Monitoring the meridional overturning circulation in the North Atlantic: A model-based array design study

A monitoring system for the meridional overturning circulation (MOC) is deployed into an eddy-permitting numerical model (FLAME) at three different latitudes in the North Atlantic Ocean. The MOC is estimated by adding contributions related to Ekman transports to those associated with the zonally integrated vertical velocity shear. Ekman transports are inferred from surface wind stress, whereas the velocity shear is derived from continuous density observations, principally near the eastern and western boundaries, employing thermal wind balance. The objective is to test the method and array setups for possible real observation in the ocean at the chosen latitudes and to guide similar tests at different latitudes. Different mooring placements are tested, ranging from a minimal setup to the theoretical maximum number of measurements. A relatively small number of vertical density profiles (about 10, the exact number depending on the latitude) can achieve a reconstruction of the MOC similar to one achieved by any larger number of profiles. However, the main characteristics of the MOC can only be reproduced at latitudes where bottom velocities are small, here at 26N and 36N. For high bottom velocities, in FLAME at 53N, the array fails to reproduce the strength and variability of the MOC because the depth-averaged flow cannot be reconstructed accurately. In FLAME, knowledge of the complete bottom velocity field could substitute for the knowledge of the depth-averaged velocity field

On the influence of submarine ridges on translation and stability of Agulhas rings

A series of experiments with a quasi‐geostrophic model have been carried out to investigate the influence of topographic obstacles on the translatory movement of Agulhas rings. The rings were initialized as Gaussian‐shaped anomalies in the stream function field of a two‐layer ocean at rest. Bottom topography consisted of a meridional ridge of constant height in the middle of the quadratic model domain. The vertical ring structure, the initial ring position, and the height of the ridge were varied. The general northwestward movement of the model eddies has been shown to be modified toward a more equatorward direction by encountering the upslope of the ridge. Sufficient topographic heights and strong slopes can even block the eddies and force them toward a pure meridional movement. During their translation the eddies lose their vertical coherence. After about 150 days the eddy can only be detected by the surface signal, while the lower layer eddy is dispersed by the radiation of Rossby waves. The passage of “young” (regarding the time between their initialization and their contact with the ridge) and energetic eddies is accompanied by the observation of along‐slope currents of significant strength. These may be due to the rectification of radiated Rossby waves at the topographic slope. Only eddies with a significant dynamic signal in the lower layer are influenced by the bottom topography. Strong, shallow eddies over deep lower layers can cross the ridge without strong modification of their translatory movement

Model simulations of CFC uptake in North Atlantic Deep Water: Effects of parameterizations and grid resolution

A series of numerical experiments with models of the Atlantic Ocean is analyzed with respect to the uptake of CFC‐11 and its export from the subpolar gyre with the North Atlantic Deep Water. We discuss the influence of parameterizations for air‐sea gas exchange and subgrid‐scale processes on the rate of CFC‐11 that enters the North Atlantic Ocean and its dependence on horizontal grid spacing in models from medium (4/3°) to eddy‐permitting (1/3°) horizontal resolution. Model results are compared with observational estimates of tracer inventories in order to evaluate to what degree the simulations capture realistic CFC distributions. While higher resolution is needed to model details of the CFC distribution, for example, in the Deep Western Boundary Current, the medium resolution models are able to simulate quantitatively satisfying CFC inventories in different water masses. Nevertheless, the inventories derived from the medium‐resolution experiments show a critical dependence on details of the parameterization of the mixing effect of mesoscale eddies and on the representation of bottom boundary layer processes. The numerical representation of eddy activity turns out to be of crucial importance in order to obtain modeled CFC inventories in agreement with observed values, which can be achieved either by carefully choosing the mixing parameterization or by applying higher horizontal resolution. The ratio of CFC‐11 being exported southward from the subpolar North Atlantic to the total CFC‐11 inventory in NADW does not vary significantly over the suite of model experiments