Interannual Flows along Australia's Western and Southern Coasts and along the Northern Coast of the Gulf of Mexico

The purpose of this dissertation is to study the interannual flows along the western and southern Australian coasts and along the northern coast of the Gulf of Mexico. Along the western and southern Australian coasts, sea levels are highly correlated with the El Ni˜no signal due to the leak in the gappy western equatorial Pacific Ocean boundary. Along the western Australian coast the coastline is nearly meridional and particle displacements near the coast undergo a change in Coriolis parameter. In order to keep the potential vorticity constant, this interannual coastal signal should propagate westward as Rossby waves with large zonal scale. TOPEX/Poseidon sea level data and coastal tide gauge measurements do show these large-scale waves off Australia's northwest coast. Along Australia's 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 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/sec) low frequency currents seaward of the shelf edge. On the shelf, inter annual flow is in the same direction as the shelf edge flow but much weaker. The anomalous flows tend to be eastward during La Ni˜na, when the western equatorial Pacific and Australian coastal sea levels are unusually high, and westward during El Ni˜no 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. Along the shelf edge south of 23oS of the western Australian coast, although the coastline is nearly meridional, high resolution satellite sea level estimates show that the inter annual sea level signal does not have the expected large spatial scale as it decreases rapidly seaward from the shelf edge. The drop in interannual sea level amplitude coincides with the mean position of the Leeuwin Current. Theory shows that a nearly meridional mean flow, as in the case of the Leeuwin Current, can induce this fall in interannual signal amplitude by altering the potential vorticity balance. The associated interannual shelf-edge flow tends to strengthen the Leeuwin Current during La Ni˜na, weaken it during El Ni˜no and may profoundly affect the recruitment of the western rock lobster. Past work has shown that the interannual wind stress curl in the North Atlantic generates Rossby waves that reach the eastern U.S. coast and affect coastal sea levels both there and along the northern coast of the Gulf of Mexico. Tide gauge and TOPEX/Poseidon satellite sea level height measurements show that this signal penetrates all the way around the Gulf shelf to the Yucatan Peninsula, local alongshore interannual wind stress increasing the signal amplitude between Pensacola and the Texas-Louisiana shelf. In accordance with theory, satellite observations show that the seaward spatial structure of the sea levels and the associated geostrophic flows depend on the angle of the coastline with respect to due north and the Loop Current mean shelf edge flow. Off the eastern boundary (small) formed by the west coast of Florida, the Loop Current distorts the potential vorticity balance and the sea level falls rapidly from the shelf edge with a scale of order the Loop Current width. Off the northern boundary ( 90o), the signal behaves as a coastal Kelvin wave, the sea level amplitude falling quickly away from the shelf edge with first baroclinic radius of deformation scale. Off the western boundary ( 180o), the interannual sea level amplitude falls rapidly seaward of the shelf edge consistent with short western boundary scales. Geostrophic shelf edge flow may reach amplitudes of order 10cms−1 but along shelf flow amplitudes are a few cm/s or less. Even so, weak shelf flows of low frequency can transport particles many hundreds of kilometers.