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Mass and volume transport variability in an eddy-filled ocean

Abstract

The possibility that the oceanic general circulation is undergoing changes as part, or the cause, of major climate shifts is being intensely discussed1, with some published results relying on data from moorings spanning the North Atlantic Ocean2,3. The circulation is, however, extremely noisy. Here, I use existing estimates of the frequency and wavenumber content of geostrophic eddies in the ocean4 to show that variations in ocean-wide integrated transport must appear even in the absence of a true long-term trend. Expected fluctuations exceed ±20×109 kg s−1 (or ±20×106 m3 s−1) and exhibit multi-year timescales. Existing knowledge of the eddy field allows predictions of observed variability and produces lower bounds on the (multi-decadal) timescale required to detect true trends of a large magnitude. Detecting and understanding the effect of climate change on the ocean circulation requires observations in three dimensions over long periods of time.

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Figure 1: Barotropic (n=0) and first two baroclinic (n=1,2) horizontal velocity and pressure modes7 labelled Fn(z),n=0,1,2,…, in the North Atlantic at 24 N, 32 W describing most variability.
Figure 2: Simulation over 1,000 months (upper panel) of the transport variation between two moorings above about 1,000 m in which the eddy variability occurs incoherently.

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Acknowledgements

Supported by the National Ocean Partnership Program (NOAA and NASA) with further funding from NASA.

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Correspondence to Carl Wunsch.

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Supplementary experimental data and supplementary figure S1 (PDF 140 kb)

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Wunsch, C. Mass and volume transport variability in an eddy-filled ocean. Nature Geosci 1, 165–168 (2008). https://doi.org/10.1038/ngeo126

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