PS113 - Weekly Report No. 2 | 14th-20th May (Whit Sunday) 2018

Whitsun Excursion with a Towed System

[23. May 2018] Since crossing the Tropic of Capricorn at 23° 26’ 05” South on Thursday this week travelling north we find ourselves, according to the astronomical definition, in the Tropics. The weather is what you would expect; water and air temperature lie between 26 and 27°C. Since we have also arrived in the zone of the SE Trade Winds there is a stiff and steady breeze and by day the sun shines from a sky with few clouds into deep blue ocean water.

Regarding our scientific programme, since leaving the Economic Zones of Argentina and the UK around the Falkland Islands, a regular daily routine has been established. Apart from the measurements, which are made continuously or at regular intervals around the clock, we stop every day around midday to make casts with a package of optical sensors (of which more next week) - and with the CTD-Sonde with its rosette sampler for water bottles, with which, apart from measurements of temperature, salinity, oxygen concentration, phytoplankton fluorescence and optical transmissivity, water samples for various investigations can be taken. Midday was chosen for the sake of the optical measurements because then, when the sun is at its highest, the light penetrates deepest into the ocean.

Fig. 1: Making the mechanical, electrical and fibre optic connection between the towing cable and Triaxus. (Photo: Alfred-Wegener-Institut)

The measurements with the optical sensors and the CTD are principally required for comparison with the instruments and sensors on the towed measurement system Triaxus/topAWI (towed ocean profiler of the AWI) which was procured last year and during this cruise is now being deployed for the first time. Additionally Triaxus/topAWI is equipped with pH and nitrate sensors, high-frequency acoustic Doppler current profilers (ADCPs) for measuring turbulent current shears and also a multi-frequency echo-sounder for detecting zooplankton. There are more sensors for the positioning and safety of the towed body. These include an acoustic transponder, which allows the position relative to the ship to be determined, upward and downward-looking altimeters which can measure the distance to the sea surface and the sea bed, and also a small radio transmitter, which can be located from the ship.

Fig. 2: Ready for deployment. (Photo: Alfred-Wegener-Institut)

The programme of work associated with the commissioning of Triaxus/topAWI requires many and various activities. These began in the harbour in Punta Arenas with the correct positioning of the towing winch on Polarstern’s afterdeck. So that the towing wire was oriented at the correct angles from the winch and to the stern A-frame a constraining block was mounted between the afterdeck and the helicopter deck above. In addition, Triaxus was first put in the water there to allow the internal buoyancy elements to be adjusted for balance. This was so that in the unlikely, and hopefully never occurring, event of a cable break Triaxus would be positively buoyant and rise to the surface.

Fig. 3: Triaxus being launched. (Photo: Alfred-Wegener-Institut)

Deployments during the cruise are used to determine, amongst other things, which depth range can be profiled for a given length of towing cable and towing speed. The greatest depth is currently limited to 350 m due to the maximum pressure rating of some of the components. In addition the depth range is also influenced by the climb and dive speeds chosen. An important feature of the system is that Triaxus can be made to shear out sideways to enable sampling of water undisturbed by the ship’s wake right up to the surface. How far and under which conditions need to be determined using Triaxus’ acoustic transponder and an array of hydrophones mounted under Polarstern’s hull.

Fig. 4: Launched smoothly! Now the towing and measurement can begin. (Photo: Alfred-Wegener-Institut)

Another important element of this initial deployment is to make colleagues from a variety of disciplines familiar with the system and its deployment at sea, as well as optimising the practices and procedures for its use. To this end a number of technical problems with which we were faced were instructive, if troublesome. This included an underwater connector which had ceased to be watertight. Given around sixty connections, some hidden in the fuselages and wings, finding this proved time consuming but revealed a number of undocumented details of the internal structure.

Fig. 5: Triaxus control centre in the winch control room on Polarstern. (Photo: Alfred-Wegener-Institut)

This new towed system will make it possible for many physical, chemical and biological parameters to be measured simultaneously and quickly with high spatial and temporal resolution and with good spatial coverage. In this way those processes can be identified which influence phytoplankton photosynthesis and primary productivity as well as CO2 uptake. Primary productivity also forms the basis of the marine food chain and ecosystem and thus determines even fishery catches. An improved understanding of the physical processes is a precondition for knowing how developments within the physical environment due to climate change will influence biological, biogeochemical and economic outcomes.

Despite the hard work, including over the Ascension and Whitsun holiday weekends, all cruise participants are in good spirits.

Many greetings from on Board Polarstern,

Volker Strass