Title

The Arrested Agulhas Retroflection

Publication Title

Jounral of Marine Research

Publication Date

2011

Document Type

Article

DOI

10.1357/002224011799849453

Disciplines

Oceanography | Oceanography and Atmospheric Sciences and Meteorology

Abstract

Paleoceanographic proxy data indicate that the Agulhas leakage into the South Atlantic was dramatically reduced during glacial times. In our former papers, we suggested that this was due to a northward shift of the zero wind stress curl that, in turn, forced the retroflection to occur farther north, where the slant of the coastline relative to the north is steep. In the present paper, we propose that strong westerlies (0.4 Pa, implying a wind speed of ∼12 m s–1 at zero degrees centigrade), which were supposedly common during glaciations, can also arrest the leakage. This arrest occurred because the wind stress opposed the momentum flux associated with the retroflection; such an arrest did not require the retroflection to shift in latitude.

We use a simple, nonlinear, “reduced gravity” model to show analytically and numerically that, under the above conditions, the eastward wind stress compensates for the zonal westward flow-force associated with the retroflection, thus avoiding the development and shedding of rings. For a nearly zonal wall, westerly winds, and small upper layer thickness along the wall, thearresting wind stress is found, theoretically, to be, τ x = 0.042α3/2 ρf0 [(2f0Q) 3 /g′]1/4, whereα is twice the retroflection eddy vorticity, ρ is the water density, Q is the Agulhas Current volume flux, and the remaining notation is conventional.