Sea gliders to shed light on ocean currents off Algoa Bay

By Dr Wayne Goschen, Dr Marjolaine Krug and Dr Seb Swart Tweet

Two SOCCO-CSIR Sea gliders deployed off Port Elizabeth during the Shelf Agulhas Glider Experiment (SAGE) were successfully recovered on 29 May 2015. They had been out at sea for over a month.

The recovery was planned for the first week of June 2015, but with massive cold fronts approaching Port Elizabeth and heavy seas predicted, it was decided to pick them up early. The glider batteries were also running low so it was crucial that they were retrieved in time.

At the time of recovery, one glider was pushed far westward, well past Cape St Francis and the other far eastward off the mouth of Algoa Bay (Figure 1):

Figure 1: The path followed by the two gliders, shown by red and black dots. Their launch position is marked as squares near the shelf break. Bathymetry is shown in metres on the scale to the left.

Satellite SST (Sea Surface Temperature) images showed that a plume from the Agulhas Current may have been the cause. Ocean adjustments to the approaching weather system could also have had an influence.

This caused concern because the gliders had drifted apart many nautical miles and it looked like it would be a hard, full day out at sea to recover both. This turned out to be true, but the trip hit the weather window perfectly, with relatively calm seas and little wind, and although it was a long day, both gliders were retrieved.

From back to front: Dr Wayne Goschen, Jean-Pierre Smit, Mfundo Bizane and Dr Shaun Deyzel recover a glider Dr Shaun Deyzel (left) and Koos Smith (skipper) operate the CTD while Mfundo Bizane (centre) deploys a plankton net The scurry to take water samples with the two recovered gliders on deck

To calibrate the gliders, conductivity, temperature and depth (CTD) casts were made with the Elwandle Node’s SeaBird instrument and water samples were taken by its Nisken bottles to measure salinity, chlorophyll and dissolved oxygen (see picture 2). The chlorophyll samples were analysed by Nelson Mandela Metropolitan University that Friday night (thanks to Prof. Eileen Campbell) and the others transported back to Cape Town for analysis. The gliders were crated and successfully couriered back to Cape Town in time for their refurbishment and deployment in the Southern Ocean.

On board the SAIAB uKwabelana research vessel was Dr Shaun Deyzel (SAEON), PhD candidate Mfundo Bizane (SAEON), Jean-Pierre Smit (STS), Dr Wayne Goschen (SAEON) and skipper Koos Smith (SAIAB).

The South African Institute for Aquatic Biodiversity (SAIAB) is thanked for the boat and skipper, the Elwandle Node for assistance in the recovery, Sea Technology Services (STS) for piloting the gliders and guiding us to them and the University of Washington for their support (see picture 3).

This concludes the fieldwork phase of the project. The data will now be analysed to discover what the ocean was doing over the outer continental shelf off Algoa Bay during April/May 2015.