C04: Coupling between atmosphere, mixed layer and pycnocline under Arctic amplification: The role of sea ice related processes
The sea ice extent and thickness in the Arctic Ocean are changing dramatically under global warming. In the Nansen Basin, there is evidence that the inflow of warm and saline Atlantic water (AW) has reduced the upper ocean vertical stratification in recent years enough, such that convective wintertime deepening of the mixed layer (ML) may have caused a sizeable upward heat flux from the subsurface AW layer toward the sea surface. This shift to conditions more similar to those found in the subpolar North Atlantic further south is called “Atlantification” of the Arctic Ocean and it may further reduce the sea ice cover. In the more permanently ice-covered central regions of the Arctic Ocean, it is thought that the halocline still prevents vertical mixing and thus, does not allow strong upward heat fluxes year-round. However, wintertime measurements hardly exist in leads within the central Arctic Ocean, where sea-to-air heat fluxes are strong. The feedback between fluxes across the halocline and base of the mixed layer and the sea ice for events like opening leads, passing storms, ice melt, passing oceanic eddies, or the decay of fronts in the upper ocean are not well understood. Our planned research will provide vertical heat fluxes by combining integral estimates from dedicated helium and neon isotopes, tritium, CFC, and SF6 measurements with observations of turbulence, currents, stratification, and meteorological data. To contrast the situation in the ice-covered Arctic Ocean with a scenario of Atlantification, we will obtain trace gas samples during the overwintering MOSAiC expedition as well as from a study near the current ice edge in Fram Strait, where AW and halocline waters form large horizontal gradients. The sampling during MOSAiC will resolve the upper few hundred meters with a focus on events such as lead openings and storms. The frontal decay in Fram Strait will be sampled in 4D with repeat parallel sections obtained with a towed system, complemented by mooring-based, year-round continuous measurements. Ultimately, our results will be combined with statistics for the occurrence of the studied events to assess the impact of Arctic amplification on the feedback between atmosphere, ocean, and sea ice.
Physical vertical and horizontal processes within the Arctic Ocean mixed layer are critical for the vertical heat flux between atmosphere and Atlantic layer, and thus for the change in Arctic sea ice and Arctic amplification.
Specifically, we will address the questions:
- What are the vertical fluxes between the atmosphere, the mixed layer and the pycnocline during events like storms, opening of leads and frontal decay?
- What is the role of meso- and submesoscale horizontal processes driven by sea ice-related and frontal processes in modifying the vertical exchanges across the mixed layer?
- What is the potential of ocean mixed layer processes in the changing Arctic Ocean in modifying Arctic amplification?
Role within (AC)³
Prof. Dr. Monika Rhein
University of Bremen
Institute of Environmental Physics
Prof Dr. Torsten Kanzow
Alfred-Wegener-Institute Helmholtz Center for Polar and Marine Research (AWI)
Am Handelshafen 12
Hofmann, Z., von Appen, W.-J., & Wekerle, C., 2021. Seasonal and Mesoscale Variability of the Two Atlantic Water Recirculation Pathways in Fram Strait. J. Geophys. Res. Oceans, 126, e2020JC017057. https://doi.org/10.1029/2020JC017057