Capturing the Transport Variability of a Western Boundary Jet: Results from the Agulhas Current Time-Series Experiment (ACT)

摘要

The volume transport of the Agulhas Current was measured over a 3-yr period by an array of seven current meter moorings and four current-and pressure-recording inverted echo sounders (CPIES) deployed at 34 degrees S. CPIES extended the array farther offshore in order to capture, for the first time, the full Agulhas Current during meander events. Transports derived fromCPIES are well correlated with overlapping current meter transports (0.89). The Eulerian mean current is 219 km wide and 3000m deep, with peak surface speeds of 1.8ms 21 and a weak northward undercurrent on the continental slope below 1200m. A new algorithm to capture the western boundary jet transport at each time step T is defined as the poleward transport out to the first maximum of the vertically integrated velocity beyond the half-width of the mean jet. The mean transport of the Agulhas Current jet, so defined, is -84 Sverdrups (Sv; 1 Sv equivalent to 10(6)m(3)s(-1)) with a standard error of 2 Sv. Sampling and instrumental errors are explicitly estimated and amount to an additional 9 Sv. A more traditional estimate, based on net transport integrated to a fixed distance offshore T-box, gives a mean transport of -77 +/- 5 Sv. This transport is 10 Sv greater than an equivalent transport at 32 degrees S, corresponding to a latitudinal increase equal to that predicted by Sverdrup dynamics. The time series of T and Tbox show important differences during solitary me ander events and at longer time scales. In terms of an annual cycle, the Agulhas Current appears strongest during austral summer, a similar phase to the Gulf Stream and Kuroshio.