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Wednesday, 30 September 2015

Good Mooring!

Since the late 1990s, scientists at the Norwegian Polar Institute (NPI) have conducted annual cruises across the Fram Strait: the widest, deepest and most important exit point for sea ice in the Arctic. One of the main aims of the Fram Strait cruise (FS2015) is to recover, service and redeploy a sprinkling of oceanographic moorings- current profiling instruments and buoys tethered to hundreds of meters of cable, anchored to the seafloor. These have been continuously measuring the velocities of water masses within the East Greenland Current at preset depths. With continuous data over decadal timescales, the NPI are hoping to understand how the nature of the Arctic freshwater budget changes in an increasingly warming climate, how this will impact biological processes, and how it will affect other water masses on a broader scale as they interact in new ways.

1 of 6 oceanographic moorings being recovered for
servicing on FS2015.  Image credit: Laura de Steur /
Norwegian Polar Institute
I was lucky enough to lend a helping hand this September; my second cruise with the NPI after having had a blast working on the Norwegian Young Sea ICE 2015 (N-ICE2015) cruise for a month back in April 2015. After flying into Longyearbyen, Svalbard, and seeing the Research Vessel Lance waiting in the harbour for a second time, it felt very odd not seeing the ship surrounded by 1.3m thick pack-ice, which is how I’d left it after N-ICE2015. It wasn’t until I dropped my bag off in my cosy cabin and heard the familiar roar of the engines warming up (and having my room located right at the back of the ship I really mean roar…) that it felt like I was returning to my home away from home.

The mooring aspect of the cruise this time introduced a different dimension of risks that had to be accommodated: namely by the presence of sea ice above many of the moorings that needed to be recovered. This gave us an occupational risk that obviously only presents itself at the poles! On the N-ICE2015 cruise the engine didn’t have a huge part to play as we were passively drifting with the Arctic pack ice. This time round, whilst navigating the ice floes across Fram Strait towards Eastern Greenland, the Lance was actively smashing through and breaking up the ice above mooring sites to ensure that the mooring returned to the surface without being blocked on its ascent. As ice coverage can alter rapidly, it’s up to chance whether or not these moorings will be readily accessible. In the best case, there will be little to no coverage, so one only has to send a command to the mooring via radio signaling and the cable is released and brought to the surface- buoys and instruments attached. In a moderate case, ice will be extensive enough that the ship will have to meander round, breaking up the ice floes as best it can. For this reason underlying current speed and ascent rate of the mooring has to be considered carefully. It’s always a tense minute or two waiting for the buoys and expensive instrumentation to reach the surface, knowing it may never arrive if it gets stuck on an unfortunately located ice floe! In the worst case, the floes will be so thick and expansive that the mooring recovery process may have to be abandoned all together. For this reason, daily satellite images of ice extent were a very valuable necessity.

As well as observing the physical properties of the Atlantic and Polar Waters spilling southward into the Atlantic, extensive tracer sampling took place at and around the mooring locations by way of collecting water at standard depths. While it is common practice for oceanographers to measure parameters like salinity soon after the water is collected (the on-board salinometer quickly became a very close friend of mine, with 528 samples needing to be analysed during the cruise!) other tracers such as coloured dissolved organic matter (CDOM), nutrients, and 18Oxygen isotope will be analysed ashore. These tracers can tell us something about the source of this water, and by looking at its isotopic composition whether it comes from melted sea ice or from other meteoric sources- that is, water derived from precipitation and runoff. Precipitated water at high latitudes is strongly depleted in 18O, while sea ice meltwater is slightly enriched in it. By looking at the mass of ice loss in the Arctic and how much of it is flowing through the Fram Strait year after year, we’re able to gauge how much is entering the Atlantic or staying in the Arctic basin [1].

The thickness of the ice flowing through Fram Strait has decreased by about 1/3 since 1990 [2]. Part of this melting is related to inflowing, relatively warm Atlantic waters travelling northwards via the West Spitsbergen Current. However, the amount of melt-water that is exported through Fram Strait hasn’t changed very significantly in the past decade. Evidence suggests that the melt water is being stored in the Beaufort gyre- a clockwise-rotating mass of water in the Arctic [3]. While the flux of melt-water into the Arctic Basin has increased in the past couple of decades, tracer analyses tell us the main mechanisms by which fresh water is supplied is by runoff from North American and Eurasian rivers, and by relatively fresh Pacific inflow through the Bering Strait, between Russia and Alaska [1].
The large-scale circulation around the Arctic Ocean.
Figure: Paul Dodd / Norwegian Polar Institute.
It is possible that with inter-annual changes in Arctic wind forcing this growing reservoir of cold, fresh water could be directed southwards across Fram Strait, where it could disrupt the thermohaline circulation of the Atlantic.
Routine sea ice stations were also carried out on suitable ice floes, giving us the chance to stretch our legs and take some ice cores for further tracer sampling. Once analysed, these will allow us to see how the chemical compositions compare with that of the underlying waters. Working 6-hours on, 6-hours off could get pretty exhausting, so it was nice to unwind with the occasional sled race across the floe or by sharpening our ‘selfie skills’ to let the world of social media know how our research was going. All in the name of science…

The FS2015 team and I (centre), exploring an ice floe.

This blog is by Adam Cooper, Earth Sciences graduate at the University of Bristol.

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