Coupling between atmosphere, mixed layer, and pycnocline in a changing Arctic: First results from tracer observations during MOSAiC.

Abstract:

The sea ice extent and thickness in the Arctic Ocean are changing dramatically under global warming. The shift to conditions more similar to those found in the subpolar North Atlantic is called Atlantification and it may further reduce the sea ice cover. In the ice-covered central regions of the Arctic Ocean, the halocline still prevents vertical mixing and thus, does not allow strong upward heat fluxes year-round. In the Nansen Basin, closer to the inflow of warm and saline Atlantic water, there is already evidence this inflow reduces the upper ocean vertical stratification, and feedback between fluxes across the halocline for events like opening leads, passing storms, ice melt, oceanic eddies, or the decay of fronts in the upper ocean may cause upward heat flux from the subsurface Atlantic water layer toward the sea surface and ice.

Trace gases that do not interact biogeochemically in the ocean are valuable tools to study and assess the role of physical processes. In the mixed layer, time scales that modify the tracer distributions are of the order of 1 week, while in deeper layers, longer time periods are involved. Inactive trace gases come in two classes: The first are the so-called transient tracers (e.g. CFCs), that are anthropogenic in origin and have changing atmospheric concentrations with time. The second are the steady state tracers like the noble gases and their isotopes. Some of the processes unique for ocean-cryosphere interaction involve especially He and Ne, e.g. the formation and melt of sea ice. Tracer derived estimates of ventilation, water mass formation, spreading, upwelling, or fluxes in and out of the mixed layer are an integral over the involved time scale. In view of the complex situation in the Arctic ocean through the involvement of sea ice, parallel measurement of transient and steady state tracers are needed to separate the processes.

Here, we will use the first results from tracer distributions (He and Ne isotopes, tritium, CFC, SF6) obtained during the MOSAiC expedition and at the current ice edge in Fram Strait, to contrast the situation in the ice-covered Arctic Ocean with a scenario of Atlantification. The observations will bring new insights into the exchange processes between the Arctic mixed layer and the Atlantic layer below.