Satellite Altimeter Data and the SA Agulhas' recent trip to Marion Is.

Sep 16, 2005
Author: Ian Hunter, South African Weather Services

As the onboard South African Weather Services’ staff will testify, the recent voyage to Marion Island on board the SA Agulhas was tempestuous to say the least. Although the Agulhas may - and has - run into major mid-latitude storms during the normal relief voyages (April to Marion and September to Gough), the in-between winter months are generally more severe.

With the rebuilding of the base at Marion Island, additional voyages have had to be fitted into the Agulhas' busy schedule. Inevitably, there had to be a shift into the winter months. Thus the Agulhas left for Marion on 16 August to offload workers and equipment.

By the following morning she was experiencing a NW'ly gale of 50 kts at a position roughly 60 nm south of Cape Agulhas (the land-based observation was NW 10 kts !). The south-westerly swell was steadily building. Conditions did not really improve until the afternoon of the 19th. In the intervening 48 hours, wind speeds of up to 53 knots were recorded and the swell was estimated to have reached 15m.

The vessel was 'hove to' - i.e. trying to simply maintain her position and reduce the (severe) swell-induced motion. Yet she moved over 150 nm to the south-west - i.e. upwind

The attached image, compiled from satellite altimeter data, simply identifies the long-term mean position of the warm rings which detach themselves from the main flow in the Agulhas Current retroflection zone (i.e. where the Current does an 'about-turn' to flow eastwards).

Both of the above phenomena may well have been related to the presence of one of these warm rings - i.e. the explosive cyclogenesis due to the significantly increased heat fluxes associated with the ring. And the south-westerly drift due to the circulation around the ring.

When the ship resumed her voyage on the afternoon of the 19th, the sea temperature was a warm 20°C (the Agulhas was already south of 40S). That night it dropped to 13.2°C as she crossed the sub-tropical convergence zone.