Description: A high-resolution model of the wind-driven and thermohaline circulation in the North and equatorial Atlantic Ocean is used to study the structure and variability of the boundary current system at 26°N, including the Florida Current, the Antilles Current, and the Deep Western Boundary Current (DWBC). The model was developed by Bryan and Holland as a Community Modeling Effort of the World Ocean Circulation Experiment. Subsequent experiments have been performed at IfM Kiel, with different friction coefficients, and different climatologies of monthly mean wind stress: Hellerman–Rosenstein (HR) and Isemer–Hasse (IH). The southward volume transports in the upper 1000 m of the interior Atlantic, at 26°N, are 25.0 Sv (Sv ≡ 106m3s−1) for HR, and 34.9 Sv for IH forcing, in good agreement with the transport from the integrated Sverdrup balance at this latitude (23.9 Sv for HR, 35.6 Sv for IH). The return flow of this wind-driven transport, plus the southward transport of the DWBC (6–8 Sv), is partitioned between the Florida Current and Antilles Current. With HR forcing, the transport through the Straits of Florida is 23.2 Sv; this increases to 29.1 Sv when the wind stresses of IH are used. The annual variation of the simulated Florida Current is very similar to previous, coarse-resolution models when using the same wind-stress climatology (HR); the annual range (3.4 Sv) obtained with HR forcing is strongly enhanced (6.3 Sv) with IH forcing. The meridional heat transport at 26°N, zonally integrated across the basin, is in phase with the Florida Current; its annual range increases from 0.44 PW (HR) to 0.80 PW (IH). The annual signal east of the Bahamas is masked by strong transport fluctuations on a time scale of O(100 days), caused by an instability of the Antilles Current. By averaging over several model years, an annual cycle is extracted, which is in phase with the wind stress curl over the western part of the basin.

Description: To avoid an explicit simulation of the overflows across the Greenland-Scotland ridge, many models of the large-scale ocean circulation seek to include the net effect of the inflowing dense water masses by restoring temperature and salinity near the ridge to observed conditions. In this paper the authors examine the effect of different datasets for the northern restoring condition in two versions, eddy resolving and non-eddy resolving, of the model of the North and equatorial Atlantic that has been developed in recent years as a Community Modeling Effort for WOCE. It is shown that the use of smoothed climatological fields of temperature and salinity south of the Denmark Strait leads to strong deficiencies in the simulation of the deep flow field in the basin. A switch to actual hydrographic data from the Denmark Strait ignites a rapid dynamic response throughout the North Atlantic, affecting the transport and vertical structure of the deep western boundary current and, by virtue of the JEBAR efffect, the transport of the horizontal gyres. Meridional overturning and northward heat transport too weak in the cases with climatological boundary conditions, increase to more realistic levels in the subtropical North Atlantic. The initial response to switches in the high-latitude thermohaline forcing is mediated by fast waves along the westurn boundary, leading to changes in the deep western boundary current in low latitudes after about two years in the non-eddy-resolving cast. The initial timescale depends on the horizontal grid spacing of the model; in the high-resolution case, the first signal reaches the equator in a few months. The adjustment to a new, dynamic quasi equilibrium involves Kelvin waves along the equator and Rossby wave in the interior and is attained in less than two decades throughout the North Atlantic. It is suggested that these fast dynamic adjustment processes could play an important role in possible fluctuations of the thermohaline circulation, or transitions between different equilibrium states of the coupled ocean–atmosphere system, and may have determined the timescale of the observed climatic transitions before and during the last deglaciation.

Description: Global mean and eddy fields from a four-year experiment with a 1/6° × 1/5° horizontal resolution implementation of the CME North Atlantic model are presented. The time-averaged wind-driven and thermohaline circulation in the model is compared to the results of a 1/3° × 2/5° model run in very similar configuration. In general, the higher resolution results are found to confirm that the resolution of previous CME experiments is sufficient to describe many features of the large-scale circulation and water mass distribution quite well. While the increased resolution does not lead to large changes in the mean flow patterns, the variability in the model is enhanced significantly. On the other hand, however, not all aspects of the circulation have improved with resolution. The Azores Current Frontal Zone with its variability in the eastern basin is still represented very poorly. Particular attention is also directed toward the unrealistic stationary anticyclones north of Cape Hatteras and in the Gulf of Mexico.

Description: The ocean takes up a large fraction of the pertubation C02 that enters the atmosphere by human activity. A realistic representation of this uptake in numerical models is essential for future climate studies. Uptake of C02 or other atmospheric trace gases is strongly influenced by oceanic physical variability at spatial scales between 20 and 100 km. Our main goal is to study the effect of this mesoscale variability on the cumulative uptake of anthropogenic C02 and chlorofluorocarbons using an existing model of the ocean circulation in the Atlantic that resolves a significant part of that variability explicitly because of its grid spacing of about 20 km. Results are compared with simulated trace gas distribution obtained from a model with coarser resolution.

Description: The Agulhas system all the interface between the Indian and Atlantic Ocean is an important region in the global oceanic circulation with a recognized key role in global climate and climate change. The simulation of the Agulhas system was performed by a high-resolution regional model nested in a global coarse-resolution ocean model. It is shown that this model simulates all characteristics of the Agulhas regime in a highly realistic manner. Due to the two-way coupling of both models the importance of the Agulhas leakage on the large-scale thermohaline circulation was demonstrated.

Description: Satellite observations and output from a high-resolution ocean model are used to investigate how the Loop Current in the Gulf of Mexico affects the Gulf Stream transport through the Florida Straits. We find that the expansion (contraction) of the Loop Current leads to lower (higher) transports through the Straits of Florida. The associated surface velocity anomalies are coherent from the southwestern tip of Florida to Cape Hatteras. A simple continuity-based argument can be used to explain the link between the Loop Current and the downstream Gulf Stream transport: as the Loop Current lengthens (shortens) its path in the Gulf of Mexico, the flow out of the Gulf decreases (increases). Anomalies in the surface velocity field are first seen to the southwest of Florida and within 4 weeks propagate through the Florida Straits up to Cape Hatteras and into the Gulf Stream Extension. In both the observations and the model this propagation can be seen as pulses in the surface velocities. We estimate that the Loop Current variability can be linked to a variability of several Sverdrups (1Sv = 10(6) m(3) s(-1)) through the Florida Straits. The exact timing of the Loop Current variability is largely unpredictable beyond a few weeks and its variability is therefore likely a major contributor to the chaotic/intrinsic variability of the Gulf Stream. However, the time lag between the Loop Current and the flow downstream of the Gulf of Mexico means that if a lengthening/shortening of the Loop Current is observed this introduces some predictability in the downstream flow for a few weeks.

Description: Oceanic eddies are an important component in preconditioning the central Labrador Sea (LS) for deep convection and in restratifying the convected water. This study investigates the different sources and impacts of Eddy Kinetic Energy (EKE) and its temporal variability in the LS with the help of a 52-year long hindcast simulation of a 1/20° ocean model. Irminger Rings (IR) are generated in the West Greenland Current (WGC) between 60 and 62°N, mainly affect preconditioning and limit the northward extent of the convection area. The IR exhibit a seasonal cycle and decadal variations linked to the WGC strength, varying with the circulation of the subpolar gyre. The mean and temporal variations of IR generation can be attributed to changes in deep ocean baroclinic and upper ocean barotropic instabilities at comparable magnitudes. The main source of EKE and restratification in the central LS are Convective Eddies (CE). They are generated by baroclinic instabilities near the bottom of the mixed layer during and after convection. The CE have a mid-depth core and reflect the hydrographic properties of the convected water mass with a distinct minimum in potential vorticity. Their seasonal to decadal variability is tightly connected to the local atmospheric forcing and the associated air-sea heat fluxes. A third class of eddies in the LS are the Boundary Current Eddies shed from the Labrador Current (LC). Since they are mostly confined to the vicinity of the LC, these eddies appear to exert only minor influence on preconditioning and restratification.

Description: A primitive equation model of an idealized ocean basin, driven by simple, study wind and buoyancy forcing at the surface, is used to study the dynamics of mesoscale eddies. Model statistics of a six-year integration using a fine grid (1/6° × 0.2°), with reduced coefficients of horizontal friction, are compared to those using a coarser grid (1/3° × 0.4°), but otherwise identical configuration. Eddy generation in both model cases is primarily due to the release of mean potential energy by baroclinic instability. Horizontal Reynolds stresses become significant near the midlatitude jet of the fine-grid case, with a tendency for preferred energy transfers from the eddies to the mean flow. Using the finer resolution, eddy kinetic energy nearly doubles at the surface of the subtropical gyre, and increases by factors of 3–4 over the jet region and in higher latitudes. The spatial characteristics of the mesoscale fluctuations are examined by calculating zonal wavenumber spectra and velocity autocorrelation functions. With the higher resolution, the dominant eddy scale remains approximately the same in the subtropical gyre but decreases by a factor of 2 in the subpolar areas. The wavenumber spectra indicate a strong influence of the model friction in the coarse-grid case, especially in higher latitudes. Using the coarse grid, there is almost no separation between the energetic eddy scale and the scale where friction begins to dominate, leading to steep spectra beyond the cutoff wavenumber. Using the finer resolution an inertial subrange with a k−3 power law begins to emerge in all model regions outside the equatorial belt. Despite the large increase of eddy intensity in the fine-grid model, effects on the mean northward transport of heat are negligible. Strong eddy fluxes of heat across the midlatitude jet are almost exactly compensated by changes of the heat transport due to the mean flow.