Hobart

Seminar Abstract

Friday 10 September 2010, 11.30am (Tas time)
CSIRO Auditorium, Hobart

George Cresswell
Honorary Fellow
CSIRO Marine and Atmospheric Research
Hobart

Upwellings, downwellings, wind forcing, and the East Australian Current, EAC, near and downstream from easternmost Australia in summer 1989/90

In 1911 Gerald Harnett Halligan, NSW Hydrographer and keen student of currents along the NSW coast – and elsewhere –said that in February, 1908, he “successfully measured the current velocity off the entrance of Newcastle harbor in a hard southerly gale which had been blowing almost continuously for 5 days. The wind velocity at the time was 60 miles per hour, the height of the waves from trough to crest was 27 feet, and the observations were made at the low-water slack. The surface velocity during calm weather was 1 ¼ knot per hour to the south, but during this gale the speed was 0.9 knot to the north. Two days after the gale had abated, the current resumed its normal speed and direction. Up to the present, no periodic observations during a southerly gale have been taken, but such data would be interesting and valuable, as showing the retarding effect of wind upon flowing streams. The difficulties in the way are not insurmountable, but the work is by no means pleasant.” Well, with a well-found ship, Franklin, enthusiastic colleagues and crew, and a large suite of instrumentation – and a lot of luck in capturing events – we’ve taken a step towards assembling the sort of data that Halligan would have liked… and more. We used a score of instrument systems off eastern Australia in 1989/90. These included shipboard CTD/rosette, ADCP, towed Seasoar, XBT, 6 moored current meters, 7 satellite-tracked drifters, a tide gauge, the NCEP wind stress archive, and the AVHRR satellite radiometer. This resulted in over fifty data streams: for a long time overwhelming and frustrating, but ultimately rewarding. Upwelling occurred when the EAC was flowing strongly at the same time as the wind stress was near-zero or upwelling-favourable (northerly). Southerly winds drove downwelling and northward nearshore currents. The southerlies also appeared to retard the southward flow of the EAC above both the shelf and upper continental slope. While switching from downwelling to upwelling over several days near Evans Head, the 20°C isotherm (a convenient upper marker of cool, salty, nutrient-rich, oxygen-poor waters) moved 12 km shoreward across the shelf, as well as upward from 30 m depth to the surface. The move was actually at a very slight angle to the bottom topography and the water was from upstream on the shelf rather than from the continental slope. The EAC in late November 1989 had a subsurface maximum at 100-150 m above the slope with speeds up to 1.7 ms-1. During the ship survey the EAC accelerated over several days near Cape Byron and Evans Head and it also accelerated as it flowed southward from Pt Lookout to Evans Head. Satellite thermal imagery and drifters (7 released along a 30 km-long line out from Cape Byron) showed how small meanders on the edge of the EAC evolved into submesoscale cyclonic eddies. The drifters furthermore followed submesoscale anticyclonic eddies on the edge of a large anticyclonic mesoscale eddy. One drifter appeared to follow the Tasman Front out to 164°E.

Co-authors: Jan Peterson and Lindsay Pender.

Seminar recording

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Location:
CSIRO = Marine Laboratories Auditorium, Castray Esplanade, Hobart

For further information, or to schedule a seminar, contact:
Andrew Meijers, (Oceanographic seminars) CSIRO Marine and Atmospheric Research (03) 6232 5335
Natalie Kelly, (Biology/Modelling seminars) CSIRO Marine and Atmospheric Research 0438 452 483
Jillian Enraght-Moony, (seminar administrator) CSIRO Marine and Atmospheric Research (03) 6232 5320
Communications Manager, Antarctic Climate and Ecosystems CRC (03) 6226 7888
Tracey Cochrane, Institute for Marine and Antarctic Studies (IMAS), University of Tasmania (03) 6226 2937

Last updated 17/09/10