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10 April 2010

The US Research Vessel Knorr – the Agulhas Current – and SAWS

It is seldom that the South African Weather Service receives metocean (VOS) observations from a ship every 3 hours. Via its onboard AWS the RV Knorr is providing hourly observations of wind, pressure and SST (in addition to dry bulb and dew point). Early this morning for example, these observations provided proof of just how far offshore the land breeze can extend, as it undercut the large scale NE’ly circulation ahead of the incoming coastal low.

TheKnorr is owned by the US Navy but operated by the Woods Hole Oceanographic Institution, situated in Cape Cod (she was the vessel involved in finding the wreck of the Titanic)

The present project involves setting a line of current meter moorings across the Agulhas Current. This line extends in a southeasterly direction from Keiskamma Pt (~ Hamburg, Eastern Cape) – see reference map below. Note that the last set of measurements is located well into the deep ocean abyss.
Abnormal Wave Predictions
What has all this to do with the South African Weather Service (apart from the fact that it is getting valuable SHIP data) ?
• Firstly, one of the main metocean prediction parameters for shipping is sea state. The effect of the Agulhas Current on wave height (and steepness) off our east coast is notorious
• From a maritime safety angle, one of the events which SAWS would like to be able to predict as accurately as possible is the potential for ‘freak’ (abnormal/ rogue) waves. Thanks mainly to the expertise of the local marine salvage industry, damage to fully-laden tankers in the Current has not (yet) resulted in pollution on an extreme scale - to the adjacent east coast and downstream of the casualty
SAWS does not presently take into account the dynamics of the Agulhas Current when issuing an Abnormal Wave Warning. The data collected by WHOI will help to improve our understanding of how the Agulhas Current changes over time. For example the rapid offshore movement of the core, associated with the Natal Pulse, a cyclonic perturbation that propagates down the east coast at approximately 20 km per day. The NLOM oceanographic model run by the Naval Research Laboratory (NRL) in the United States actually indicates such a feature at the present moment, off our east coast :

The generation of abnormal waves in the Agulhas Current is not simply a function of long period SW’ly swell opposing the Current. There is also the issue of curvature in the Current – focusing wave energy, much as an optical lens will do with light. The diagram below paints the general picture :
I.E one needs to have a semi-realtime picture of the Agulhas Current (AND the full energy spectrum of the waves entering the Current)

The above Very Large Crude (oil) Carrier VLCC Atlas Pride had to transship its cargo to another tanker – at sea..

Ian Hunter – SAWS– 09-04-10