A time-average ocean: Thermal wind and flow spirals

Abstract

Using a 26-year average of a dynamically consistent ECCO state-estimate, an effort is made to find a few simple descriptive, but quantitative, patterns of properties of the ocean circulation that are near-globally applicable outside the Arctic regions. The conceptual assumption is made that such an average is physically meaningful. Even with a 26-year average, complex spatial variations in the flow field remain, particularly below about 2000 m where the intricate structure of the underlying topography becomes manifest. Nonetheless, certain constructs do describe the great bulk of the ocean. These constructs consist of thermal wind balance (quasi-geostrophy), spiral-like flow behavior in the near-surface boundary layers with orientation analogous to that of an Ekman layer—abruptly changing sign across the equator. In contrast, evidence for beta-spirals is very thin, consistent with the spatially complex meridional and vertical velocities. As expected, integration so as to remove spatial dependence in one coordinate (e.g. zonal) does produce much simplified structures, albeit in the process suppressing diverse dynamical regimes. Predominantly zonal structures persist in the zonal velocity at depth, and are presumed sensitive to the (parameterized) mean eddy fluxes. An unanswered question, and one perhaps unanswerable at the present time, is whether a much longer averaging interval would significantly further simplify the upper-ocean circulation. The abyssal circulation almost everywhere appears dominated by the topography and slopes and whose structure likely would persist in averages of arbitrary duration.