Cape of Storms - August 2005

Oct 4, 2005
Author: Ian Hunter
Principal Researcher, South African Weather Service

During the period 18-21 August 2005 a very intense vortex was located south-east of South Georgia in the Southern Ocean, with the pressure recorded by two drifting buoys dropping below 940 hPa. The fact that the low was semi-stationary increased the duration factor associated with the wave generation and in the next 4-5 days these waves traveled almost 3000 n.miles to reach the Cape by Friday evening, 26 August. They arrived in the form of a long-period swell with peak energy period (Tp) in excess of 18s (ex Slangkop Waverider buoy, offshore Cape Town).

To complicate matters another deep vortex formed in the interim, much closer to Cape Town on 24 August. This second low was associated with an upper cut-off low and thus also moved very slowly relative to the speed of a normal mid-latitude frontal trough. Thus in this case the duration factor for wave generation was also increased. A rough calculation indicates that both wave components would have arrived off Cape Town at approximately the same time. The result was a significant wave height of 10m (33 ft) - by far the highest since the start of the winter - at around 8 pm.

It is a well-known phenomenon in False Bay (on the eastern side of the Cape Peninsula) for long-period swell to be refracted and focused onto certain parts of the coastline. This is particularly the case at the small fishing harbour of Kalk Bay. The harbour master had the foresight to send the trawlers along to the safer haven of Simonís Town. However two tourists inexplicably decided to take a stroll along the breakwater and both ended up in the sea. Fortunately both were rescued.

Despite being on a major trade route, VOS reports off the Southern African coast tend to be very sparse at times. Thus in the preamble to every Coastal and Deep Seas Bulletin, the South African Weather Service calls upon vessels to report unexpected, extreme conditions via Cape Town Radio. This has had a very good response. The tanker Theano (SZTD) sent through a plain text report early on Saturday morning from a position just west of Cape Agulhas - wind north-west 60 to 70 kts, swell (estimated) up to 10m (33 ft). Compare this with the 0600 UTC land-based observation from Cape Agulhas - north-west 10 kts! - and the value of offshore wind observations should become abundantly clear, even to the layman.

At 12.00 UTC on Saturday 27 August a formal VOS report was received from the container vessel Safmarine Agulhas (ELSM9) just south of Cape St Francis. Swell was estimated at 12m (40 ft) with a 25s period. This period does appear to be a bit high, but swell period is generally estimated too low, so the observer was at least witnessing an unusually long swell component. Furthermore the swell had traveled a long distance (high frequencies lost) and (accelerometer-based) Waverider buoys are unable to measure swells longer than ~ 20 sec accurately.

At the FA gas production platform some 120 nm to the east, the wave height peaked at around 10 m during the night. Unfortunately the radar sensor was giving spurious values.

Surprisingly, no major problems were reported by vessels at sea during this storm. The previous week there were three crew from two different ships crossing the Agulhas Bank who had to be helivac'd to shore following injuries relating to an earlier storm. Yet the associated sea conditions were by no means as extreme as those described above. Sadly, one of the two vessels also lost a crew member overboard.

Further up the coast, just to the south of East London, the ongoing attempt to salvage the cargo of West African timber from the stranded Kiperousa was also adversely affected by the heavy swell moving up the coast. She had already been declared a total loss following several attempts to refloat her after she grounded on 7 June. The long-period wave component peaked at 5 am on Sunday 28 August.

Note 1. In October 1999 a log carrier, the Sanaga sank south of Madagascar spilling her cargo of huge 20 ton logs into the ocean. These logs presented a major navigation hazard to smaller vessels as they floated down the east coast, beaching at various places all the way between Inhambane (Mozambique) and False Bay (over 1200 nm)

Note 2. SA Weather Service is grateful to the National Ports Authority of South Africa for making their Cape Town Waverider data available via Environmentek (CSIR)

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