Ocean surface waves in an ice-free Arctic Ocean

The retreat of the Arctic ice edge implies that global ocean surface wave models have to be extended at high latitudes or even to cover the North Pole in the future. The obstacles for conventional latitude-longitude grid wave models to cover the whole Arctic are the polar problems associated with their Eulerian advection schemes, including the Courant-Friedrichs-Lewy (CFL) restriction on diminishing grid length towards the Pole, the singularity at the Pole and the invalid scalar assumption for vector components defined relative to the local east direction. A spherical multiple-cell (SMC) grid is designed to solve these problems. It relaxes the CFL restriction by merging the longitudinal cells towards the Poles. A round polar cell is used to remove the singularity of the differential equation at the Pole. A fixed reference direction is introduced to define vector components within a limited Arctic part in mitigation of the scalar assumption errors at high latitudes. The SMC grid has been implemented in the WAVEWATCH III model and validated with altimeter and buoy observations, except for the Arctic part, which could not be fully tested due to a lack of observations as the polar region is still covered by sea ice. Here, an idealised ice-free Arctic case is used to test the Arctic part and it is compared with a reference case with real ice coverage. The comparison indicates that swell wave energy will increase near the ice-free Arctic coastlines due to increased fetch. An expanded Arctic part is used for comparisons of the Arctic part with available satellite measurements. It also provides a direct model comparison between the two reference systems in their overlapping zone.