Hydrography and water masses off the Western Australian coast

The water mass characteristics of the eastern Indian Ocean margin between latitudes 21 degrees and 35 degrees S, adjacent to the Western Australian coast, are described using field measurements. The results indicated the presence of five different water masses as interleaving layers of salinity and dissolved oxygen concentrations in the upper 1000 m of the ocean. These water masses included (i) lower salinity tropical surface water (TSW), (ii) higher salinity south Indian central water (SICW), (iii) higher dissolved oxygen subantarctic mode water (SAMW), (iv) lower salinity Antarctic intermediate water (AAIW), and (v) lower oxygen north-west Indian intermediate (NWII) water. Data collected in 2000 and 2003 were compared with historical data (1987), and interannual variability in the tropical surface water and subantarctic mode water masses were identified and linked to El Nino-southern oscillation (ENSO) events. In the study region, the circulation pattern, known as the Leeuwin current system, consists of three major currents: the Leeuwin current (LC), the Leeuwin undercurrent (LU), and shelf current systems consisting of the Capes and Ningaloo currents. In the study area's northern region, geopotential gradients were found driving the Leeuwin current (LC) and Leeuwin undercurrent (LU), with a (negative) sea surface slope of 4 x 10(-7) driving the LC poleward, and a (positive) slope of 1 x 10(-7) beneath the LC driving the LU equatorward. Cross-shelf geopotential anomalies revealed the surface layer (in the upper 300 m) sloped seaward at a gradient of 1.7 x 10(-6), and the subsurface layer (between depths of 300 and 730 m) sloped coastward at a gradient of 6.3 x 10(-7). This arrangement of geopotential slopes, together with the positions of the LU relative to the LC, indicated a dynamical relationship between the LU and the LC. The data indicated the LU's core was located at a depth of 400m and it transported subantarctic mode water (SAMW) equatorward. The presence of the Leeuwin current and Leeuwin undercurrent at the continental shelf break and slope, respectively, influenced the water mass distribution at the continental margin. Leeuwin Current induced downwelling caused the surface and subsurface water mass (SICW) and the SAMW's upper edge to slope downward toward the shelf break while subsurface upwelling beneath the LU moved the Antarctic intermediate water (AAIW) and the SAMW's bottom edge higher in the water column. (C) 2008 Elsevier Ltd. All rights reserved.