Coastline Direction, Interannual Flow, and the Strong El Niño Currents along Australia's Nearly Zonal Southern Coast

Abstract

The western equatorial Pacific Ocean El Niño signal leaks through the gappy western equatorial Pacific Ocean boundary to the western and southern coasts of Australia. Ocean Topography Experiment (TOPEX)/Poseidon sea level data and coastal tide gauge measurements show that off the northwest coast the low-frequency signal propagates westward as large-scale Rossby waves. However, along the nearly zonal southern coast, particle displacements are nearly zonal near the coast and experience no planetary vorticity change. As a consequence, the Rossby wave mechanism fails, and theory suggests that the signal should decay from the shelf edge with baroclinic Rossby radius-of-deformation scale. High-resolution along-track TOPEX/Poseidon sea level heights show that the interannual height signal does decay rapidly seaward of the shelf edge with this scale. The sharp fall in sea level and geostrophic balance imply strong (?10 cm s?1) low-frequency currents seaward of the shelf edge. On the shelf, interannual flow is in the same direction as the shelf-edge flow but is much weaker. The anomalous flows tend to be eastward during La Niña, when the western equatorial Pacific and Australian coastal sea levels are unusually high, and westward during El Niño when coastal sea levels tend to be anomalously low. The anomalous low-frequency flows can transport larvae large distances, enhancing the recruitment of Australian salmon to nursery grounds in the eastern part of the southern coast when the coastal sea level is higher than normal and decreasing recruitment when it is lower than normal.