Eddy-edge baroclinic instability, offshore from the Leeuwin Current

J.S. Godfrey, R. Fiedler and G.R. Cresswell
IMAS Sandy Bay seminar room
Wednesday 14th December, 12:00 – 12:50

Abstract:
In the 1970’s, the track of a satellite-tracked buoy turned a sharp corner on leaving an anticyclonic eddy. The buoy moved rapidly for 400 km towards the Western Australian coast, and joined the Leeuwin Current. Flow speeds – up to 0.9 m/sec – were greatest about 400 km from shore. Thus the Leeuwin Current can gain its kinetic energy, as well as its volume transport, from such large distances offshore. Modern, high-resolution ocean models now spontaneously generate such phenomena. In the Ocean Forecast Australia Model (OFAM), such rapid onshore flows originate from baroclinic instability around the edges of large eddies. In a particular OFAM example, bands of warm shelf-edge water are carried rapidly southwestward from 27°S along the northwest edge of a large eddy. This flow turns eastward along the south side of the eddy near 30°S, about 500 km west and 300 km south of its starting point. Lines of upward heat/buoyancy transfer lie along the warm side of the SST front accompanying this jetlike zonal flow, releasing kinetic energy into it and accelerating it. Diurnal variations in convective overturn and vertical heat diffusion within the deep surface mixed layer appear to play a strong role in the instability mechanism, provisionally referred to as “Convectively-Assisted Baroclinic Instability” (CABI). A sharp SST cusp also forms in this jet. Cold water, accelerated along the south edge of the eddy, sinks beneath the cusp, displacing warmer water upwards. CABI develops to the north and east of the SST cusp, accelerating the eastward jet towards the Western Australian shelf.

The talk will close by displaying and discussing the evolution (in OFAM) of a “striation”, from its April 2004 origins in a kink in the Leeuwin Current at 30°S, to a jet at least 600 km long at that latitude about four months later.

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