Flux comparison of Eulerian and Lagrangian estimates of Agulhas leakage: A case study using a numerical model

• Published in: Deep-sea research. Part I, Oceanographic research papers Vol. 57; no. 3; pp. 319 - 327
• Main Authors: van Sebille, Erik, van Leeuwen, Peter Jan, Biastoch, Arne, de Ruijter, Wilhelmus P.M.
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier Ltd 01.03.2010; Elsevier; Pergamon Press Inc
• Subjects: Agulhas leakage; Atlantic Ocean; Confidence intervals; Dynamics of the ocean (upper and deep oceans); Earth, ocean, space; Eddy formation; Estimates; Estimating; Eulers equations; Exact sciences and technology; External geophysics; Flux; Interocean exchange; Leakage; Marine; Mathematical models; Model validation; Mooring array design; Numerical analysis; Numerical lagrangian drifters; Ocean circulation; Oceanography; Oceans; Physics of the oceans; Studies; Thresholds; South Atlantic; Indian Ocean; Southeast Atlantic; Cape Basin; Atlantic Ocean; Agulhas current; ISW, West Indian Ocean, Agulhas Current; ISW, Indian Ocean; ASE, Atlantic, Cape Basin; Agulhas leakage; Interocean exchange; Mooring array design; Model validation; Eddy formation; Numerical lagrangian drifters; Euler equation; Observational network; Mooring; Marine current volume; digital simulation; Lagrange equation; Nested model; Drifting float; Exchange flow; Observation data; Computing method; high resolution
• ISSN: 0967-0637; 1879-0119
• DOI: 10.1016/j.dsr.2009.12.006

Estimating the magnitude of Agulhas leakage, the volume flux of water from the Indian to the Atlantic Ocean, is difficult because of the presence of other circulation systems in the Agulhas region. Indian Ocean water in the Atlantic Ocean is vigorously mixed and diluted in the Cape Basin. Eulerian integration methods, where the velocity field perpendicular to a section is integrated to yield a flux, have to be calibrated so that only the flux by Agulhas leakage is sampled. Two Eulerian methods for estimating the magnitude of Agulhas leakage are tested within a high-resolution two-way nested model with the goal to devise a mooring-based measurement strategy. At the GoodHope line, a section halfway through the Cape Basin, the integrated velocity perpendicular to that line is compared to the magnitude of Agulhas leakage as determined from the transport carried by numerical Lagrangian floats. In the first method, integration is limited to the flux of water warmer and more saline than specific threshold values. These threshold values are determined by maximizing the correlation with the float-determined time series. By using the threshold values, approximately half of the leakage can directly be measured. The total amount of Agulhas leakage can be estimated using a linear regression, within a 90% confidence band of 12 Sv. In the second method, a subregion of the GoodHope line is sought so that integration over that subregion yields an Eulerian flux as close to the float-determined leakage as possible. It appears that when integration is limited within the model to the upper 300 m of the water column within 900 km of the African coast the time series have the smallest root-mean-square difference. This method yields a root-mean-square error of only 5.2 Sv but the 90% confidence band of the estimate is 20 Sv. It is concluded that the optimum thermohaline threshold method leads to more accurate estimates even though the directly measured transport is a factor of two lower than the actual magnitude of Agulhas leakage in this model.