Description: The time dependent circulation of the North Brazil Current is studied with three numerical ocean circulation models, which differ by the vertical coordinate used to formulate theprimitive equations. The models are driven with the same surface boundary conditions and their horizontal grid-resolution (isotropic, 1/3° at the equator) is in principle fine enough topermit the generation of mesoscale eddies. Our analysis of the mean seasonal currents concludes that the volume transport of the North Brazil Current (NBC) at the equator isprincipally determined by the strength of the meridional overturning, and suggests that the return path of the global thermohaline circulation is concentrated in the NBC. Models whichsimulate a realistic overturning at 24°N of the order of 16-18 Sv also simulate a realistic NBC transport of nearly 35 Sv comparable to estimates deduced from the most recentobservations. In all models, the major part of this inflow of warm waters from the South Atlantic recirculates in the zonal equatorial current system, but the models also agree on theexistence of a permanent coastal mean flow to the north-west, from the equator into the Carribean Sea, in the form of a continuous current or a succession of eddies. Importantdifferences are found between models in their representation of the eddy field. The reasons invoked are the use of different subgrid-scale parameterisations, and differences instability of the NBC retroflection loop because of differences in the representation of the effect of bottom friction according to the vertical coordinate that is used. Finally, even ifdifferences noticed between models in the details of the seasonal mean circulation and water mass properties could be explained by differences in the eddy field, nonetheless themajor characteristics (mean seasonal currents, volume and heat transports) appears to be at first order driven by the strength of the thermohaline circulation.

Description: Three different, eddy-permitting numerical models are used to examine the seasonal variation ofmeridional mass and heat flux in the North Atlantic, with a focus on the transport mechanisms inthe subtropics relating to observational studies near 25°N. The models, developed in theDYNAMO project, cover the same horizontal domain, with a locally isotropic grid of 1/3° resolutionin longitude, and are subject to the same monthly-mean atmospheric forcing based on athree-year ECMWF climatology. The models differ in the vertical-coordinate scheme(geopotential, isopycnic, and sigma), implying differences in lateral and diapycnic mixingconcepts, and implementation of bottom topography. As shown in the companion paper ofWillebrand et al. (2001), the model solutions exhibit significant discrepancies in the annual-meanpatterns of meridional mass and heat transport, as well as in the structure of the western boundarycurrent system.Despite these differences in the mean properties, the seasonal anomalies of the meridional fluxesare in remarkable agreement, demonstrating a robust model behavior that is primarily dependenton the external forcing, and independent of choices of numerics and parameterization. The annualrange is smaller than in previous model studies in which wind stress climatologies based on marineobservations were used, both in the equatorial Atlantic (1.4 PW) and in the subtropics (0.4-0.5PW). This is a consequence of a weaker seasonal variation in the zonal wind stresses based onthe ECMWF analysis than those derived from climatologies of marine observations.The similarities in the amplitude and patterns of the meridional transport anomalies betwen thedifferent model realizations provide support for previous model conclusions concerning themechanism of seasonal and intraseasonal heat flux variations: they can be rationalized in terms of atime-varying Ekman transport and their predominantly barotropic compensation at depth. Analysisfor 25°N indicates that the net meridional flow variation at depth is concentrated near the westernboundary, but cannot be inferred from transport measurements in the western boundary currentsystem, because of significant and complex recirculations over the western half of the basin. Themodel results instead suggest that the main requirement for estimating the annual cycle of heatflux through a transoceanic section, and the major source of error in model simulations, is anaccurate knowledge of the wind stress variation.

Description: Seasonal changes in eddy energy are used to investigate the role of high-frequency wind forcingin generating eddy kinetic energy in the oceans. To this end, we analyze two experiments of aneddy-permitting model of the North Atlantic driven by daily and monthly mean wind stress fields,and compare results with corresponding changes in the variance of the wind fields, and relatedresults from previous studies using altimeter and current meter data.With daily wind-stress forcing the model is found to be in general agreement with altimetricobservations and reveal a complex pattern of temporal changes in variability over the NorthAtlantic. Observations and the model indicate enhanced levels of eddy energy during wintermonths over several areas of the northern and, particularly northeastern North Atlantic. Since thewind-generated variability is primarily barotropic, its signal can be detected mostly in thelow-energy regions of the northern and north-eastern North Atlantic, which are remote frombaroclinically unstable currents. There the winter-to-summer difference in simulated eddy kineticenergy caused by the variable wind forcing is 〈0.5cm2 s2 between 30° and 55°N, and is 1-3cm2 s2north of 55°N. Seasonal changes in kinetic energy are insignificant along the path of the NorthAtlantic current and south of about 30°N.The weak depth dependence of the seasonal changes in eddy energy implies that the relativeimportance of wind-generated eddy energy is maximum at depth where the general (baroclinic)variability level is low. Accordingly, a significant correlation is found between the seasonal cycle inthe variance of wind stress and the seasonal cycle in eddy energy over a substantially wider areathan near the surface, notably across the entire eastern North Atlantic between the North AtlanticCurrent and the North Equatorial Current.

Description: A systematic intercomparison of three realistic eddy-permitting models of the North Atlanticcirculation has been performed. The models use different concepts for the discretization of thevertical coordinate, namely geopotential levels, isopycnal layers, terrain-following (sigma)coordinates, respectively. Although these models were integrated under nearly identicalconditions, the resulting large-scale model circulations show substantial differences. The resultsdemonstrate that the large-scale thermohaline circulation is very sensitive to the modelrepresentation of certain localised processes, in particular to the amount and water massproperties of the overflow across the Greenland-Scotland region, to the amount of mixing withina few hundred kilometers south of the sills, and to several other processes at small or sub-gridscales. The different behaviour of the three models can to a large extent be explained as aconsequence of the different model representation of these processes.

Description: Im Rahmen der Satellitenmission GRACE werden die ermittelten Massenvariationen gegen Ozeanbodendruckanomalien, die einerseits aus Ozeanmodellen und andererseits aus in-situ Bodendruckdaten gewonnen werden, validiert. Bei den Ozeanmodellen handelt es sich zum einen um das Finite Element Sea Ice-Ocean Model FESOM, ein globales 3D Ozeanmodell, das die primitiven Gleichungen (Navier-Stokes-Gleichungen) in der hydrostatischen Approximation und die Advektions-Diffusionsgleichungen für Temperatur und Salzgehalt auf einem horizontal formal unstrukturierten Gitter löst, und zum anderen um das Hamburg Large Scale Geostrophic Model LSG, ein globales 3D Ozeanmodell, das ebenfalls auf den primitiven Gleichungen basiert, aber im Gegensatz zu FESOM Satellitendaten und Ozeanmessungen assimiliert. Für die Validierung durch in-situ Daten sind vom Alfred-Wegener-Institut (AWI) an verschiedenen Positionen im Antarktischen Zirkumpolarstrom und im tropischen Atlantik Drucksensoren am Meeresboden ausgelegt worden. Aufgrund der Zusammenarbeit mit Chris Hughes (POL, Liverpool) stehen weitere in-situ Daten aus einer weltweiten Ozeanbodendruck-Datenbank für die GRACE-Validierung bereit. Durch die Bestimmung von Kohärenzradien um die Positionen der Druckrekorder mit Hilfe der Ozeanmodelle lassen sich die über die entsprechenden Gebiete gemittelten GRACE Daten mit den Punktmessungen vergleichen. Um den Einfluss von synoptischer Variabilität der Atmosphäre auf den Ozeanbodendruck für verschiedene Zeitskalen zu untersuchen, wurde eine Hierarchie von Simulationen durchgeführt. Erste Ergebnisse weisen darauf hin, dass GRACE zirkulationsbedingte Bodendruckanomalien nur eingeschränkt, aber globale und (bedingt) hemisphärische Massenvariationen gut wiedergibt.

Description: A finite-element coupled sea ice--ocean model (FESOM) has been developed at the Alfred Wegener Institute for Polar and Marine Research.The sea-ice component is a dynamic-thermodynamic model with an elastic-viscous-plastic rheology.The ocean component is the primitive-equation Finite Element Ocean Model (FEOM).An eight-compartment model of the marine ecosystem, featuring nitrate and silicate cycles and considering possible iron limitation, has been implemented.The coupled model has been configured (1) in a circumpolar domain covering the Southern Ocean between the coast of Antarctica and 48S, and (2) on a global grid with $1.5^\circ$ mean resolution.Multi-decadal simulations have been performed in both configurations using atmospheric forcing data from the NCEP reanalysis. The circumpolar model has also been integrated with atmospheric forcing from the ECHAM5-MPIOM coupled climate model.Results are analysed with regard to ice concentration and thickness, as well as ocean hydrography and circulation.All model setups yield stable integrations and give quite reasonable results. In simulations forced by NCEP reanalysis data, summer ice coverage in the western Weddell Sea is substantially underestimated.This deficiency is cured by using forcing from the ECHAM5-MPIOM simulations, which contain substantially lower temperatures and a different wind pattern in this area.This indicates that the underestimation of ice concentration and thickness in the northwestern Weddell Sea, which is typical to many models of the Southern Ocean, is not due to model deficiences but produced by a poor representation of the Antarctic Peninsula in the NCAR/NCEP reanalysis model.For winter conditions, however, especially outside the peninsula region, NCEP forcing data yield very good results.Results from a wide range of sensitivity studies confirm the crucial importance of a carefully chosen adaptive mixing scheme to parameterize vertical and horizontal mixing.Specifically, to reproduce Southern Ocean hydrography and circulation at the same time, it turned out to be essential to use isopycnic (instead of horizontal) diffusion and to scale lateral diffusivities with the horizontal grid scale.

Description: A finite-element coupled sea ice--ocean model (FESOM) has been developed at the Alfred Wegener Institute for Polar and Marine Research.The sea-ice component is a dynamic-thermodynamic model with an elastic-viscous-plastic rheology.The ocean component is the hydrostatic, primitive-equation Finite Element Ocean Model (FEOM).An eight-compartment model of the marine ecosystem, featuring nitrate and silicate cycles and considering possible iron limitation, has been implemented.The coupled model has been configured in a circumpolar domain covering the Southern Ocean between the coast of Antarctica and 48S, and on a global grid with $1.5^\circ$ mean resolution.Multi-decadal simulations have been performed in both configurations with a surface forcing derived from atmospheric reanalysis datasets.The model features a realistic representation of sea-ice coverage and large-scale ocean circulation.Results from a wide range of sensitivity studies confirm the crucial importance of a carefully chosen adaptive mixing scheme to parameterize vertical and horizontal mixing.Artificial passive tracers are used to identify water mass formation pathways.With the right choice of parameters and parameterizations, the model is able to reproduce observed water mass properties and formation processes realistically.