Characteristics of two counter-rotating eddies in the Leeuwin Current system off the Western Australian coast

Abstract

Abstract A multi-disciplinary R.V. Southern Surveyor cruise was conducted in October 2003 to quantify upper ocean productivity within two adjacent, counter-rotating mesoscale eddies (an eddy pair off the coast of Western Australia (WA)) in the southeast Indian Ocean. In this study, a combination of satellite data (altimeter, sea surface temperature, and chlorophyll a) and shipboard measurements (acoustic Doppler current profiler (ADCP) and conductivity-temperature-depth (CTD)) were used to characterize the temporal evolution and spatial structures of the eddy pair. Satellite data show that the eddy pair evolved from meander structures of the poleward-flowing Leeuwin Current (LC) in May 2003 and fully detached from the current in late August–early September. Gaussian fits to the ADCP velocities show that the anticyclonic eddy had a maximum azimuthal velocity of 65 cm s−1 at 63 km from the eddy centre (the apparent radius), and the cyclonic eddy had a maximum azimuthal velocity of 60 cm s−1 at the apparent radius of 49 km. The mixed-layer temperature and salinity properties in the cores of the eddy pair were quite stable over the period of the cruise. Temperature–salinity diagrams indicate that the mixed-layer water in the core of the anticyclonic eddy was composed mainly of LC water, while in the core of the cyclonic eddy the mixed layer water was similar to surrounding open-ocean water. The total volume of the LC water retained in the anticyclonic eddy was 5.6×1012 m3, equivalent to 19 days’ annual mean transport of the LC. The chlorophyll a concentration in the surface layer of the anticyclonic eddy (LC water) was higher than in the cyclonic eddy (open-ocean water), which was likely a result of the entrainment of the productive water from the continental shelf during the eddy formation stage. The offshore transport of the chlorophyll a biomass may be important for the pelagic ecosystem off the coast of WA.