Description: Many models of the large-scale thermohaline circulation in the ocean exhibit strong zonally integrated upwelling in the midlatitude North Atlantic that significantly decreases the amount of deep water that is carried from the formation regions in the subpolar North Atlantic toward low latitudes and across the equator. In an analysis of results from the Community Modeling Effort using a suite of models with different horizontal resolution, wind and thermohaline forcing, and mixing parameters, it is shown that the upwelling is always concentrated in the western boundary layer between roughly 30° and 40°N. The vertical transport across 1000 m appears to be controlled by local dynamics and strongly depends on the horizontal resolution and mixing parameters of the model. It is suggested that in models with a realistic deep-water formation rate in the subpolar North Atlantic, the excessive upwelling can be considered as the prime reason for the typically too low meridional overturning rates and northward heat transports in the subtropical North Atlantic. A new isopycnal advection and mixing parameterization of tracer transports by mesoscale eddies yield substantial improvements in these integral measures of the circulation.

Description: The authors use different versions of the model of the wind- and thermohaline-driven circulation in the North and Equatorial Atlantic developed under the WOCE Community Modeling Effort to investigate the mean flow pattern and deep-water formation in the subpolar region, and the corresponding structure of the basin-scale meridional overturning circulation transport. A suite of model experiments has been carded out in recent years, differing in horizontal resolution (1° × 1.2°, 1/3° × 0.4°, 1/6° × 0.2°), thermohaline boundary conditions, and parameterization of small-scale mixing. The mass transport in the subpolar gyre and the production of North Atlantic Deep Water (NADW) appears to be essentially controlled by the outflow of dense water from the Greenland and Norwegian Seas. in the present model simulated by restoring conditions in a buffer zone adjacent to the boundary near the Greenland–Scotland Ridge. Deep winter convection homogenizes the water column in the center of the Labrador Sea to about 2000 m. The water mass properties (potential temperature about 3°C, salinity about 34.9 psu) and the volume (1.1×1053 km3) of the homogenized water are in fair agreement with observations. The convective mixing has only little effect on the net sinking of upper-layer water in the subpolar gyre. Sensitivity experiments show that the export of NADW from the subpolar North Atlantic is more strongly affected by changes in the overflow conditions than by changes in the surface buoyancy fluxes over the Labrador and Irminger Seas, even if these suppress the deep convection completely. The host of sensitivity experiments demonstrates that realistic meridional overturning and heat transport distributions for the North Atlantic (with a maximum of 1 PW) can be obtained with NADW production rates of 15–16 Sv, provided the spurious upwelling of deep water that characterizes many model solutions in the Gulf Stream regime is avoided by adequate horizontal resolution add mixing parameterization.

Description: Analyses of ocean observations and model simulations suggest that there have been considerable changes in the thermohaline circulation (THC) during the last century. These changes are likely to be the result of natural multidecadal climate variability and are driven by low-frequency variations of the North Atlantic Oscillation (NAO) through changes in Labrador Sea convection. Indications of a sustained THC weakening are not seen during the last few decades. Instead, a strengthening since the 1980s is observed. The combined assessment of ocean hydrography data and model results indicates that the expected anthropogenic weakening of the THC will remain within the range of natural variability during the next several decades

Description: A model of the subpolar North Atlantic Ocean is used to study different aspects of ventilation and water mass transformation during a year with moderate convection intensity in the Labrador Sea. The model realistically describes the salient features of the observed hydrographic structure and current system, including boundary currents and recirculations. Ventilation and transformation rates are defined and compared. The transformation rate of Labrador Sea Water (LSW), defined in analogy to several observational studies, is 6.3 Sv (Sv ≡ 106 m3 s−1) in the model. Using an idealized ventilation tracer, mimicking analyses based on chlorofluorocarbon inventories, an LSW ventilation rate of 10 Sv is found. Differences between both rates are particularly significant for those water masses that are partially transformed into denser water masses during winter. The main export route of the ventilated LSW is the deep Labrador Current (LC). Backward calculation of particle trajectories demonstrates that about one-half of the LSW leaving the Labrador Sea within the deep LC originates in the mixed layer during that same year. Near the offshore flank of the deep LC at about 55°W, the transformation of LSW begins in January and is at a maximum in February/March. While the export of transformed LSW out of the central Labrador Sea continues for several months, LSW generated near the boundary current is exported more rapidly, with maximum transport rates during March/April within the deep LC.

Description: The causes and characteristics of interannual–decadal variability of the meridional overturning circulation (MOC) in the North Atlantic are investigated with a suite of basin-scale ocean models [the Family of Linked Atlantic Model Experiments (FLAME)] and global ocean–ice models (ORCA), varying in resolution from medium to eddy resolving (½°–1/12°), using various forcing configurations built on bulk formulations invoking atmospheric reanalysis products. Comparison of the model hindcasts indicates similar MOC variability characteristics on time scales up to a decade; both model architectures also simulate an upward trend in MOC strength between the early 1970s and mid-1990s. The causes of the MOC changes are examined by perturbation experiments aimed selectively at the response to individual forcing components. The solutions emphasize an inherently linear character of the midlatitude MOC variability by demonstrating that the anomalies of a (non–eddy resolving) hindcast simulation can be understood as a superposition of decadal and longer-term signals originating from thermohaline forcing variability, and a higher-frequency wind-driven variability. The thermohaline MOC signal is linked to the variability in subarctic deep-water formation, and rapidly progressing to the tropical Atlantic. However, throughout the subtropical and midlatitude North Atlantic, this signal is effectively masked by stronger MOC variability related to wind forcing and, especially north of 30°–35°N, by internally induced (eddy) fluctuations.

Description: The authors present the first quantitative comparison between new velocity datasets and high-resolution models in the North Atlantic subpolar gyre [1/10° Parallel Ocean Program model (POPNA10), Miami Isopycnic Coordinate Ocean Model (MICOM), ° Atlantic model (ATL6), and Family of Linked Atlantic Ocean Model Experiments (FLAME)]. At the surface, the model velocities agree generally well with World Ocean Circulation Experiment (WOCE) drifter data. Two noticeable exceptions are the weakness of the East Greenland coastal current in models and the presence in the surface layers of a strong southwestward East Reykjanes Ridge Current. At depths, the most prominent feature of the circulation is the boundary current following the continental slope. In this narrow flow, it is found that gridded float datasets cannot be used for a quantitative comparison with models. The models have very different patterns of deep convection, and it is suggested that this could be related to the differences in their barotropic transport at Cape Farewell. Models show a large drift in watermass properties with a salinization of the Labrador Sea Water. The authors believe that the main cause is related to horizontal transports of salt because models with different forcing and vertical mixing share the same salinization problem. A remarkable feature of the model solutions is the large westward transport over Reykjanes Ridge [10 Sv (Sv ≡ 106 m3 s−1) or more]

Description: This paper describes, and establishes the dynamical mechanisms responsible for, the large-scale, time-mean, midlatitude circulation in a high-resolution model of the North Atlantic basin. The model solution is compared with recently proposed transport schemes and interpretations of the dynamical balances operating in the sub-tropical gyre. In particular, the question of the degree to which Sverdrup balance holds for the subtropical gyre is addressed. At 25°N, thermohaline-driven bottom flows cause strong local departures from the Sverdrup solution for the vertically integrated meridional mass transport, but these nearly integrate to zero across the interior of the basin. In the northwestern region of the subtropical gyre, in the vicinity of the Gulf Stream, higher-order dynamics become important, and linear vorticity dynamics is unable to explain the model's vertically integrated transport. In the subpolar gyre, the model transport bears little resemblance to the Sverdrup prediction, and higher-order dynamics are important across the entire longitudinal extent of the basin. The sensitivity of the model transport amplitudes, patterns, and dynamical balances are estimated by examining the solutions under a range of parameter choices and for four different wind stress forcing specifications. Taking into account a deficit of 7–10 Sv (Sv ≡ 106 m3 s−1) in the contribution of the model thermohaline circulation to the meridional transports at 25°N, the wind stress climatology of Isemer and Hasse appears to yield too strong of a circulation, while that derived from the NCAR Community Climate Model yields too weak of a circulation. The Hellerman and Rosenstein and ECMWF climatologies result in wind-driven transports close to observational estimates at 25°N. The range between cases for the annual mean southward transport in the interior above 1000 m is 14 Sv, which is 40%–70% of the mean transport itself. There is little sensitivity to the model closure parameters at this latitude. At 55°N, in the subpolar gyre, there is little sensitivity of the model solution to the choice of either closure parameters or wind climatology, despite large differences in the Sverdrup transports implied by the different wind stress datasets. Large year to year variability of the meridional transport east of the Bahamas makes it difficult to provide robust estimates of the sensitivity of the Antilles and deep western boundary current systems to forcing and parameter changes.