By: Angelika Renner // Institute of Marine Research. Øyvind Lundesgaard and Arild Sundfjord // Norwegian Polar Institute
The Atlantic Water inflow is the main source of heat for the Arctic Ocean. Part of it flows in through the Barents Sea while another part moves along the west coast of Svalbard before turning eastward and following the continental slope north of Svalbard into the Arctic Ocean proper. This branch is the focus of A-TWAIN. At the start of the project in 2011, we knew little about the properties and behaviour of the Atlantic Water inflow at the entrance to the Arctic due to a lack of long-term observations between Fram Strait upstream and the Siberian Seas downstream. Major uncertainty was connected to circulation at the Yermak Plateau northwest of Svalbard.
Does more Atlantic Water move around or across the plateau? What happens further downstream – does it form one or two current cores? The A-TWAIN array was designed to capture a cross section of the entire inflow, with moorings placed across the boundary current from the shelf northward to the deep basin. The array was placed as far east as possible without making annual service logistically unfeasible: at 31°E north of Kvitøya.
Since the first mooring deployments in 2012 we have learned a lot more about the Atlantic Water pathways and current structure. It turns out that 31°E is a great place to monitor the Atlantic inflow. This far east, it has formed a strong and coherent boundary current fed mostly by Atlantic Water that goes over the Yermak Plateau instead of around. This boundary current follows the continental slope and can be traced downstream along the Siberian Seas. The core of the current sits on average above the 800 m depth contour, making our “800 m mooring” a critical tool to investigate its variability. The first years of ship-borne and mooring data allowed us to describe the structure of the current from shelf to deep basin and the seasonal and interannual variability in the current core. Complementary modelling studies showed how eddies can spin off the current, transporting warm and salty Atlantic Water into the cold and deep Nansen Basin.
In A-TWAIN we have had the pleasure of collaborating with several international institutions over the past ten years. Partners from Poland, France, UK, and USA have deployed moorings during our cruises. This has given much better coverage of the Atlantic Water inflow than what we would have managed on our own. Consistency in carrying out cruises and allowing space and time on board for collaborators has enabled better planning and use of the critical resource that an ice-going research vessel is. Joint data analysis similarly increased the value of the observations.
Historically, there are many reports of the region being sea-ice-free for large parts of the year despite being so far north. This is closely tied to the ocean circulation around Svalbard. Previous studies showed how warm Atlantic Water reaches the surface when it flows past the west coast of Spitsbergen. Our data revealed that this also is the case northeast of Svalbard from summer to early winter, until a cold freshwater layer can form on top of the Atlantic Water as sea ice moves onto and melts above the warm water. Only when that fresher surface layer is in place can sea ice that is blown in by wind survive, or new sea ice form during winter freeze-up.
The latest A-TWAIN publication looks closer into drivers of sea ice coverage in the A-TWAIN region, making use what had by then become an eight-year time series from the shelf mooring, together with satellite observations of sea ice. The satellite record shows how the region north of Svalbard is kept ice-free unusually often along the inflow pathway of relatively warm water – and increasingly so during recent decades. There are large year-to-year variations in the sea ice coverage which do not seem to be simply a result of changes in ocean temperature. Instead, the amount of sea ice that drifts into the area from the north and east determines whether there is little or much sea ice in the region in a given year. Sea ice drift is mostly driven by wind, and the amount of available sea ice has a lot to do with the air temperature. Since winds and air temperatures vary more than the ocean and can be quite different from year to year in this part of the Arctic, the amount of sea ice that moves into the A-TWAIN area is equally variable.
We also quickly learned that the continental slope and especially the 800 m isobath are dangerous places for moorings. The Atlantic Water boundary current varies a lot in strength and speed. Especially in autumn, high velocities in the core can “blow over” a mooring: the floats on the mooring line, intended to keep the rope upright, are not sufficient and the mooring dives. Sometimes the current is so strong that the mooring moves or breaks and we don’t find it again.
So what did we do on that cruise in 2019 when the 800 m mooring was gone? Of course we put out a new one in the same spot in the core of the current! And this autumn, back onboard RV Kronprins Haakon, we successfully recovered the mooring and with it two more years of crucial observations in our quest to build up a long-term monitoring time series in this inhospitable but important region. As the sea ice cover dwindles, continued observations here at the entrance to the Arctic Ocean are essential to understand drivers of changes in the climate and environment of the Eurasian shelves and in the interior basins. Future Fram Centre research will help us understand the emerging blue Arctic Ocean and facilitate safe operations and sound management of industries and resources.
Lundesgaard Ø, Sundfjord A, Renner AHH (2021) Drivers of interannual sea ice concentration variability in the Atlantic Water inflow region north of Svalbard. Journal of Geophysical Research: Oceans, 126(4), e2020JC016522, doi:10.1029/2020JC016522