Does the Wind Control the Import and Export of the South Atlantic?

Abstract

A different way of examining the meridional flux of warm and intermediate water (σθ < 27.50) from the Southern Ocean into the South Atlantic is proposed. The method considers the Americas to be a “pseudo island” in the sense that the continent is entirely surrounded by water but has no circulation around it. It is shown that, although the northern connection between the Atlantic and the Pacific (via the Bering Strait) is weak, it imposes severe limitations on the sea level in the Atlantic basin: so much so that it allows one to compute the meridional transport without finding the detailed solution to the complete wind–thermohaline problem. The method employs an integration of the linearized momentum equations along a closed contour containing the Americas, Greenland, the Atlantic, and parts of the Arctic Ocean. First, an idealized rectangular model involving three layers, an active continuously stratified upper layer containing both thermocline (σθ < 26.80) and intermediate water (26.80 < σθ < 27.80), an inert deep layer (27.80 < σθ < 27.90), and a southward moving bottom layer (σθ > 27.90) is considered. In this idealized model, the Americas are represented by the pseudo island. Deep-water formation is allowed (in the northern part of the basin east of the Americas and south of the gap connecting the Atlantic–Arctic basin to the Pacific), but the cooling rate need not be specified. The basin is subject to both zonal winds and heat exchange with the atmosphere [i.e., ρ = ρ(x, y, z)], but, for simplicity, (temporarily) meridional winds are not allowed. A simple analytical expression for the transport of the meridional overturning cell is derived, and process-oriented numerical experiments that were conducted (using a primitive equation layer-and-a-half isopycnic model) are in excellent agreement with the theory. The theory is then extended to a more convoluted geography subject to both zonal and meridional winds. The surprising result is found that, even for the complex situation, the northward transport of upper and intermediate water is given simply by [fy917,1]) τl dl/ρ/f0, where f0 is the average Coriolis parameter along a line connecting the southern tip of the Americas with the southern tip of Africa and τl is the wind stress along the integration path (l). This implies that, although the amount of high-latitude cooling is responsible for the location and manner in which bottom water is formed, it has very limited effect on the net meridional mass flux (which constitutes the so-called conveyor). Detailed application of the above formula to the Atlantic using actual geography and spherical coordinates as well as actual meridional and zonal winds (adopted from 40-yr averages given by NCEP) gives the reasonable estimate of 9 Sv (Sv ≡ 106 m3 s−1) for the transport of the conveyor upper limb.