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

In this project we are studying the role of ocean surface gravity waves on climate. These waves are generated by winds and they mediate the mechanical and thermal coupling between all components of the climate system over the world ocean. Climate models have traditionally been formulated under the assumption that an instantaneous state of statistical equilibrium between ocean surface waves and near-surface winds exists everywhere at all times. This assumption however is systematically erroneous under specific conditions, realised at times everywhere in the mid-latitude storm tracks, and in specific regions of the global ocean which are significantly and permanently affected by remotely generated swell. Therefore the effects of waves on climate can differ from the effects that can be inferred from a direct statistical connection with local, instantaneous surface winds only. We have indeed demonstrated that this is the case on the basis of model simulations with the Norwegian Earth System Model (NorESM) where we have included a dynamical surface wave component, WaveWatch 3 (ww3), in the model configuration, and explicitly used the output of time-dependent, propagating surface wave fields to replace the common semi-empirical statistical-equilibrium parametrisations in order to represent the effect of surface waves on surface stress, surface heat fluxes, and ocean-surface mixing. The differences are most notable in regions and regimes notoriously difficult for climate models to represent faithfully, namely the Southern Ocean, the Eastern Tropical South Pacific and the Eastern Tropical South Atlantic, and high-wind regimes in the North Atlantic and North Pacific. In particular, deeper ocean mixed layers and enhanced air-sea fluxes under high winds generally result in warming in the high latitudes and in the Equatorial Pacific “cold tongue” which have been long-standing biases of the simulations with NorESM up to now.

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.