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The role of meridional density differences for a wind-driven overturning circulation

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Abstract

Experiments with the coupled climate model CLIMBER-3α, which contains an oceanic general circulation model, show deep upwelling in the Southern Ocean to be proportional to the surface wind stress in the latitudinal band of Drake Passage. At the same time, the distribution of the Southern Ocean upwelling onto the oceanic basins is controlled by buoyancy distribution; the inflow into each basin being proportional to the respective meridional density difference. We observe approximately the same constant of proportionality for all basins, and demonstrate that it can be directly related to the flow geometry. For increased wind stress in the Southern Ocean, the overturning increases both in the Atlantic and the Indo-Pacific basin. For strongly reduced wind stress, the circulation enters a regime where Atlantic overturning is maintained through Pacific upwelling, in order to satisfy the transports set by the density differences. Previous results on surface buoyancy and wind stress forcing, obtained with different models, are reproduced within one model in order to distill a consistent picture. We propose that both Southern Ocean upwelling and meridional density differences set up a system of conditions that determine the global meridional overturning circulation.

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Notes

  1. We use the term ‘driving mechanism’ in the sense of the mechanism providing the input of potential energy required to sustain a deep overturning circulation.

  2. Here, we neglect spatial variation of ν hor . In the model, the latitudinal variation of ν hor is small in low and middle latitudes.

  3. An example for a 2-dimensional quadratic profile would be v(x,z) = 16 v max x(x−L x )(z−D)(z−D−δ)/L 2 x δ2. An example for a sine profile would be v(x,z) = v max sin(πx/L x ) sin(π (z−D)/δ).

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Acknowledgment

We thank A. Naveira Garabato for a helpful discussion on eddy momentum fluxes. J. S. was funded by the Heinrich Böll Foundation.

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Authors and Affiliations

  1. Earth System Analysis, Potsdam Institute for Climate Impact Research, Telegrafenberg A62, 14473, Potsdam, Germany

    J. Schewe & A. Levermann

  2. Physics Institute, Potsdam University, Potsdam, Germany

    J. Schewe & A. Levermann

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  1. J. Schewe
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  2. A. Levermann
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Correspondence to J. Schewe.

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Schewe, J., Levermann, A. The role of meridional density differences for a wind-driven overturning circulation. Clim Dyn 34, 547–556 (2010). https://doi.org/10.1007/s00382-009-0572-1

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