The Cut-off Low of 17-21 March 2007 off the East Coast of South Africa

Ian Hunter, South African Weather Service

Meteosat-8

Figure 1. Meteosat-8 visible image - taken at 0600 UTC on 19 March, 2007.
(Click the image to enlarge.)

Cut-off lows are one of the most important synoptic-scale weather systems when it comes to the generation of heavy seas off the South African east coast. A cut-off low developed rapidly on Sunday 18 March 2007 whilst tracking in a north-easterly direction between Port Elizabeth and Durban. Figure 1 shows the vortex, approximately 300 nm south-east of Durban on 19 March. Note how little movement there is between Sunday 1800 UTC and Tuesday 1800 UTC (Figure 2). Winds on the south-western and western side of the low were averaging over 60 kts (Figure 3).


Meteosat-8

Figure 2. Track of cut-off low with Meteosat 8 images superimposed.
(Click the image to enlarge.)

Fortunately, the only significant event at sea was a relatively minor collision between two bulkers off Richards Bay on Sunday 18 March. WSW'ly winds on this coast averaged over 40 kts throughout Sunday, with gusts to over 60 kts. By early Monday morning the wind was slowly easing, but significant wave height peaked at over 8m along the coast. This was the highest since February 1984 when Tropical Cyclone 'Imboa' moved unusually far to the south-west after exiting the Mozambique Channel.


 

QuikSCAT's radar scatterometer

Figure 3. Sea surface wind field derived from QuikSCAT's radar scatterometer - evening passes on 18 March, 2007.
(Click the image to enlarge.)

 

 

With the broad and intense fetch area remaining essentially stationary for 48 hours, wave generation was considerably enhanced, as can be seen in the wave height analysis, courtesy ECMWF (Figure 4). Apart from extensive damage to coastal infrastructure (amounting to over $100m), the maritime industry also incurred significant losses. The port of Richards Bay suffered damage due to the heavy swell and unusually high water levels (equinoctial spring high + wind set-up + wave set-up + wave run-up). But there was also a considerable loss in revenue as a result of the closure of all 3 ports – East London, Durban and Richards Bay - for 2 days. There were 14 container ships waiting outside Durban by the time it was reopened on 22 March.

Analysis from the ECMWF global wave model

Figure 4. Analysis from the ECMWF global wave model - for 0000 UTC on 19 March, 2007.
(Click the image to enlarge.)

Finally, this seems a good opportunity to thank all mariners that provide Voluntary Observing Ship reports (VOS). MV ‘Serenata’, bound for South America, provided regular observations as she tracked close to the centre of the vortex. Their barometric pressures were very useful for the sea level pressure analysis of the cut-off low, and their wind observations helped to verify the satellite-derived sea surface winds from QuikSCAT. The master was even kind enough to comply with a request to send some photos of sea conditions!