Category: Spring 2022

Polynyas are areas of open water or thin ice that are surrounded by sea ice and can stretch over hundreds of kilometres. They are created by concurrent upper-ocean preconditioning (weakened stratification) and meteorological perturbations (storms). Polynyas play a crucial role in the dense water formation in the Southern Ocean. For this, two mechanisms can be distinguished: First and predominantly, the formation of cold, dense Antarctic Bottom Water (AABW) due to sea-ice production above the continental shelf within coastal polynyas; and secondly, open-ocean deep convection within open-ocean (sensible heat) polynyas. In the present-day climate, the first mechanism is the dominant contributor to AABW formation, which accounts for 30-40% of the global ocean mass and is a key driver of the global overturning circulation. Under climate change, two competing effects for the formation of polynyas are expected to occur. A freshening of the surface waters would act to increase the stratification and thereby decrease the likelihood of polynya-formation. Opposingly, an intensification of the westerly winds around Antarctica would increase the upwelling of warm, salty deep water and by that intensify the creation of polynyas. It is uncertain which of these opposing factors will dominate in the future.

The Arctic Ocean circulation is characterised by an inflow of relatively warm and saline water from the Pacific and Atlantic Oceans and an outflow of cold fresh Polar Water. The Polar Water derives from river runoff and ice-melt. Historically, large parts of the Arctic Ocean has been covered in perennial sea ice. With anthropogenic emissions of carbon to the atmosphere, air and ocean temperatures are rising. This effect is especially seen in the Arctic Ocean. The increased heat content in both causes a decrease in sea ice extent. Atlantification of the water masses in the European inflow regions of the Arctic Ocean enhances this effect. The consequences of fossil fuel burning impacts both the biology and the biogeochemistry of the Arctic Ocean. An example is the Pacific diatom species Neodenticula seminae which for the first time in 800 000 years has been observed in the Nordic Seas. It is hypothesised that it has been transported via the Transpolar Drift. This is made possible due to the reduced sea ice extent. Another consequence is the acidification. The pH has decreased in the Nordic Seas since the start of the industrial revolution and is projected to decrease further. Atlantic Water cools down in the Nordic Seas on its way to the Arctic Ocean. The acidification of the Nordic Seas may therefore propagate into the Arctic Ocean. For an oceanic region vulnerable to changes in pH, this is dire news. 

As the sea ice edge is retreating northwards, the deep convection in the north Atlantic and the Nordic Seas connected to the strong heat fluxes close to the sea ice edge, follows. Moreover, with the projected disappearance of sea ice in the Arctic in the recent future, the deep convection in the North Atlantic and Nordic Seas is projected to disappear. Currently, deep convection has been observed in an until recently ice covered region north of Svalbard. One of the many immediate consequences is acidification of the Nordic Seas due to the recent emergence of deep convection affecting cold water corals in this region. Conversely, as the ice cover is retreating towards the east coast of Greenland, currents previously insulated by sea ice are ventilated and densified as they flow southwards leading to new deep convection.