Monday 22 August 2005

P6
1000-1230 hours

045
Interannual coastal flow and examples of its influence on Australian marine populations
Clarke, Allan¹, Li, JiAnke¹
1 Florida State University, USA
Author email: clarke@ocean.fsu.edu
The western equatorial Pacific El Nio signal leaks through the gappy western equatorial Pacific Ocean boundary to the western and southern coasts of Australia. Satellite and coastal sea level data 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. Consequently 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 satellite 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 (approximately 10 cm/sec) low frequency currents seaward of the shelf edge. On the shelf, interannual flow is in the same direction as the shelf edge flow but much weaker. The anomalous flows tend to be eastward during La Nia, when the western equatorial Pacific and Australian coastal sea levels are unusually high, and westward during El Nio 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. Off Australia's southwest coast satellite sea level estimates suggest that the Leeuwin Current strongly affects both the interannual shelf edge flow and the interannual flow toward the shelf edge. This appears to affect the number of lobster larvae reaching the shelf edge from the deep sea and hence the number that are able to cross the shelf and settle in the shallow coastal reefs.

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