Abstract

Arecent paper (Hu et al., 2020, Sci. Adv.) has raised the interesting question of whether the ocean circulation has been “speeding up” in the last decades. Their result contrasts with some estimates of the lack of major trends in oceanic surface gravity waves and wind stress. In general, both the increased energy and implied power inputs of the calculated circulation correspond to a small fraction of the very noisy background values. An example is the implied power increase of about 3×108W, as compared to wind energy inputs of order 1012W. Here the problem is re-examined using a state estimate that has the virtue of being energy, mass, etc. conserving. Because it is an estimate over an entire recent 26-year interval it is less sensitive to the strong changes in observational data density and distribution, and it does not rely upon non-conservative “reanalyses.” The focus is on the energy lying in the surface layers of the ocean. A potential energy increase is found, but it is almost completely unavailable—arising from the increase in mean sea level. A weak increase in kinetic energy in the top layer (10m) is confirmed, corresponding to an increase of order 1cm/s/year over 26 years: An estimate of kinetic energy in the full water column shows no monotonic trend; but the changes in the corresponding available PE are not calculated here.

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