The Pulse of Heat in the North Atlantic

The moment in which the acoustic release signal is transmitted toward the seafloor is one that Wilken von Appen has experienced dozens of times. Nevertheless, the AWI oceanographer can’t simply lean back and wait on the bridge of the German research icebreaker Polarstern; there’s too much at stake. If he doesn’t see a handful of yellow and orange buoys, with a mooring attached to them, start surfacing within the next 60 seconds, it could mean he just lost the data recorded over the past year – and scientific equipment that cost as much as several compact cars.

45 seconds go by … 50 seconds. Still nothing. Wilken von Appen gets out his binoculars and starts scanning the water’s surface. In the back of his mind, he starts playing out various scenarios. Maybe the plastic cable, more than two kilometres long, got tangled under one of the ice floes? After all, plenty of them were floating near the ship. In that case, the team could lower a deep-sea robot with a camera into the water to search for the mooring. If the mooring’s release mechanism somehow malfunctioned, that would be a much more difficult situation: the three massive railway wheels attached to the acoustic release are lying on the ocean floor, 2,500 metres below the surface – definitely too deep for the robot to reach.

The mooring is part of one of the Alfred Wegener Institute’s most important long-term projects. Since 1997 AWI oceanographers have been monitoring the pulse of the Atlantic current system in the Fram Strait, the region between Svalbard and the northeast coast of Greenland. “There are three reasons why the Fram Strait is so important to us. Firstly, it represents one of the two gateways through which water and heat from the Atlantic Ocean are transported to the Arctic Ocean, which contributes to the melting of sea ice. So if we want to know how much heat is transported from the Atlantic to the Arctic, we have to measure it at these two points,” says Wilken von Appen.

Secondly, the Fram Strait is a key region for global oceanic circulation. On its way north, the comparatively salty surface water from the Atlantic releases large quantities of heat into the atmosphere, becoming cooler and denser in the process. In the 600-kilometre-wide Fram Strait, the water sinks to a depth of between 200 and 800 metres and flows back south along the edge of Greenland’s eastern shelf. South of Iceland, it then crashes down into the deep-sea basins of the North Atlantic in gigantic waterfalls. Climate researchers refer to this overturning as ‘convection‘ and are paying close attention to the deep water formation. Why? Because half of the Gulf Stream, a powerful force for transporting heat from the tropics toward central and northern Europe, is part of this circulation – and when a single factor along the conveyor belt’s route changes, it could affect the entire system.

The third reason for AWI’s focus on the Fram Strait is the retreat of glaciers on Greenland’s eastern coast. “We believe the glaciers are shrinking not just because of the rising air temperature, but also because warm Atlantic water from the Fram Strait is finding its way under their ice tongues, melting them from below. We’re currently studying the 79° North Glacier to determine how this warm water travels from the Fram Strait to underneath the glaciers,” says Wilken von Appen.

No matter whether deep water formation or melting glaciers – the starting point for every research question is the dataset from the network of moorings, which reaches back more than 20 years. The grid consists of 16 individual moorings, which AWI researchers and their Norwegian colleagues deployed at intervals of 10 to 30 kilometres. Like a string of pearls along the latitude of 79°N, this observatory reaches all the way across the deep section of the Fram Strait, which is roughly 300 km wide. It measures the temperature, flow speed and salinity of the inflowing and outflowing water masses, 365 days a year.

“At the water’s surface, the Fram Strait’s system of currents is a bit like a highway. In the right-hand lane, salty Atlantic water at three to six degrees Celsius flows north in the West Spitsbergen Current; in the left-hand lane, sea ice and less salty water at 1.8 degrees flow from the Arctic into the North Atlantic via the East Greenland Current. Between the two, there are also forks in the road, where part of the Atlantic water heads west and joins the water moving in the opposite direction; the remainder continues to flow northward, straight into the Arctic Ocean,” von Appen explains.

Today, this system transports significantly warmer water into the Arctic than when record keeping began. According to Wilken von Appen, “On average, today the water masses in the West Spitsbergen Current are one degree warmer than they were in 1997. That also means the water masses that leave the West Spitsbergen Current headed for Greenland, some of which reach its glaci­ers, have also grown warmer – a change with far-reaching consequences for Greenland’s ice masses.” The rising temperatures can also be felt far below the surface. “The circulation of the wa­ter masses in the Greenland Sea no longer reaches the seafloor; the water is only mixed to a depth of roughly 1,000 metres. What that will mean for the Atlantic circulation system remains to be seen,” says von Appen.

In the past few years, the oceanographer has focused his efforts on identifying which forces make the Atlantic water from the West Spitsbergen Current veer off west – because, upon closer inspection, the physics of seawater are highly complex, with their fair share of apparent contradictions. His favourite example is that, on their own, different water masses don’t actually mix. As the AWI researcher explains, “The temperature and salinity differences between any two water masses create layers and fronts that are hard to break through. As a result, within a given current, the water basically flows just as if it were in a pipe. Normally, it shouldn’t be able to veer off to the left or right.” Nevertheless, AWI scientists have observed water masses splitting in the Fram Strait. “That’s possible because of eddies in the West Spitsbergen Current. They are several kilometres wide and several hundred metres deep, sometimes even extending down to the seafloor,” says Wilken von Appen.

And their spinning motion makes the water fronts unstable. “For one thing, we know this produces the westward flow in the Fram Strait. Yet we also believe the same mechanism to be the reason why the warm and dense Atlantic water manages to slip below the ice-cold water in the East Greenland Current.” The computer models created by his AWI colleagues Dr Claudia Wekerle and Dr Tore Hattermann allow Wilken von Appen to see what these eddies most likely look like. But tracking them down in the open sea to confirm the team’s hypothesis is quite a different challenge. “As part of the AWI’s major infrastructure initiative FRAM, in the summer of 2016, we installed new moorings at exactly those points where the model estimates part of the Atlantic water turns off to the west. We currently assume there are two such turning points, but we still need observational data for confirmation,” explains Wilken von Appen.

If only retrieving that data weren’t so ­nerve­-wracking. It’s now been a minute and five seconds since the signal was transmitted. Suddenly, the officer on duty calls out, “Buoys in sight,” and points toward a pair of ice floes drifting alongside the research icebreaker. Wilken von Appen breathes a short sigh of relief, then switches his routine to autopilot: Make a written record of the retrieval, clean the measuring equipment, transfer and check their data, maybe even glean the first scientific insights – his work schedule for the next 48 hours is already full. And that was just the first of more than 20 moorings!

Text: Sina Löschke