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MARINFORSKHAV-Marine ressurser og miljø - havmiljø

MixED Layer hEterogeneitY

Awarded: NOK 4.5 mill.

The ocean's surface is continuously influenced by the wind, which produces waves, stirring and mixing the uppermost part of the ocean. This continuous stirring creates a surface layer in the ocean where properties such as temperature, salinity, and nutrients are uniform across the whole layer. This so-called mixed layer is a hot spot for biology and a central part of heat and momentum transfer between the ocean and the atmosphere. Properly capturing the physics of the mixed layer is, however, challenging. Today's ocean and climate models struggle to reproduce the mean state of the mixed layer correctly, such as its depth. In most cases, they don't capture this variability well enough. In the models, the mixed-layer depth and other characteristics change slowly in space, giving an overly smooth - or not heterogeneous enough - representation of this important feature of the surface ocean. The over-arching goal of MEDLEY is to investigate the heterogeneity of the mixed layer and how better to reproduce that in models. Nowhere is this heterogeneity of the mixed layer more pronounced than in the Arctic, and nowhere do our models fail as badly at reproducing it. In MEDLEY, we propose that the reason is that the sea-ice models used generally don't simulate narrow openings in the ice, called leads. The Norwegian partner in MEDLEY is the Nansen Center and the Center's sea-ice modelling group. This group has developed a new sea-ice model that reproduces leads much better than traditional models. In the project, we have used the new sea-ice model to understand better how the oldest Arctic sea ice is lost due to climate change. This information will also be valuable concerning the mixed layer because we expect the mixed layer to behave differently under multi-year ice compared to first-year ice. This is related to the difference in thickness between the two ice types but also difference in salinity. We have also used the model coupled with an ocean model to investigate the effects leads have on the ice and ocean. We have used the model to gain insights into the role of leads in sea-ice formation in winter. We have shown that between 20 and 30% of the ice formed in winter forms in leads. This fits well with observational estimates, but our estimate has the added value of showing also an increasing trend in this over the last 20 years. We expect this localised ice formation to impact the mixed layer substantially. Our current investigations focus on how an extreme breakup event in the Beaufort Sea in 2013 may have impacted the upper ocean. Observations of the ocean during this event are scarce, but they indicate that the mixed layer grows deeper as the ice breaks up - and we also see this in the model. Our hypothesis is that this happens because when the ice breaks up the wind can more easily stir the surface of the ocean which will deepen the mixed layer. This is an exciting prospect, but there are still details to be teased out of this event, and this will be our focus for the remainder of the MEDLEY project.

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The ocean surface mixed layer mediates the transfer of heat, freshwater, momentum and trace gases between atmosphere, sea ice and ocean. Thus, the mixed layer transfer function must be represented accurately in climate models, especially in the North Atlantic and in the Arctic oceans which are, respectively, hotspots of anthropogenic CO2 storage and warming. Large discrepancies in mixed layer depths are found in low resolution CMIP5 models, in part because these models do not parameterize properly the spatial heterogeneities induced by the presence of a discontinuous and very dynamic sea ice cover, mesoscale eddies and submesoscale fronts and filaments at the kilometer-scale. The region of interest ranging from the North Atlantic to the Arctic ocean is especially relevant to future changes of the European climate. MEDLEY brings together state of the art observational datasets, groundbreaking submesoscale-resolving basin scale models, an innovative sea-ice model, and the latest generation of climate models with an eddying ocean component participating in the HighResMIP intercomparison. By pooling their expertise, MEDLEY members will produce the most complete evaluation of the mixed layer dynamics in state of the art climate models, from the North Atlantic to the Arctic ocean. MEDLEY will improve our understanding of the relationship between air- sea fluxes and mixed layer properties, taking into account the mediation of the fluxes induced by the fractured sea ice cover. MEDLEY will evaluate the effect of spatial heterogeneities on mixed layer properties, including currents and kinetic energy, as well as the relationship between the mixed layer and the interior through the stratified transition layer. Building on interdisciplinary collaborations of its members, MEDLEY will take advantage of the most recent data analysis methods (e.g., machine learning-based classification).

Publications from Cristin

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Funding scheme:

MARINFORSKHAV-Marine ressurser og miljø - havmiljø