ALBERT

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

Description: Seasonal changes in eddy energy are used to investigate the role of high-frequency wind forcing in generating eddy kinetic energy in the oceans. To this end, we analyze two experiments of an eddy-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 related results from previous studies using altimeter and current meter data. With daily wind-stress forcing the model is found to be in general agreement with altimetric observations and reveal a complex pattern of temporal changes in variability over the North Atlantic. Observations and the model indicate enhanced levels of eddy energy during winter months over several areas of the northern and, particularly northeastern North Atlantic. Since the wind-generated variability is primarily barotropic, its signal can be detected mostly in the low-energy regions of the northern and north-eastern North Atlantic, which are remote from baroclinically unstable currents. There the winter-to-summer difference in simulated eddy kinetic energy caused by the variable wind forcing is 〈0.5 cm2 s2 between 30° and 55°N, and is 1–3 cm2 s2 north of 55°N. Seasonal changes in kinetic energy are insignificant along the path of the North Atlantic current and south of about 30°N. The weak depth dependence of the seasonal changes in eddy energy implies that the relative importance 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 in the variance of wind stress and the seasonal cycle in eddy energy over a substantially wider area than near the surface, notably across the entire eastern North Atlantic between the North Atlantic Current and the North Equatorial Current.

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

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 the primitive 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 to permit 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 is principally 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 which simulate 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 recent observations. 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 the existence 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. Important differences are found between models in their representation of the eddy field. The reasons invoked are the use of different subgrid-scale parameterisations, and differences in stability 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 if differences 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 the major characteristics (mean seasonal currents, volume and heat transports) appears to be at first order driven by the strength of the thermohaline circulation.

Description: This paper presents some research developments in primitive equation ocean models which could impact the ocean component of realistic global coupled climate models aimed at large-scale, low frequency climate simulations and predictions. It is written primarily to an audience of modellers concerned with the ocean component of climate models, although not necessarily experts in the design and implementation of ocean model algorithms.