Scientists at the Institute of Marine Research and the Bjerknes Centre for Climate Research in Bergen have found a new answer to an old climate question. Among other things it has been demonstrated that low-pressure systems that go further north than normal cause greater oceanic heat loss than normal.
The Norwegian Atlantic Current, often referred to as the Gulf Stream, brings warm and saline water northwards along the Norwegian coast and accounts for the majority of oceanic heat transport towards the Arctic. At the entrance to the Barents Sea, the current faces a choice of pathways: will it continue northwards and flow into the Arctic through the deep Fram Strait or will it swing eastwards and enter through the shallow Barents Sea?
Ever since Fridtjof Nansen sailed across the Arctic Ocean on Fram we have known that part of the current continues northwards, while the other part swerves off and flows through the Barents Sea. Today we also know that these two inflow branches are approximately equally as strong. However, there is an important difference between these two routes. In the Fram Strait the current persists beneath an insulating layer of colder and fresher surface water in the Arctic Ocean, thereby retaining much of its heat. In the shallow Barents Sea, mixing by winds and tides prevent such a mechanism. Consequently, the Barents Sea branch gives off heat to the atmosphere equivalent to around 3500 times Norway’s total energy consumption – before it reaches the Arctic Ocean. In other words, the two inflow branches control how much of the residual oceanic heat from the Gulf Stream enters the Arctic Ocean. Until now no one has been able to provide a good explanation of what determines the relative strength between the two.
Towards the right
On his voyage with Fram, Nansen observed that the icebergs had a weak drift to the right relative to the prevailing wind. He engaged a young Swedish man by the name of Vagn Walfrid Ekman, who was a student of Wilhelm Bjerknes, to find an explanation of this phenomenon. Ekman arrived at the fact that the wind-driven water transport in the upper layer of the ocean will be to the right of the prevailing wind (in the Northern Hemisphere, to the left in the Southern Hemisphere). This is attributed to friction and the earth’s rotation (the Coriolis force). In the atmosphere the wind blows anticlockwise around a low-pressure system. Therefore, an atmospheric storm while located above the shallow areas of the Barents Sea will cause divergence in the ocean below and the sea level will drop. The result of this is that the Fram Strait branch is weakened, while the inflow to the Barents Sea receives an extra contribution (see figure).
In order to study this mechanism, the scientists have utilized both direct measurements of the current just south of Bear Island and a regional ocean model in which temperature, salinity and current were calculated in a 4 by 4 km horizontal resolution grid. The observations clearly show “pulses” in which the current south of Bear Island changes direction and flows towards the east. According to the model, there is a clear correlation between these pulses and the low-pressure activity causing lower sea levels over the entire northern Barents Sea. The model also shows a clear response in the form of a change in the circulation encircling the entire northern Barents Sea shelf.By utilizing this model it is also possible to estimate the effect of such events. According to both the model and observations, the temperature difference between the Fram Strait branch and the Barents Sea branch is approximately 3 °C when the two branches converge again in the Arctic Ocean. A reduction of between a half and one Sverdrup of the Fram Strait branch in favour of the Barents Sea branch is equivalent to heat in the order of 500 times Norway’s energy consumption disappearing up into the atmosphere above the Barents Sea instead of flowing into the Arctic Ocean. (1 Sv = 106 m3/s; after the Norwegian oceanographer Harald U. Sverdrup.)
Can impact on access to food
The extra inflow branch means that the warm Atlantic Ocean is coming closer to the polar front and the ice edge in the northern Barents Sea. The Atlantic water is not only bringing heat with it, but it is also transporting large amounts of nutrients and zooplankton from the Norwegian Sea and into the Barents Sea. Consequently, changes in the inflow pattern determine the spatial distribution and access to food for everything from phytoplankton up to commercial fish stocks. Therefore, events in which relatively warm Atlantic water flows eastwards along the slope south of Bear Island can also have both climatic and ecological consequences locally in the Barents Sea.
Reference:Lien VS, Vikebø FB and Skagseth, Ø. One mechanism contributing to co-variability of the Atlantic inflow branches to the Arctic. Nature Communications, 4, 1488 (2013). doi: 10.1038/ncomms2505