Wave-mediated atmosphere-ocean-sea-ice interactions and their climatic impacts in the Nordic Seas and eastern Arctic

Everyone’s experience of the ocean is associated with surface waves. They not only dominate its appearance, but also control its interactions with the atmosphere and with floating bodies between the two, such as ships or sea-ice. Yet climate models do not include ocean surface waves when modelling the interactions between air or sea-ice and the ocean, or they do so in an implicit way whereby a certain sea state is assumed. As a result, sea roughness associated with storms is underestimated. In our proposed research, we will include explicit calculations of sea-state and its effects on air-sea fluxes and on sea-ice dynamics in the Norwegian Earth System Model (NorESM). It will be the first time this is done in a comprehensive climate model. Currently, some operational forecasts routinely employ such calculations for air-sea interaction, but not for sea-ice. Also, their short duration imply that the climate implications of ocean waves have so far not been explored. We will test in particular a new hypothesis, viz that the effect of sea-state causes additional warming of the Arctic. Several reasons lead us to believe so. First, the rough sea-state associated with storms in the North Atlantic tends to encourage storm formation both by localising jet-stream momentum dissipation and by increasing the amount of oceanic heat taken up by the atmosphere. As the storms wander northwards, they export water vapour to the Arctic where it acts to increase surface temperatures. Second, waves increase vertical mixing in the ocean, thereby making the heat stored in the deep mixed layer of the North Atlantic available to the surface and the atmosphere. Third, swell propagating into the Arctic from stormy regions can delay sea-ice formation, and break up thin ice, thereby preventing surface cooling. These three effects together can have a sizable impact on the simulated surface climatology of the Arctic and on its sensitivity to climate change, and we will quantify this impact.

The North Atlantic - Arctic Ocean is a key region of the global Northern-Hemisphere storm track, characterized by strong air-sea exchange and associated meridional flux of enthalpy and water vapour. Regionally it determines much of the climate of western Eurasia and of the Nordic Seas, with strong effects on the Arctic sea-ice cover. The ocean state in particular is affected not only by the effects of strong diabatic and mechanical forcing, but notably also by the large activity of surface waves forced by the intense storms. Even though it is expected that the presence of surface waves has an important impact on the coupling between atmosphere, ocean, and sea ice, to date there is no quantitative assessment on the implications for the climate of the European/North-Atlantic region and for its evolution under natural or anthropogenic forcing. Indeed, surface waves are at present excluded from the representation of surface interactions in Earth System Models (ESMs). To understand the implications of this, we will, for the first time, use an ESM with a fully coupled wave model covering the northern North Atlantic - Arctic domain to carry out a systematic study of the effects of full coupling between ocean surface waves, atmosphere, ocean, and sea-ice on the climate in the Nordic Seas and eastern Arctic region. The project will thereby go beyond the state of the art and provide the scientific justification for a next generation ESM with a fully coupled wave model in the global domain. The rationale for a regional focus is two-fold. The first concerns the critical role of the marginal ice zone on the Northern-Hemisphere climate, an area that is particularly exposed to the effects of global warming and intrinsically exposed to wave action. The second rationale concerns the scientific prioritization of model development paths for the next-generation Norwegian Earth System Model. Understanding regional effects is a prerequisite to evaluate potential global climate impacts.