The mean state of the Leeuwin Current system between North West Cape and Cape Leeuwin

摘要

The circulation and water mass characteristics offshore Western Australia between North West Cape and Cape Leeuwin (21-35° S) are described using findings from past studies as well as additional field data. The circulation pattern in the study region is dominated by the Leeuwin current system, which includes three main currents: the Leeuwin current (LC), the Leeuwin undercurrent (LU), and shelf current systems consisting of the Capes and Ningaloo currents. Localised discharges of fresh water and high salinity water were present in the shelf regions offshore Perth and Shark Bay, respectively. Localised upwelling also affected the water masses in the Geelvink Channel. Eight water masses were identified as interleaving layers of salinity and dissolved oxygen concentrations: (i) low salinity tropical surface water (TSW); (ii) high salinity south Indian central water (SICW); (iii) high dissolved oxygen subantarctic mode water (SAMW); (iv) low salinity Antarctic intermediate water (AAIW); (v) low oxygen north-west Indian intermediate (NWII) water; (vi) low oxygen upper circumpolar deep water (UCDW); (vii) high salinity low circumpolar deep water (LCDW); and (viii) cold (< 2 °C), high oxygen Antarctic bottom water (AABW). The LC consisted of tropical surface water and south Indian central water. In the northern region of the study area (between North West Cape and the Abrolhos Islands), SICW was present as a subsurface salinity maximum beneath the TSW. Between 28 and 29.5° S, the depth of the 35.7 salinity contour decreased from -150 m to the surface. South of 29.5° S, SICW was present at the surface. Along the continental slope, the LU was present beneath the LC at depths between 300 and 800 m, with its core (lowest oxygen level) at 450 m. The LU transported SAMW north while the wind drove the continental shelf currents. Southerly, upwelling-favourable winds pushed the , LC farther offshore (a process known as Ekman transport), which forced cold, upwelling water onto the continental shelf. The Ningaloo current mainly consisted of TSW and extended north from Shark Bay to beyond North West Cape; the Capes current consisted of SICW and was present as a continuous current system on the continental shelf, extending north from Cape Leeuwin to Geraldton- a distance of some 750 km.