Yanli Jia

Circulation characteristics in three eddy-permitting models of the North Atlantic

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.

Formation of an Azores Current Due to Mediterranean Overflow in a Modeling Study of the North Atlantic

A mechanism for the formation of the Azores Current is proposed. On the basis of observations and model results, it is argued that the primary cause of the Azores Current is the water mass transformation associated with the Mediterranean overflow in the Gulf of Cadiz. Observations show that the transport of the Mediterranean outflow water through the Strait of Gibraltar increases significantly as it descends the continental slope by entraining the overlying North Atlantic Central Water. This entrainment process introduces a sink at the eastern boundary to the ocean upper layer in addition to the inflow into the Mediterranean. Such a sink is capable of inducing strong zonal flows such as the Azores Current. This mechanism is confirmed by numerical experiments with and without the representation of the Mediterranean overflow process. The numerical model is based on the Miami Isopycnic Coordinate Ocean Model. The model does not include the Mediterranean overflow explicitly, but restores the model density fields in the Gulf of Cadiz toward the observations. This restoring condition produces a reasonable representation of the water mass transformation deduced from observations. The formation of the Azores Current in response to the water mass transformation in the Gulf of Cadiz suggests that the Mediterranean overflow is not only a source of warm and saline water at depth, but also has a strong dynamic impact on the ocean upper layer. This study emphasizes the need to improve the representation of the Mediterranean overflow process in general circulation models in order to capture the correct characteristics of the flow fields and water masses in the subtropical eastern North Atlantic.

Open‐ocean convection in the Irminger Sea

Open-ocean deep convection is known to occur in a very few locations in the present climate. Convection is important for the ventilation of the oceans, and for the operation of the meridional overturning circulation. Using data from ships and profiling floats, we present evidence for the occurrence of convection in the Irminger Sea of the North Atlantic, south-east of Greenland. Confirmation of this convective site in the North Atlantic will influence our understanding of the connection of the atmosphere to the ocean depths, and of the mechanisms of climate variability.

On the seasonal variability and eddies in the North Brazil Current: insights from model intercomparison experiments

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.

On the role of the Azores Current in the ventilation of the North Atlantic Ocean

Three high-resolution ocean circulation models of the North Atlantic, differing chiefly in their description of the vertical coordinate, are used to examine the ventilation of the North Atlantic subtropical gyre. All the models produce mode waters of realistic densities in the Sargasso Sea and the European Basin, but have Azores Currents of differing strengths, which are categorised as strong (of realistic transport), intermediate, and weak. These differences have a critical impact upon the ventilation of the gyre. Most importantly, the strong Azores Current forms an effective barrier to the southward ventilation of Eastern North Atlantic Water from the northern European Basin, initially driving it southwestwards into the central gyre, before turning it back eastwards again in a general cyclonic circulation north of the Current. The intermediate and weak Azores Currents instead allow the southward ventilation of this water mass near the European and African coasts. The situation in Nature appears to be intermediate between these two cases, with the Azores Current acting as a partial block. The study also raises the possibility of the Azores Current forming an advective connection between the Sargasso Sea Mode Waters in the western basin and modes of similar densities found in the eastern basin on the southern side of the Current. Although there are high levels of variability in the extent of these lighter modes in the eastern basin in Nature, this postulate is supported by a number of observational studies. In addition, the present study also provides some support for the local production of Madeira Mode Water in the eastern basin, associated with retroflecting current branches on the southern side of the Azores Current. Overall, the Azores Current is, therefore, likely to have a critical impact upon the ventilation of the subtropical gyre over a large area, rather than just locally, affecting the potential vorticity and density structure of the upper ocean between subtropical latitudes and the northern European Basin. The study also contributes to an ongoing community effort to assess the realism of our current generation of ocean models.

Seasonal cycle of meridional heat transport in the subtropical North Atlantic: a model intercomparison in relation to observations near 25°N

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.

Dispersion of Tracers by Ocean Gyres

Abstract The dispersion of a tracer by a two-dimensional gyre circulation is studied using simple numerical models. Two approaches are taken: a random walk model formulated in a streamline coordinate system and the numerical solution of the advection-diffusion equation. A number of different gyres are considered. Attention is focused on the characteristics of the gyre that determine the spreading and mixing time of the tracer. The authors find that the dispersion by a given gyre can be characterized in terms of a bulk Peclet number and the three length scales: L the horizontal width of the gyre, l the width of the boundary current, and L the length of the boundary current. By taking into account the length of the boundary layer, gyre dispersion is found to conform moderately well with previous analytic models, in particular the partitioning between weak and strong diffusive regimes, even though the shear characteristics may be quite variable across the gyre. The analytic models become less valid as the le...

Chlorofluorocarbon constraints on North Atlantic ventilation

The North Atlantic Ocean vigorously ventilates the ocean interior. Thermocline and deep water masses are exposed to atmospheric contact there and are sequestered in two principal classes: Subtropical Mode Water (STMW: 26.5 {le} {sigma}{thetas} {le} 26.8) and Subpolar Mode Water (SPMW: 26.9 {le} {sigma}{thetas} {le} 27.8). These ventilation rates and pathways are uncertain, and a powerful way to estimate them is to monitor the penetration of chlorofluorocarbon (CFC) tracers. Here, a CFC dataset of over 44 000 observations, taken between 1982 and 1998, is combined with a non-eddy-resolving ( resolution) general circulation model of the North Atlantic Ocean. The CFC data are assimilated with the model by optimizing the uncertain air{ndash}sea CFC flux. The assimilated CFC fields are then systematically compared with the observations to identify the best fit and hence the most realistic ventilation. Three GCM experiments are performed this way to find the dependence on model thickness diffusivity. Each GCM solution is close to being statistically consistent with the CFC observations and likely sources of error. Lower diffusivity gives the best match to data although some systematic bias in sequestering tropospheric CFC remains. Lower diffusivity, around 150 m2 s{ndash}1, permits a stronger circulation with a more realistic North Atlantic Current. For this experiment, the subduction rate is around 16 Sv (Sv {equiv} 106 m3 s{ndash}1) in the subtropics and eastern subpolar Atlantic (26.35 {le} {sigma}{thetas} {le} 27.13) averaged over 1975{ndash}95. Around 26 Sv is formed in the Labrador and Irminger Seas (27.58 {le} {sigma}{thetas} {le} 27.8). Only about 40{percnt} of the CFC carried into the subpolar interior by this flux remained there in 1998, however. The rest was returned to the subpolar mixed layer after an average period of 6{ndash}8 yr. In contrast, 70{percnt} of the CFC subducted into the subtropical interior remained there in 1998

Tritium distributions in an isopycnic model of the North Atlantic

An ocean general circulation model with an isopycnic coordinate in the vertical is used to simulate the North Atlantic circulation. Tritium and helium are injected into the model after an initial spin-up phase to assist our understanding of the model response and to validate the model itself by comparing the model data with available tracer observations. In this paper, the distribution of tritium is presented and compared with observations in 1972 (Geochemical Ocean Sections Study) and 1981 (Transient Tracers in the Ocean (TTO)). The vertical penetration of tritium in the model compares well with observations, reflecting the ventilation patterns in the subtropical and subpolar gyres and the deep overflows from the Greenland and Norwegian basin through the Denmark Strait and across the Iceland-Faeroes rise. However, the distributions of tritium on various isopycnic layers do show discrepancies with observations, and the magnitude of tritium concentration is too high in the model. It is found that the model produces a pronounced maximum tritium concentration in the interior of the North Atlantic subtropical gyre in 1972, which is not observed. The presence of this maximum suggests that the ratio of diffusion to advection timescales (the Peclet number) is too high in the model. A simple two-dimensional advection-diffusion model is used to explore the relationships between the distributions of tritium and the timescales of advection and diffusion processes. These experiments suggest an upper bound on the Peclet number. A further experiment using the Atlantic isopycnic model with a decreased Peclet number shows some improvement to the distribution of tritium on the isopycnic layers. Comparisons of tritium-helium age in 1981 between the model results and TTO data show that the model has a too rapid circulation. The lack of mixing by eddies in the model is also believed to be partially responsible for the discrepancies between the modeled tritium distributions and the observations. The good correspondence between our tritium simulation with the Atlantic isopycnic model and observations leads us to believe that the model captures the major pathways of deep ocean circulation in the North Atlantic. The model should be useful for other tracer sequestration studies. Combining models of varying complexity has proved useful in identifying and (partially) correcting deficiencies in the model and will be used in future tracer studies.

A model analysis of the behavior of the Mediterranean Water in the North Atlantic

Abstract The behavior of the Mediterranean Water in the North Atlantic Ocean sector of a global ocean general circulation model is explored, starting from its entry point at the Strait of Gibraltar. The analysis focuses primarily on one experiment in which explicit watermass exchange between the Mediterranean Sea and the Atlantic at the Strait of Gibraltar is permitted. The model produces an exchange rate of approximately 1 Sv (Sv ≡ 106 m3 s−1). This is comparable to estimates derived from field measurements. The density of the Mediterranean outflow, however, is lower than observed, mainly because of its high temperature (more than 2°C higher than in reality). The lower density of the outflow and the model’s inadequate representation of the entrainment mixing in the outflow region cause the Mediterranean Water to settle in a depth range ∼800–1000 m in the North Atlantic, about 200 m shallower than observed. Here an interesting current system forms in response to the intrusion of the Mediterranean Water, i...