Description: We estimate 3-dimensional ocean currents from hydrographic (ARGO) data by determining the associated circulation in an inverse model. While velocities are treated diagnostic as an instantaneous steady state response temperature and salinity are allowed to change slowly with their inter-annual variability. Annual mean solutions are presented for 1999 to 2008. Altimetry referenced to a geoid provides a mean dynamic topography that determines the large scale surface circulation. AGRO data extend this information further into the ocean. It appeared useful to regularize the solution by constraining deep velocities to be small or as in our case to be close to a prognostic model simulation. Altimetry alone already improves temperature and salinity fields while ARGO data are less useful in constraining the dynamic topography. Heat, volume and overturning transports are in general agreement with previous work. Their inter-annual variability appears to be large in comparison to possible trends. Transport variances are estimated by perturbing the input data in a Monte Carlo simulation. They are smaller than the changes between consecutive years. ARGO data coverage seems to be reliable for this work after the year 2002.

Description: The coupled sea-ice ocean model FESOM is based on the finite element method and hydrostatic primitive equa- tions. Both the ocean and ice modules are discretized on the same triangular surface meshes, allowing direct exchange of fluxes and fields between the two components. Recently, this model has been employed to resolve multi-scale dynamics in the Arctic Ocean. This work is conducted using global simulations with local mesh re- finement in the Arctic Ocean. The global background resolution is 1.5 degree. Two simulations with different resolutions of 24km and 9km in the Arctic region have been conducted. The model performance in the Arctic Ocean is assessed comparing the available observational data. The comparison results demonstrate that the model can well simulate the general ocean circulation and important ocean and sea ice processes, although model param- eters and/or forcing for the focus of the Arctic Ocean modeling need to be further optimized in the future work. The sensitivity of the model performance to local mesh refinement in the Arctic Ocean is analyzed.

Description: Das Finite Element Sea ice Ocean Model FESOM ist mit dem atmosphärischen Zirkulationsmodell ECHAM gekoppelt worden. Durch das unstrukturierte Gitter des Ozeanmodells ist es möglich, das Gitter des Ozeanmodells sehr variabel zu gestalten, um Schlüsselregionen oder Regionen von besonderem Interesse in hoher Auflösung zu simulieren, während andere Regionen niedriger aufgelöst bleiben. In unseren Sensitivitätsstudien nutzen wir verschieden hohe Auflösungen in der Arktis und in den nördlichen mittleren Breiten. Kombiniert mit variierten Modellparametern in der Atmosphäre und im Ozean wie z.B. Treibhausgaskonzentrationen, Schwerewellenwiderstand und Meereisalbedo erreicht das gekoppelte System zwei unterschiedliche Zustände, einen mit stark ausgeprägter atlantischer meridionaler Umwälzzirkulation und deutlichem Irminger-Strom und einen mit schwach ausgeprägter atlantischer meridionaler Umwälzzirkulation und schwachem Irminger-Strom. Im ersten Zustand beträgt die Stärke der atlantischen meridionalen Umwälzzirkulation zwischen 20 und 25 Sverdrup während der zweite Zustand zeitweise nur zwischen 5 und 10 Sverdrup aufweist. Die Beobachtungen liegen mit durchschnittlich 18 Sverdrup dazwischen. Um physikalische Ursachen für diese Unterschiede feststellen zu können, werden verschiedene Zeitpunkte der Simulation mit stark ausgeprägter atlantischer meridionaler Umwälzzirkulation ausgewählt. Der Zustand des Ozeans und der Atmosphäre an diesen ausgewählten Zeitpunkten wird verwendet, um Modellsimulationen mit der Modellversion, mit der eine schwach ausgeprägte atlantische meridionale Umwälzzirkulation simuliert wird, zu initialisieren. Anschließend wird die mittlere Abweichung der auf diese Weise initialisierten Modellsimulationen von der Stammsimulation nach Tagen, Wochen, Monaten und Jahren analysiert, um festzustellen, in welchen Gebieten sich die ersten Unterschiede einstellen und wie sich diese ausbreiten. Dies wird anschließend wiederholt, indem in weiteren Simulationen sukzessive die einzelnen Unterschiede zwischen den Modellversionen zugeschaltet werden, um herauszufinden, welche Modelländerung den größten Einfluss hat.

Description: Unstructured meshes offer geometric flexibility. In the context of large-scale ocean modeling they enable simulations with a regional focus in an otherwise global setup, without nesting or open boundaries. We follow this concept by developing and exploiting FESOM, the Finite-Element Sea ice-Ocean circulation Model. A brief review of current FESOM-assisted research will be given, to illustrate what is possible to achieve in the framework of this concept. In particular, we present results from studies of freshwater transport through the Canadian Arctic Archipelago, simulations of the impact of Greenland ice sheet melting on the sea level, results from the high-resolution (about 9 km) Arctic runs, as well as results related to the dense water formation around Antarctica. In most case we are dealing with coarse global setups (around 1 degree), refined to 3 - 20 km in the area of interest. FESOM is now coupled to ECHAM5 and ECHAM6 atmospheric models and is also used in climate studies, with the same basic concept of focus on regional dynamics. There are numerous challenges both on computational and numerical sides which have to be solved to ensure wider acceptance of unstructured meshes by the oceanographic community. First, the numerical efficiency of unstructured-mesh models has to be essentially improved, which includes data storage, solvers, domain decomposition, load balance and other related issues. Second, we see the need for more accurate (less dissipative) numerical transport algorithms. FESOM is based on the stabilized P1-P1 discretization. We are exploring two closely related finite-volume discretizations (vertex-vertex and cell-vertex) which promise higher numerical efficiency. A brief review of related efforts will be given.

Description: The Finite Element Sea-ice Ocean Model (FESOM) is formulated on unstructured meshes and offers geometrical flexibility which is difficult to achieve on traditional structured grids. In this work, the performance of FESOM in the North Atlantic and Arctic Ocean on large time scales is evaluated in a hindcast experiment. A water-hosing experiment is also conducted to study the model sensitivity to increased freshwater input from Greenland Ice Sheet (GrIS) melting in a 0.1-Sv discharge rate scenario. The variability of the Atlantic Meridional Overturning Circulation (AMOC) in the hindcast experiment can be explained by the variability of the thermohaline forcing over deep convection sites. The model also reproduces realistic freshwater content variability and sea ice extent in the Arctic Ocean. The anomalous freshwater in the water-hosing experiment leads to significant changes in the ocean circulation and local dynamical sea level (DSL). The most pronounced DSL rise is in the northwest North Atlantic as shown in previous studies, and also in the Arctic Ocean. The released GrIS freshwater mainly remains in the North Atlantic, Arctic Ocean and the west South Atlantic after 120 model years. The pattern of ocean freshening is similar to that of the GrIS water distribution, but changes in ocean circulation also contribute to the ocean salinity change. The changes in Arctic and sub-Arctic sea level modify exchanges between the Arctic Ocean and subpolar seas, and hence the role of the Arctic Ocean in the global climate. Not only the strength of the AMOC, but also the strength of its decadal variability is notably reduced by the anomalous freshwater input. A comparison of FESOM with results from previous studies shows that FESOM can simulate past ocean state and the impact of increased GrIS melting well.

Description: We estimate 3-dimensional ocean currents from hydrographic (ARGO) data by determining the associated circulation in an inverse model. While velocities are treated diagnostic as an instantaneous steady state response temperature and salinity are allowed to change slowly with their inter-annual variability. Annual mean solutions are presented for 1999 to 2008. Altimetry referenced to a geoid provides a mean dynamic topography that determines the large scale surface circulation. AGRO data extend this information further into the ocean. It appeared useful to regularize the solution by constraining deep velocities to be small or as in our case to be close to a prognostic model simulation. Altimetry alone already improves temperature and salinity fields while ARGO data are less useful in constraining the dynamic topography. Heat, volume and overturning transports are in general agreement with previous work. Their inter-annual variability appears to be large in comparison to possible trends. Transport variances are estimated by perturbing the input data in a Monte Carlo simulation. They are smaller than the changes between consecutive years. ARGO data coverage seems to be reliable for this work after the year 2002.

Description: We describe the global configuration of a coupled atmosphere/ocean model. The atmosphere is simulated by the ECHAM5 and the ocean by the Finite-Element Sea-Ice Ocean Model (FESOM), which supports unstructured meshes and allows for variable resolution. Coupling between structured and unstructured meshes is a technically challenging task due to different geometry, resolution and representation of coastlines in both components. This has been achieved via the parallel OASIS4 coupler and additional use of a regular exchange mesh. The latter has been introduced in the ocean model. The conservation of flux moments requires additional care since model grids are different in both components. The heat and moisture fluxes are computed in the atmospheric model so that the flux variance is defined by the resolution in the atmosphere. Since it is problematic to downscale the variance onto the fine resolved parts of the ocean, a few interpolation techniques are suggested. We validate the coupled setup on the basis of an integration run for 300 years.