Alongshore currents and mesoscale variability near the shelf edge off northwestern Australia

[1] Shelf break conditions and alongshore flow off northwestern Australia are studied during the strongly evaporative conditions of austral winter 2003. Present results, along with those of previous authors, confirm that a poleward, fresh Leeuwin current core is normally found near the shelf break. Salinity increases alongshore toward the southwest. Although there is no obvious shelf break front, there is a persistent offshore upward tilting of isopycnals in the depth range of 100 - 150 m. Repeated mesoscale surveys were made at the shelf edge during 8 days when the shelf break flow was equatorward. Waters offshore of the shelf break contain a rich baroclinic ageostrophic eddy field, with typically 10 km length scales, and the eddy patterns in the upper 60 m are uncorrelated with those below 90 m. The two depth horizons yield energy transfers from mean to eddy potential energy and appear to represent distinct finite amplitude instabilities on upper ocean and 180 m mean velocity cores, respectively. A linear stability model supports the existence of the two vertically separated instability types.