The time‐mean circulation in the Agulhas region determined with the ensemble smoother

The time-mean circulation in the Agulhas Retroflection area is determined by combining TOPEX/POSEIDON data and a two-layer quasi-geostrophic model using the ensemble smoother. By taking the time-mean circulation as the unknown in the data assimilation procedure, the time-varying altimeter signal is used to constrain the time-mean field. The quasi-geostrophic model is applied as a strong constraint, with only the time-mean circulation containing errors. Inspection of the posterior penalty function showed that the inversion was successful. The errors in the time-mean sea surface topography reduced from about 10 to about 3 cm. A cyclonic recirculation cell over the Agulhas Plateau was found, related to the northward meander of the Agulhas Return Current. Another cyclonic recirculation cell was found west of Africa, probably related to the passage of anticyclonic Agulhas Rings south of it. The new field is compared with advanced very high resolution radiometer infrared satellite data, confirming the northward meander of the Agulhas Return Current.

[1]  A. Gordon,et al.  Origins and variability of the Benguela Current , 1996 .

[2]  Lee-Lueng Fu,et al.  Sea level variabilities in the Gulf Stream between Cape Hatteras and 50°W: A Geosat study , 1990 .

[3]  Brian D. Beckley,et al.  Global mesoscale variability from collinear tracks of SEASAT altimeter data , 1983 .

[4]  K. Ichikawa,et al.  Life history of a cyclonic ring detached from the Kuroshio Extension as seen by the Geosat altimeter , 1994 .

[5]  W. R. Holland,et al.  Modelled time-dependent flow in the Agulhas retroflection region as deduced from altimeter data assimilation , 1991 .

[6]  Francis J. Lerch,et al.  Dynamic sea surface topography, gravity, and improved orbit accuracies from the direct evaluation of Seasat altimeter data , 1990 .

[7]  P. Visser.  The use of satellites in gravity field determination and model adjustment , 1992 .

[8]  P. Leeuwen,et al.  A new method to determine the mean sea surface dynamic topography from satellite altimeter observations , 1998 .

[9]  R. Nerem,et al.  Variations of global mesoscale eddy energy observed from Geosat , 1990 .

[10]  Sarah T. Gille,et al.  Gulf Stream surface transport and statistics at 69°W from the Geosat altimeter , 1990 .

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

[12]  M. Srokosz,et al.  Seasonal Variations in the Region of the Agulhas Retroflection: Studies with Geosat and FRAM , 1993 .

[13]  G. Evensen,et al.  Analysis Scheme in the Ensemble Kalman Filter , 1998 .

[14]  G. Evensen,et al.  Assimilation of Geosat altimeter data for the Agulhas current using the ensemble Kalman filter with , 1996 .

[15]  A. Gordon,et al.  Southern Ocean fronts from the Greenwich Meridian to Tasmania , 1996 .

[16]  Andrew F. Bennett,et al.  Inverse Methods in Physical Oceanography: Frontmatter , 1992 .

[17]  Southern ocean atlas , 1982 .

[18]  Byron D. Tapley,et al.  Determination of the ocean circulation using Geosat altimetry , 1990 .

[19]  Ralph F. Milliff,et al.  A modified capacitance matrix method to implement coastal boundaries in the Harvard open ocean model , 1990 .

[20]  A. Lorenc.  Optimal nonlinear objective analysis , 1988 .

[21]  A. Tarantola.  Inverse problem theory : methods for data fitting and model parameter estimation , 1987 .

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

[23]  C. Koblinsky,et al.  An error covariance model for sea surface topography and velocity derived from TOPEX/POSEIDON Altimetry , 1994 .

[24]  Chang-Kou Tai,et al.  Geosat crossover analysis in the tropical Pacific: 1. Constrained sinusoidal crossover adjustment , 1988 .

[25]  Michael J. Caruso,et al.  The mean sea surface height and geoid along the Geosat subtrack from Bermuda to Cape Cod , 1991 .

[26]  W. Teague,et al.  A Comparison Between the Generalized Digital Environmental Model and Levitus climatologies , 1990 .

[27]  Chester J. Koblinsky,et al.  Reflecting on the first three years of TOPEX/POSEIDON , 1996 .

[28]  J. Willebrand,et al.  Verification of Geosat sea surface topography in the Gulf Stream extension with surface drifting buoys and hydrographic measurements , 1990 .

[29]  A. Bennett.  Inverse Methods in Physical Oceanography , 1992 .

[30]  P. Visser,et al.  GEOSAT and ERS-1 radar altimetry over the North Atlantic , 1993 .

[31]  G. Evensen.  Sequential data assimilation with a nonlinear quasi‐geostrophic model using Monte Carlo methods to forecast error statistics , 1994 .

[32]  S. Drijfhout.  Ring Genesis and the Related Transports of Heat, Momentum and Vorticity: A Parameter Study , 1990 .

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

[34]  A. Gordon,et al.  Agulhas Eddies: A Synoptic View Using Geosat ERM Data , 1995 .

[35]  M. L. Gründlingh,et al.  Eddy fluxes of heat and salt from the southwest Indian Ocean into the southeast Atlantic Ocean : a case study , 1994 .

[36]  B. Qiu,et al.  Mean flow and variability in the Kuroshio Extension from Geosat altimetry data , 1991 .

[37]  G. Evensen,et al.  Data assimilation and inverse methods in terms of a probabilistic formulation , 1996 .

[38]  Sarah T. Gille,et al.  Mean sea surface height of the Antarctic circumpolar current from Geosat data : method and application , 1994 .

[39]  A. Gordon,et al.  Convective modifications of water masses in the Agulhas , 1992 .