PS124 – Weekly Report No. 8 | 22 - 28 March 2021

Our long-term ocean observatories

[29. March 2021] 

The southern Weddell Sea is home of the Filchner-Ronne Ice Shelf (FRIS). FRIS is the largest (by volume) floating ice shelf on the planet and therefore quite an important feature in the Antarctic Ice Sheet system, which is directly relevant for our global climate and sea level.

As mentioned in previous reports, one of the PS124-focii lies on the ocean circulation of the region, which features several important aspects. First of all, we would like to understand how, where, and why any warm water currents from the deep Weddell Sea get close to FRIS, as warm water leads to enhanced ice shelf melting.

 

FRIS melt rates are still moderate, and in order to better predict future scenarios at FRIS, we aim to understand the dominant drivers of these warm water inflows. Earlier posts already brought up that water sampling and shipboard measurements are important techniques to measure and analyze the various parameters of the ocean and the seafloor. However, similar to changing weather patterns, ocean currents are also variable, although generally on long time scales compared with the weather. Therefore, in order to capture this variability and understand potential forcing mechanisms that influence currents and water masses, it is important to measure these parameters over periods of up to several years. This is commonly done with moored ocean observatories, simply referred to as moorings.

A mooring consists of a long mooring line with measuring devices attached to record parameters such as ocean salinity, temperature or currents (see Fig. 1). At the top of the line, we attach large floats (Fig. 2), which want to rise straight up to the surface. However, the floats and the line are kept in place by a bottom anchor, which typically consists of old train wheels weighing up to a metric ton. Such a long sturdy line stands on the seafloor and extends from few hundred meters to sometimes kilometers long into the water column, depending on its purpose and the water depth.

The mooring line is attached to the anchor via an acoustic release (Fig. 1). Upon receipt of an acoustic command from the ship, the hook releases and detaches from the bottom weight. Then, the floats make the whole package rise to the surface, where the folks on deck are eagerly waiting for the recovery. The moment before a mooring breaks through the surface is often exciting, and many pairs of eyes are needed on the bridge to spot the mooring, especially in rougher weather or in ice covered waters. The first sighting is generally awarded a bottle of sparkling wine, a little extra motivation to clean the glasses and pay attention! Most of the moorings we recovered were in the water for 3 or 4 years, and upon successful recovery a wealth of valuable data are downloaded from the instruments. During PS124 we recovered 16 and deployed 22 new moorings as a collaboration between AWI and our French and Norwegian partners, who are on board with us.

Mooring operations require an entire suite of people on deck, including the chief mate, the bosun and his deck’s crew, working closely together with several members from the scientific party and, most importantly, the officers on the bridge who navigate the ship (Fig. 3). The mooring operations during this expedition provided an excellent example of what can be accomplished with teamwork. Polarstern and her captain and crew have carried out mooring work in Polar Oceans for decades now and is among the most experienced vessels in the world to do this kind of operation.

The 22 moorings we deployed during PS124 are distributed in distinct focus regions in order to cover specific aspects of the circulation system. The northernmost moorings cover the continental slope region, where a front separating cold shelf water from the warmer offshore waters determines how much of the warm water reaches the continental shelf (2 moorings, University of Bergen).  A transect of 7 moorings (NORCE, Norway and LOCEAN, France) is placed on a 600 m-deep sill, separating the more than 1000-m deep Filchner Trough from the continental slope. The sill is for once a region that features an inflow of modified Warm Deep Water (MWDW) on the eastern side and an outflow of very cold water in the west. With its ~-1°C, MWDW is significantly warmer than the near-freezing waters we usually observe in this region, and could lead to enhanced melt when getting in contact with the ice shelf base. The southward inflow is the focus of a mooring array (6 moorings) at the eastern side of Filchner Trough at 76°S, that is maintained by AWI since 2013. The inflow occurs seasonally and generally lasts for a few months. Model studies predict that the inflow of warm water to the ice shelf might increase in the coming decades, which would then strongly increase ice shelf melting in this region. All of the “traditional” moorings are equipped with sensors to record ocean salinity, temperature, and currents. In addition, we deployed a number of specialized moorings, like a row of 3 moorings carrying sound sources, which provide underwater navigation for autonomous floats that sample the water mass properties under the sea ice. These moorings are further equipped with sound recorders, which allow to study the abundance and behavior of marine mammals such as seals and whales. The last mooring deployed on this expedition carries a suite of different sensors to study the biogeochemical properties of the ocean, in particular, to quantify the different aspects of the carbon cycle. The study is an initiative from different scientists across several disciplines and once more exemplifies the multi-disciplinary character of PS124. Overall, we accomplished a lot of work and will come home with a wealth of new experiences and data that will allow further insights into this region, important for the global ocean.

 

PS124 says good-bye to all our readers. We hope that we have informed thoroughly about our exciting work and life on board. However, now we are looking forward to being with our loved ones in a week from now.

Hartmut H. Hellmer (Chief Scientist)

Contact

Science

Hartmut Hellmer
+49(471)4831-1794
Hartmut.Hellmer@awi.de

Scientific Coordination

Ingo Schewe
+49(471)4831-1709
Ingo Schewe

Assistant

Sanne Bochert
+49(471)4831-1859
Sanne Bochert