Description: The characteristics of a global set-up of the Finite-Element Sea-Ice Ocean Model under forcing of the period 1958-2004 are presented. The model set-up is designed to study the variability in the deep-water mass formation areas and was therefore regionally better resolved in the deep-water formation areas in the Labrador Sea, Greenland Sea, Weddell Sea and Ross Sea. The sea-ice model reproduces realistic sea-ice distributions and variabilities in the sea-ice extent of both hemispheres as well as sea-ice transport that compares well with observational data. Based on a comparison between model and ocean weather ship data in the North Atlantic, we observe that the vertical structure is well captured in areas with a high resolution. In our model set-up, we are able to simulate decadal ocean variability including several salinity anomaly events and corresponding fingerprint in the vertical hydrography. The ocean state of the model set-up features pronounced variability in the Atlantic Meridional Overturning Circulation as well as the associated mixed layer depth pattern in the North Atlantic deep-water formation areas.

Description: The Canadian Arctic Archipelago (CAA) is one of the main pathways for freshwater exiting the Arctic Ocean. Freshwater exported to the North Atlantic may influence the deep water formation in the Labrador Sea, and thus the meridional overturning circulation. Modeling ocean and sea ice conditions of the CAA is difficult because of narrow straits and complex coastlines. The Finite-Element Sea-ice Ocean circulation Model (FESOM) configured on a global mesh is applied to assess the volume, freshwater and sea ice transports through the CAA.With a mesh resolution of 5 km in the CAA we are able to accurately resolve complex coastlines. Outside the CAA the mesh is refined to 24 km north of 55N with a global background resolution of 1.5degrees. In this study, first, it is shown that the transports modeled with FESOM correlate well with the available observational data. Second, the model is used to learn about the impact of different atmospheric forcing datasets differing in spatial and temporal resolution (CORE 2 and the Reforecast dataset from Environment Canada). The CORE 2 dataset is on the T62 grid, which is coarse compared to the Reforecast dataset with grid resolution of 0.45degrees longitude and 0.3degrees latitude. The temporal resolution of the Reforecast dataset is higher than the CORE 2 dataset (one hourly and 6-hourly data, respectively, for wind, surface temperature and specific humidity fields). The representation of sea ice in the CAA can be improved by using the high resolution atmospheric forcing.

Description: The volume and freshwater transports through the Canadian Arctic Archipelago (CAA) are assessed using the unstructured-mesh Finite-Element Sea-ice Ocean circulation Model (FESOM) in a global setup with the CAA resolved at the 5 km scale. The hindcast simulation realistically represents fluxes through the main gates of the Arctic Ocean and the Arctic sea ice conditions. During the period 1968-2007 the mean volume transports through Lancaster Sound and Nares Strait amount to 0.86 Sv and 0.91 Sv, respectively. The monthly mean volume transport through western Lancaster Sound is highly correlated with the observational estimate (r=0.81). The seasonal variability of the Lancaster Sound transport is well represented in the model. The simulated mean CAA freshwater export rate is 123 mSv, slightly higher than the observational estimate. The interannual variability of CAA volume transports is determined by sea surface height (SSH) gradients between the Arctic Ocean and northern Baffin Bay. The sea level upstream of Lancaster Sound is mainly determined by that along the Beaufort Sea coast, which can be explained by changes in the wind regimes (cyclonic vs. anticyclonic) associated with release or accumulation of freshwater in the Beaufort Gyre. Sea level variations downstream of Lancaster Sound and Nares Strait are connected to SSH variations in the eastern Baffin Bay and in the Labrador Sea, which can be attributed to the variability of ocean-atmosphere heat fluxes. Both processes upstream and downstream of the CAA are linked with the North Atlantic Oscillation type of atmospheric variability. The local mesh refinement of ~5 km allows us to investigate the contribution of individual narrow straits to the Parry Channel volume transport. The volume transports through these straits show a very similar variability.

Description: A review is given of existing efforts and future challenges in large-scale ocean modeling on unstructured meshes. Because of large integration time the large-scale ocean circulation models require more attention with respect to conservation and accuracy than their coastal counterparts, and deal with different dynamics. Numerous discretizations of finite-element and finite-volume type have been proposed and explored, but only simplest approaches (like P1-P1 of FESOM or cell-vertex of FVCOM) are realized as working tools. While the results of first applications performed in global context are very encouraging and show the feasibility of local refinement by a factor of 30-50 in a global setup, they also indicate that further efforts are needed if one aims at simulating the ocean in eddying regimes. These efforts must concentrate on numerical efficiency of unstructured-mesh codes and ensure much higher accuracy of advection schemes than currently available on linear elements. Since the selection of discretization introduces obvious limitations on unstructured meshes, one has to re-consider it from the perspective that takes into account the already available lessons, and not only the representation of linear wave dynamics in the shallow-water context. In particular, too large velocity spaces of many mixed discretizations as a rule lead to difficulties in the performance of momentum advection in eddying regimes, and continuous representation of scalar quantities creates many inconveniences in hydrostatic codes. While the need for improved advection schemes tells on its own in favor of either discontinuous Galerkin methods or high-order reconstructions with finite volumes, the key question is how to implement them without significant loss of efficiency. The area of large-scale ocean simulations is almost fully dominated by structured-mesh models which are currently much more accurate and efficient per degree of freedom than models on unstructured meshes. The acceptance of unstructured-mesh technology on this `large-scale background' depends on our progress with this question as well as ability to show on practical examples that refinement is more practical and consistent than nesting.