Seasonal Climatology of the Leeuwin Current-Capes Current System Off Southwest Australia From Long-Term Moored Observations

Abstract

In this study, we have analyzed 13+ years of moored observations of ocean current, temperature, and salinity on the Rottnest Shelf off southwest Australia, to describe the characteristics of the Leeuwin Current, the Capes Current, and associated seasonal variability. The observations capture the Leeuwin Current component anchored on the continental shelf and slope, which extends to ∼300 m, being stronger in the austral winter (peak in June) and weaker in summer (December/January), with an estimated annual mean, poleward volume transport of ∼1.8 Sv (10⁶ m³s⁻¹). The estimated mean transport is 2.4 Sv when adjusted with a data assimilating model. The variations of the Leeuwin Current are closely associated with thermocline depth variability at the shelf break, with seasonal maximum subsurface temperatures leading the annual peak of the Leeuwin Current by 1–2 months. The 500 m mooring on the continental slope captures the Leeuwin Undercurrent in the 300–400 m depth range. The offshore volume flux in this depth range, likely linked to the offshore bottom Ekman flows on the shelf, is rather weak at this latitude. The Capes Current flows northward, mostly confined in the middle shelf (20–50 m), with a mean equatorward transport of 0.07 Sv from November to March, forced by strong southerly winds. The cross-shelf flows associated with the Capes Current are weak on the middle shelf and appear strong off Rottnest Island on the edge between the outer and the middle shelf, where active upwelling tends to occur in the summer months.