The influence of bottom topography on the decay of modeled Agulhas rings

• Published in: Deep-sea research. Part I, Oceanographic research papers Vol. 56; no. 4; pp. 471 - 494
• Main Authors: de Steur, L., van Leeuwen, P.J.
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier Ltd 01.04.2009; Elsevier; Pergamon Press Inc
• Subjects: Agulhas rings; Cartography; Dynamics of the ocean (upper and deep oceans); Earth, ocean, space; Energy conversion; Exact sciences and technology; External geophysics; Marine; Numerical modeling; Ocean bottom; Oceanography; Oceans; Passive tracer; Physics of the oceans; Topographic ridge; South Atlantic; Walvis Ridge; Southeast Atlantic; Atlantic Ocean; Cape Basin; Agulhas current; AS, South Atlantic, Walvis Ridge; ISW, West Indian Ocean, Agulhas Current; Numerical modeling; Passive tracer; Topographic ridge; Agulhas rings; Energy conversion; ocean currents; mid-ocean ridges; kinetic energy; stabilization; Anticyclonic eddy; Space remote sensing; topography; Ring vortex; Primitive equation; Satellite observation; Ocean bottom; Ocean model; Rossby wave; Observation data; Barotropic wave; ocean circulation; altimetry; Baroclinic waves
• ISSN: 0967-0637; 1879-0119
• DOI: 10.1016/j.dsr.2008.11.009

The influence of a large meridional submarine ridge on the decay of Agulhas rings is investigated with a 1 and 2-layer setup of the isopycnic primitive-equation ocean model MICOM. In the single-layer case we show that the SSH decay of the ring is primarily governed by bottom friction and secondly by the radiation of Rossby waves. When a topographic ridge is present, the effect of the ridge on SSH decay and loss of tracer from the ring is negligible. However, the barotropic ring cannot pass the ridge due to energy and vorticity constraints. In the case of two-layer ring the initial SSH decay is governed by a mixed barotropic–baroclinic instability of the ring. Again, radiation of barotropic Rossby waves is present. When the ring passes the topographic ridge, it shows a small but significant stagnation of SSH decay, agreeing with satellite altimetry observations. This is found to be due to a reduction of the growth rate of the m = 2 instability, to conversions of kinetic energy to the upper layer, and to a decrease in Rossby-wave radiation. The energy transfer is related to the fact that coherent structures in the lower layer cannot pass the steep ridge due to energy constraints. Furthermore, the loss of tracer from the ring through filamentation is less than for a ring moving over a flat bottom, related to a decrease in propagation speed of the ring. We conclude that ridges like the Walvis Ridge tend to stabilize a multi-layer ring and reduce its decay.