Mass and volume transport variability in an eddy-filled ocean

The possibility that the oceanic general circulation is undergoing changes as part, or the cause, of major climate shifts is being intensely discussed(1), with some published results relying on data from moorings spanning the North Atlantic Ocean(2,3). The circulation is, however, extremely noisy. Here, I use existing estimates of the frequency and wavenumber content of geostrophic eddies in the ocean(4) 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 x 10(9) kg s(-1) (or +/- 20 x 10(6) m(3) 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.