Mass and volume transport variability in an eddy-filled ocean

Abstract

Eddy activity in the North Atlantic ocean produces fluctuations in ocean-wide volume transport on the order of 20×106 cubic metres per second, on multi-year timescales. Such background noise makes it impossible to detect possible trends in the ocean circulation due to a changing climate without multi-decadal observations in three spatial dimensions. 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.