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: Experiments with a suite of North Atlantic general circulation models are used to examine the sources of eddy kinetic energy (EKE) in the Labrador Sea. A high-resolution model version (112°) quantitatively reproduces the observed signature. A particular feature of the EKE in the Labrador Sea is its pronounced seasonal cycle, with a maximum intensity in early winter, as already found in earlier studies based on altimeter data. In contrast to a previously advanced hypothesis, the seasonally varying eddy field is not related to a forcing by high-frequency wind variations but can be explained by a seasonally modulated instability of the West Greenland Current (WGC). The main source of EKE in the Labrador Sea is an energy transfer due to Reynolds interaction work (barotropic instability) in a confined region near Cape Desolation where the WGC adjusts to a change in the topographic slope: Geostrophic contours tend to converge upstream of Cape Desolation, such that the topographically guided WGC narrows as well and becomes barotropically unstable. The eddies spawned from the WGC instability area, dominating the EKE in the interior Labrador Sea, are predominantly anticyclonic with warm and saline cores in the upper kilometer of the water column, while the few cyclones originating as well from the instability area show a more depth-independent structure. Companion experiments with a ⅓° model exhibit the strength of the WGC, influenced by either changes in the wind stress or heat flux forcing, as a leading factor determining seasonal to interannual changes of EKE in the Labrador Sea

Description: A suite of high-resolution models of the Atlantic Ocean circulation is used to study the deep seasonal current variability in the equatorial regime, with a particular emphasis on its manifestation in the variability of the interhemispheric transports near the western boundary. The basic experiment has a resolution of 1/3∘1/3∘ horizontally and 45 vertical levels, and is subject to a monthly mean atmospheric forcing based on ECMWF flux fields. Sensitivity experiments explore the effects of higher horizontal resolution (1/12∘1/12∘), and alternative mixing parameterizations. The model behavior near the equator confirms previous suggestions based on solutions of the WOCE Community Modelling Effort (“CME”) and the “DYNAMO” model intercomparison project, of the presence of a system of vigorous seasonal current oscillations, spanning the whole water column and nearly the whole zonal extent of the basin. The patterns of the primarily zonal current anomalies are fairly robust across the range of model cases investigated, i.e., show relatively little sensitivity to horizontal resolution/mixing, or to the different choices of vertical discretization and vertical mixing as in the DYNAMO cases. The amplitude of the seasonal variation exceeds 10 cm/s in the surface layer, and decreases to about 5 cm/s near 1000 m and 2–3 cm/s in the deep ocean in both the basic 1/3∘1/3∘- and the 1/12∘1/12∘-cases, thereby leading to seasonally reversing current signatures at all depths below the EUC. A particular aspect of the seasonal current variability concerns its manifestation in the southward transport of North Atlantic Deep Water (NADW) by the Deep Western Boundary Current (DWBC). The temporal characteristics of the DWBC variability are in agreement with moored current meter observations at 44∘W44∘W, with simulated DWBC transports varying between a maximum of more than 30 Sv in January/February, and almost vanishing transport in September. However, in contrast to the annual-mean deep water transport which is confined to the DWBC and tight, O(100) km-recirculation cells, the seasonal cycle of transport is not trapped near the boundary: the simulations show that the zonal current variations of the equatorial wave guide, near the western boundary give rise to a broad system of seasonal recirculation cells of the DWBC. Calculations of the amplitude of the seasonal variability in the deep water transport near the equator are therefore strongly dependent of the spatial extent of the cross-section considered; in particular, for being approximately representative of low-frequency variations in the net, zonally-integrated meridional transport of deep water in the equatorial regime, transport sections would need to extend over nearly the whole western basin.

Description: This study focuses on an important aspect of air–sea interaction in models, namely, large-scale, spurious heat fluxes due to false pathways of the Gulf Stream and North Atlantic Current (NAC) in the “storm formation region” south and east of Newfoundland. Although high-resolution eddy-resolving models show some improvement in this respect, results are sensitive to poorly understood, subgrid-scale processes for which there is currently no complete, physically based parameterization. A simple method to correct an ocean general circulation model (OGCM), acting as a practical substitute for a physically based parameterization, is explored: the recently proposed “semiprognostic method,” a technique for adiabatically adjusting flow properties of a hydrostatic OGCM. The authors show that application of the method to an eddy-permitting model of the North Atlantic Ocean yields more realistic flow patterns and watermass characteristics in the Gulf Stream and NAC regions; in particular, spurious surface heat fluxes are reduced. Four simple modifications to the method are proposed, and their benefits are demonstrated. The modifications successfully account for three drawbacks of the original method: reduced geostrophic wave speeds, damped mesoscale eddy activity, and spurious interaction with topography. It is argued that use of a corrected (eddy permitting) OGCM in a coupled modeling system for simulating present climate (as now becomes possible because of increasing computer power) should lead to a more realistic simulation in regions of strong air–sea interaction as compared with that obtained with an uncorrected model. The method is also well suited for the simulation of the uptake and transport of passive tracers, such as anthropogenic carbon dioxide or components of ecosystem models.

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.

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: 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.