Weather conditions during summer are characterised by long periods during which the atmospheric anticyclone is stationary over the Bight (Figure 5). The associated winds are favourable for coastal upwelling along the entire coastline and cannot explain the observed intensification of coastal upwelling in the east.
The situation was modelled with a stationary anticyclone (Figure 6) which in shallow water of uniform depth produces Ekman layers at the surface and at the bottom:
In a basin with uniform depth, flow in the surface Ekman layer converges towards the centre of the anticyclone, flow in the bottom Ekman layer diverges from the centre and produces upwelling around the edges.
The depth H of the Great Australian Bight increases slowly from the head to the open ocean, producing a nearly uniform negative bottom slope (dH/dy < 0). In the southern hemisphere (f / H < 0, f: Coriolis parameter) conservation of potential vorticity then requires (Pedlosky, 1987) that the current is intensified on the eastern side of the Bight, as was found in the velocity field derived from the model (Figure 7).
When the same atmospheric anticyclone was applied to a model bight with uniform depth the velocity field followed the coast with uniform strength around the bight; the existence of a shallow region in the west affected this pattern only slightly (Figure 8).
The process responsible for the observed coastal upwelling along the eastern coast of the Bight and its absence along the central and western coast is thus the mismatch between the atmospheric anticyclone and the resulting anticyclonic oceanic circulation in the Bight. The surface Ekman layer is directly wind forced and reflects therefore the symmetrical shape of the wind field, but the bottom Ekman layer is the result of the oceanic circulation and therefore has a much stronger transport in the east than in the west.
© 1998 M. Tomczak
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