The decomposition requires the following information: 1) a density profile wherever bathymetry changes to construct the vertical shears component, 2) the zonal-mean zonal wind stress for the Ekman component, and 3) the mean depth-independent velocity information over each isobath to construct the external mode. The decomposition is applied to the third-generation Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3) to determine the meridional variability of these individual components within the Atlantic Ocean. The external mode component is shown to be extremely important where western boundary currents impinge on topography, and also in the area of the overflows. The Sverdrup balance explains the shape of the external mode MOC component to first order, but the time variability of the external mode exhibits only a very weak dependence on the wind stress curl. Thus, the Sverdrup balance cannot be used to determine the external mode changes when examining temporal change in the MOC. The vertical shears component allows the time-mean and the time-variable upper North Atlantic MOC cell to be deduced at 25°S and 50°N. A stronger dependency on the external mode and Ekman components between 8° and 35°N and in the regions of the overflows means that hydrographic sections need to be supplemented by bottom pressure and wind stress information at these latitudes. At the decadal time scale, variability in Ekman transport is less important than that in geostrophic shears. In the Southern Hemisphere the vertical shears component is dominant at all time scales, suggesting that hydrographic sections alone may be suitable for deducing change in the MOC at these latitudes.

* Current affiliation: Physical Sciences Division, British Antarctic Survey, Cambridge, United Kingdom

Corresponding author address: Louise C. Sime, Physical Sciences Division, British Antarctic Survey, Cambridge CB3 0ET, United Kingdom. Email: lsim@bas.ac.uk

Save