We use a global 5-km resolution model to analyze the air-sea interactions during a katabatic storm in the Irminger Sea originating from the Ammassalik valleys. Katabatic storms have not yet been resolved in global climate models, raising the question of whether and how they modify water masses in the Irminger Sea. Our results show that dense water forms along the boundary current and on the shelf during the katabatic storm due to the heat loss caused by the high wind speeds and the strong temperature contrast. The dense water contributes to the lightest upper North Atlantic Deep Water as upper Irminger Sea Intermediate Water and thus to the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). The katabatic storm triggers a polar low, which in turn amplifies the near-surface wind speed due to the superimposed pressure gradient, in addition to acceleration from a breaking mountain wave. Overall, katabatic storms account for up to 25% of the total heat loss (20 January 2020 to 30 September 2021) over the Irminger shelf of the Ammassalik area. Resolving katabatic storms in global models is therefore important for the formation of dense water in the western boundary current of the Irminger Sea, which is relevant to the AMOC, and for the large-scale atmospheric circulation by triggering polar lows.

Gutjahr, O., Jungclaus, J. H., Brüggemann, N., Haak, H., & Marotzke, J. (2022). Air-sea interactions and water mass transformation during a katabatic storm in the Irminger Sea. Journal of Geophysical Research: Oceans, 127, e2021JC018075. https://doi.org/10.1029/2021JC018075

Sea ice in polar regions plays an important role in the exchange of heat and freshwater between the atmosphere and the ocean and hence for climate in general. Therefore climate models require a description (a set of equations) to express the large‐scale sea ice motion. We present a mathematical framework for describing sea ice flow in a global three‐dimensional Cartesian system. The idea is to express the sea ice motion in tangent planes. In this reference system, we solve the mathematical equations that describe the sea ice motion. The equations are approximated on a computational grid, that consists of triangles covering the surface of the sphere. On each triangle the sea ice velocity is placed at the edge midpoint. The development is motivated by the infrastructure of the ocean and sea ice model Icosahedral Non‐hydrostatic‐Ocean model. The old representation of sea ice dynamics uses a different design principle. Therefore, the communication between the sea ice and ocean model is computationally expensive. To circumvent this problem we have developed a numerical realization of sea ice dynamics that uses the same infrastructure as the ocean model. We show that the new realization of the sea ice dynamics is capable of capturing the sea ice drift.

Mehlmann, C., & Gutjahr, O. (2022). Discretization of sea ice dynamics in the tangent plane to the sphere by a CD-grid-type finite element. Journal of Advances in Modeling Earth Systems, 14, e2022MS003010. https://doi.org/10.1029/2022MS003010