On the Physics of the Agulhas Current: Steady Retroflection Regimes

From previous model studies, it has become clear that several physical mechanisms may be at work in the retroflection of the Agulhas Current. Here, a systematic study of steady barotropic flows connecting the Indian Ocean and South Atlantic Ocean in several idealized setups is performed. By solving directly for the steady circulation with continuation methods, the connection between different retroflection regimes can be monitored as external conditions, such as the wind forcing or bottom topography, as well as parameters, such as the lateral friction and layer depth, are changed. To distinguish the different steady retroflecting flows, an objective measure of the degree of retroflection, a retroflection index R, is introduced. By monitoring R along a branch of steady solutions, using the horizontal friction as control parameter, several steady retroflecting regimes are found. At large friction there exist stable steady states with viscously dominated retroflection. When friction is decreased, inertial effects become more dominant, and eventually unstable steady states with strong retroflection characteristics exist. Within this framework, different results from earlier studies can be reconciled.

[1]  D. Webb,et al.  Modelling the Agulhas current system with FRAM (fine resolution antarctic model) , 1995 .

[2]  W. Large,et al.  A Global Ocean Wind Stress Climatology Based on ECMWF Analyses , 1989 .

[3]  M. Schmeits,et al.  Physics of the 9-Month Variability in the Gulf Stream Region: Combining Data and Dynamical Systems Analyses , 2000 .

[4]  W. D. Ruijter Asymptotic Analysis of the Agulhas and Brazil Current Systems , 1982 .

[5]  D. Nof,et al.  Why Are There Agulhas Rings , 1999 .

[6]  Johann R. E. Lutjeharms,et al.  The Retroflection of the Agulhas Current , 1988 .

[7]  Compensation for the NADW Outflow in a Global Ocean General Circulation Model , 1995 .

[8]  Gerard L. G. Sleijpen,et al.  A Jacobi-Davidson Iteration Method for Linear Eigenvalue Problems , 1996, SIAM J. Matrix Anal. Appl..

[9]  A. Gordon Indian-Atlantic Transfer of Thermocline Water at the Agulhas Retroflection , 1985, Science.

[10]  Gerard L. G. Sleijpen,et al.  A generalized Jacobi-Davidson iteration method for linear eigenvalue problems , 1998 .

[11]  D. Nof,et al.  The Retroflection Paradox , 1996 .

[12]  M. L. Gründlingh,et al.  Stratification and circulation at the Agulhas Retroflection , 1987 .

[13]  W. D. Ruijter,et al.  Ring shedding in the Agulhas Current System , 1992 .

[14]  P. Holmes,et al.  Nonlinear Oscillations, Dynamical Systems, and Bifurcations of Vector Fields , 1983, Applied Mathematical Sciences.

[15]  The wind-driven circulation of the South Atlantic-Indian ocean — II. Experiments using a multi-layer numerical model , 1986 .

[16]  W. Howard,et al.  Late Quaternary Surface Circulation of the Southern Indian Ocean and its Relationship to Orbital Variations , 1992 .

[17]  C. Wunsch,et al.  How well does a 1/4° global circulation model simulate large-scale oceanic observations? , 1996 .

[18]  Wilbert Weijer,et al.  Indian‐Atlantic interocean exchange: Dynamics, estimation and impact , 1999 .

[19]  A. Ploeg,et al.  An efficient code to compute non-parallel steady flows and their linear stability , 1995 .

[20]  E. Chassignet,et al.  Dynamics of Agulhas Retroflection and Ring Formation in a Numerical Model. Part II. Energetics and Ring Formation , 1988 .

[21]  J. Lutjeharms,et al.  Topographic control in the Agulhas Current system , 1984 .

[22]  P. Leeuwen,et al.  Impact of Interbasin Exchange on the Atlantic Overturning Circulation , 1999 .

[23]  E. Chassignet,et al.  Dynamics of Agulhas Retroflection and Ring Formation in a Numerical Model. Part I: The Vorticity Balance , 1988 .

[24]  W. D. Ruijter,et al.  Separation of an Inertial Boundary Current from a Curved Coastline , 1986 .

[25]  W. D. Ruijter,et al.  The wind-driven circulation in the South Atlantic-Indian Ocean—I. Numerical experiments in a one-layer model , 1985 .

[26]  A. Gordon,et al.  Agulhas eddies invade the south Atlantic: Evidence From Geosat altimeter and shipboard conductivity‐temperature‐depth survey , 1990 .

[27]  M. Maltrud,et al.  Numerical simulation of the North Atlantic Ocean at 1/10 degrees , 2000 .