Second ASCA cruise a rollercoaster ride

By Jethan d’Hotman, ASCA Officer and Juliet Hermes, SAEON Egagasini Node Tweet

On 6 April 2016, the RV Algoa departed from Cape Town harbour for the second Agulhas System Climate Array (ASCA) cruise.

During the first ASCA cruise six moorings - four tall (A-D) and two short (C2 and C3) - were deployed into the Agulhas Current to begin the first long-term monitoring mooring array of the Agulhas Current’s volume, heat and salt transport and its variability from seasonal to interannual time scales (Figure 1).

One year later, the RV Algoa arrived at the mooring array to retrieve, service and redeploy these moorings. In addition, the ASCA array was extended further offshore, adding an additional three tall moorings (E – G) and five Current and Pressure Inverted Echo Sounders (CPIES; P1 – P5) to the array (Figure 1).

Additional objectives of the cruise were to conduct a high-resolution survey of Conductivity, Temperature and Depth (CTD) profiles; nutrient and biological samples were to be collected in conjunction with the CTD profiles, all of which complement the mooring array. The CTD survey was also to be run in conjunction with a Bongo net survey at and between each station.

Due to the vast amount of work to be done, the cruise was split into three legs - mooring recovery, service and deployment; CTD and Bongo net survey; and education (en route back to Cape Town).

Figure 1. A schematic of the ASCA mooring array through the Agulhas Current

Multi-national, multi-institutional crew

The crew on this cruise came from many different South African and international organisations, including SAEON, the Department of Environmental Affairs (DEA), Rhodes University, University of Cape Town and Cape Peninsula University of Technology. One of the chief scientists, Prof. Lisa Beal, is from the Rosenstiel School of Marine and Atmospheric Science (RSMAS) in the United States, while one of the lead technicians, Leon Wuis, hails from the Royal Netherlands Institute for Sea Research (NIOZ).

Once all gear (Figure 2) and scientific personnel (Figure 3) were on board, the RV Algoa departed from Cape Town harbour at noon on April 6, into flat seas. This was short lived, however, and by nightfall the sea state worsened, forcing many people to their beds with sea sickness. Luckily, by the time we arrived at the ASCA mooring line, the sea state had improved, allowing the work to begin.

Recovering the moorings

The first mooring to be recovered was Mooring B, which had an experimental MicroCAT tail allowing heat and salt data to be collected near the surface. During recovery we noticed this tail had broken off and two of the four MicroCATs were lost. Once the rest of the mooring was recovered, the instruments were serviced and the mooring was redeployed.

Figure 2. Deck of the RV Algoa showing all mooring equipment needed during the cruise (Picture: Kayleen McMonigal)

After a day or so of successful hard work, we arrived at the most offshore position, P5, where a CPIES was to be deployed. However, after deployment we lost communication with the CPIES, forcing us to classify the instrument as lost. A decision was made to sacrifice the CPIES at P2 and to deploy it at P5 to ensure we capture the Agulhas meanders.

The next few days were used to deploy the additional three tall moorings. These moorings are longer than any of the other moorings along the ASCA array (down to depths of around 4.5 km), creating a major challenge for the mooring technicians. These moorings, and the rest, were deployed without a hitch, thanks to favourable weather conditions, high levels of skill (from everyone on board) and great teamwork.

The recovery, servicing and deployment of these moorings were also used as an opportunity for skills sharing between the University of Miami’s mooring team, NIOZ and the South African mooring team (comprised of members from DEA, SAEON and the Bayworld Centre for Research and Education [BCRE]).

Figure 3. Leg 1 mooring team. From left: Kayleen McMonigal, Shane Elipot, Adam Houk,Cobi Christiansen, Lisa Beal, Mark Graham, Gavin Louw (front), Leon Wuis (back), Bradley Blows, Mfundo Lombi, Jethan d’Hotman and Mbulelo Makheta (Picture: Kayleen McMonigal)

Figure 4. Top floatation buoy of mooring F with Acoustic Doppler Current Profiler (Picture: Kayleen McMonigal)

Battered by inclement weather

Unfortunately, because this rollercoaster had been on a continuous upward path, the ride came crashing down shortly after the first leg on account of consecutive cold fronts (Figure 5) and some of the worst weather conditions the captain of the RV Algoa had ever seen, which eventually caused the second and third legs to be cancelled.

The RV Algoa forced its way back to Cape Town through a severe cold front, sometimes encountering waves of nine metres high, again sending many people to their beds with sea sickness. The RV Algoa arrived back in Cape Town on 1 May, ending the rollercoaster ride of the second ASCA cruise.

Fortunately, at the beginning of July 2016, there will be a dedicated cruise to the ASCA line where full-depth CTDs will be performed at each station. This will be in conjunction with the first ever SEAmester, South Africa’s floating marine classroom.

Figure 5. Synoptic chart (27 April 2016) that led to the decision to cancel the remainder of the ASCA cruise

ASCA would like to thank the captain and crew of the Algoa and the scientific and technical team who made this a truly memorable cruise.