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Significant sink of ocean-eddy energy near western boundaries


Ocean eddies generated through instability of the mean flow are a vital component of the energy budget of the global ocean1,2,3. In equilibrium, the sources and sinks of eddy energy have to be balanced. However, where and how eddy energy is removed remains uncertain3,4. Ocean eddies are observed to propagate westwards at speeds similar to the phase speeds of classical Rossby waves5, but what happens to the eddies when they encounter the western boundary is unclear. Here we use a simple reduced-gravity model along with satellite altimetry data to show that the western boundary acts as a ‘graveyard’ for the westward-propagating ocean eddies. We estimate a convergence of eddy energy near the western boundary of approximately 0.1–0.3 TW, poleward of 10° in latitude. This energy is most probably scattered into high-wavenumber vertical modes, resulting in energy dissipation and diapycnal mixing6. If confirmed, this eddy-energy sink will have important implications for the ocean circulation.

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Figure 1: Eddy energy budget for a single, anticyclonic eddy in the reduced-gravity model.
Figure 2: Eddy energy budget for a sea of eddies in the reduced-gravity model.
Figure 3: Sources (red) and sinks (blue) of eddy energy in the first baroclinic mode.

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We are grateful for funding from the UK Natural Environment Research Council. H.L.J. is supported by a Royal Society University Research Fellowship and D.P.M. acknowledges additional support from the James Martin 21st Century School, University of Oxford.

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X.Z. conducted the numerical experiments and analysis of the altimetry data. All the authors contributed to design of the study, interpretation of the results and writing of the manuscript.

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Correspondence to Xiaoming Zhai.

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The authors declare no competing financial interests.

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Zhai, X., Johnson, H. & Marshall, D. Significant sink of ocean-eddy energy near western boundaries. Nature Geosci 3, 608–612 (2010).

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