TY - JOUR
T1 - The arrested Agulhas retroflection
AU - Nof, Doron
AU - Zharkov, Volodymyr
AU - Ortiz, Joseph
AU - Paldor, Nathan
AU - Arruda, Wilton
AU - Chassignet, Eric
PY - 2011/7
Y1 - 2011/7
N2 - 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, the arresting wind stress is found, theoretically, to be, τx = 0.042α3/2 ρf0 [(2f0Q)3/g′]1/4, where α is twice the retroflection eddy vorticity, ρ is the water density, Q is the Agulhas Current volume flux, and the remaining notation is conventional.
AB - 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, the arresting wind stress is found, theoretically, to be, τx = 0.042α3/2 ρf0 [(2f0Q)3/g′]1/4, where α is twice the retroflection eddy vorticity, ρ is the water density, Q is the Agulhas Current volume flux, and the remaining notation is conventional.
UR - http://www.scopus.com/inward/record.url?scp=84858822708&partnerID=8YFLogxK
U2 - 10.1357/002224011799849453
DO - 10.1357/002224011799849453
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AN - SCOPUS:84858822708
SN - 0022-2402
VL - 69
SP - 659
EP - 691
JO - Journal of Marine Research
JF - Journal of Marine Research
IS - 4-6
ER -