Weakly depth-dependent segments of the North Atlantic circulation

Abstract

Time-averaged horizontal currents obtained from long-term moored instruments deployed in the western North Atlantic over the Sohm Abyssal Plain along 55W exhibit two segments of weakly depth-dependent flow: one, near 36N, predominantly westward and narrow or jet-like (~ 200 km wide or less); the second primarily eastward, located near 37.5N, about 200-300 km south of the mean position of the axis of the Gulf Stream (its width cannot be estimated quantitatively with the data available because only one mooring with adequate vertical coverage is clearly located in this flow regime, but an upper bound of roughly 200 km seems plausible). In both cases, long-term mean zonal currents between 600 and 4000 m depths (nominal) vary in amplitude from only 6 to 10 cm s-1 (approximately). The vertical structure of the westward recirculation varies with horizontal position, being both surface and bottom intensified. The possibility exists that the identification of these weakly depth-dependent flow regimes may point to one way of increasing the transport of the Gulf Stream. That is, flow with weak vertical shear is added offshore of the more baroclinic segment of the Stream, and possibly recirculated accordingly. This notion is generally consistent with all previous investigations which find the weakest vertical shears at the offshore edge of the Stream, wherever and however examined, and in particular with the addition of transport to the Florida Current over the Blake Plateau, after emerging from the Straits of Florida (Richardson, Schmitz, and Niiler, 1969). The horizontal patterns of the two weakly depth-dependent flow regimes found at 55W may be quite complex, containing variability on comparatively short and intermediate scales, associated to some extent with bottom topography. A specific example of the effect of bottom topography on the 55W data has been presented by Owens and Hogg (1980). It is hypothesized that the observations described here may indicate the presence of a previously unknown, weakly depth-dependent smaller scale gyre recirculating within the subtropical gyre, with the former confined between the New England Seamounts and the Grand Banks of Newfoundland. It should be emphasized that other horizontal and vertical structures may be characteristic of different locations in the recirculation of the North Atlantic. Eddy kinetic energy (Schmitz, 1978) and the off-diagonal component of Reynolds' stress are also to some extent weakly depth-dependent in each of the weakly depth-dependent mean flow regimes noted above, relative to more mid-ocean locations. At one site in particular, the off-diagonal component of the Reynolds' stress is found to be essentially depth-independent. The observation of weak depth-dependence in association with relatively strong abyssal currents for the recirculation regime could in principle help rationalize (Schmitz, 1977; Stommel, Niiler and Anati, 1978; Wunsch, 1978) some of the difficulties in geostrophica\ly balancing (at the leading order of approximation!), according to Worthington (1976), the North Atlantic Circulation in this type of region. Estimates of contributions to momentum balances (based on the available moored instrument data) involving horizontal gradients of the Reynolds' stresses, or of the momentum transport by the time-averaged flow, are typically at least an order of magnitude less than the Coriolis force associated with the zonal (or downstream) mean flow component, and possibly also the meridional (or cross-stream) flow component at most locations, thereby precluding violation of geostrophy at leading order by these effects. Geostrophic terms associated with estimates of the curvature of the Reynolds' stresses and/or mean momentum flux could be significant at the next order of approximation in the immediate vicinity of the Gulf Stream or near topographic features. Niiler (1979) has developed a model of an eddy-driven mean flow, where the eddy-terms in the vorticity equation are locally significant only in the Gulf Stream, but with a basin-wide mid-ocean flow driven in response to the noncompensated eddy-induced pressure gradient at the offshore edge of the region where eddy effects are locally significant dynamically. Two recent hydrographic sections across the Gulf Stream and recirculation along SSW were found to be in mass balance geostrophically, relative to the bottom (McCartney, Worthington and Raymer, 1980).