Abstract

In the 1970's, an intense physical oceanographic effort was focused on the MODE Area (centered at 28N, 70W) to study mesoscale eddies and their effect on the larger-scale longer-term (or general) interior ocean circulation. At that time there was considerable discussion as to the typicality of these results. It has become clear that the time-dependent field is horizontally inhomogeneous; eddies have a geography related to the general circulation. In some areas, significant temporal inhomogeneity (nonstationarity) has been observed, but this issue has not yet been clarified for the MODE Area. Recently, a collective experiment to study the effect of fronts on mixed layer dynamics was carried out near the subtropical front in the North Atlantic. This note summarizes the (subsidiary) mean flow and eddy-based results from two subsurface moorings set as support for the main experiment, focusing on MODE Center (28N, 70W) where instruments at several levels from 150 to 4000 m were deployed for 20 months. Abyssal mean flow increased by an order of magnitude. The previously most stable eddy-field observable, abyssal eddy kinetic energy, changed by more than 50% from the measurement period in the 1970's to that in the 1980's. Eddy kinetic energy and mean flow in the thermocline, expected to be the least reproducible observables, hardly changed. The directionality of the thermocline eddy field is notably different, essentially reversed from the 1970's, with the meridional twice as large as the zonal variance in the 1980's. The spectral distribution in the thermocline is less "red," with the opposite tendency at abyssal depths. In summary, the MODE Area is neither particularly representative of the rest of the ocean nor are the MODE results of the 1970's quantitatively representative of measurements there ten to fifteen years later. It does seem possible, however, that many of the differences observed could be rationalized in terms of comparatively small-scale horizontal excursions of larger-scale flow regimes, notably the subtropical frontal zone.

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