Abstract Revolutionary observational arrays, together with a new generation of ocean and climate models, have provided new and intriguing insights into the Atlantic Meridional Overturning Circulation (AMOC) over the last two decades. Theoretical models have also changed our view of the AMOC, providing a dynamical framework for understanding the new observations and the results of complex models. In this paper we review recent advances in conceptual understanding of the processes maintaining the AMOC. We discuss recent theoretical models that address issues such as the interplay between surface buoyancy and wind forcing, the extent to which the AMOC is adiabatic, the importance of mesoscale eddies, the interaction between the middepth North Atlantic Deep Water cell and the abyssal Antarctic Bottom Water cell, the role of basin geometry and bathymetry, and the importance of a three‐dimensional multiple‐basin perspective. We review new paradigms for deep water formation in the high‐latitude North Atlantic and the impact of diapycnal mixing on vertical motion in the ocean interior. And we discuss advances in our understanding of the AMOC's stability and its scaling with large‐scale meridional density gradients. Along with reviewing theories for the mean AMOC, we consider models of AMOC variability and discuss what we have learned from theory about the detection and meridional propagation of AMOC anomalies. Simple theoretical models remain a vital and powerful tool for articulating our understanding of the AMOC and identifying the processes that are most critical to represent accurately in the next generation of numerical ocean and climate models.
The nonlinear, three-dimensional behavior of baroclinic fronts in a barotropic deformation field is investigated. A major finding is that baroclinic instability of the frontal zone can play an important role in limiting frontogenesis forced by the large-scale deformation. This results in a statistically equilibrated state in which the front oscillates about a mean vertical shear and frontal width. This equilibration mechanism is effective over a wide range of parameter space and is relevant to a variety of fronts in both the ocean and the atmosphere. Sufficiently strong deformation fields, however, can stabilize the baroclinic jet, yielding the two-dimensional result in which the frontogenesis is ultimately limited by the model subgrid-scale mixing parameterization. The time-dependent three-dimensional equilibrated state is achieved for those cases in which perturbations can grow to sufficient amplitude such that the nonlinearities counteract the frontal steepening induced by the large-scale deformation field through the large amplitude baroclinic wave cycle and resulting heat flux. The regimes of the steady equilibrated state and the time-dependent equilibrated state are predicted well by an application of Bishop's linear model of time-dependent wave growth. The vertical heat flux and subduction rate are dominated by the essentially two-dimensional ageostrophic circulation resulting from the large-scale deformation field, not by the eddy heat flux associated with baroclinic instability. The ageostrophic horizontal and vertical circulations, and vertical heat flux and subduction rates, are discussed and compared to various oceanic observations.
Abstract For more than five decades, the Mediterranean Sea has been identified as a region of so‐called thermohaline circulation, namely, of basin‐scale overturning driven by surface heat and freshwater exchanges. The commonly accepted view is that of an interaction of zonal and meridional conveyor belts that sink at intermediate or deep convection sites. However, the connection between convection and sinking in the overturning circulation remains unclear. Here we use a multidecadal eddy‐permitting numerical simulation and glider transport measurements to diagnose the location and physical drivers of this sinking. We find that most of the net sinking occurs within 50 km of the boundary, away from open sea convection sites. Vorticity dynamics provides the physical rationale for this sinking near topography: only dissipation at the boundary is able to balance the vortex stretching induced by any net sinking, which is hence prevented in the open ocean. These findings corroborate previous idealized studies and conceptually replace the historical offshore conveyor belts by boundary sinking rings . They challenge the respective roles of convection and sinking in shaping the oceanic overturning circulation and confirm the key role of boundary currents in ventilating the interior ocean.
The outflow through Denmark Strait shows remarkable mesoscale variability characterized by the continuous formation of intense mesoscale cyclones just south of the sill. These cyclones have a diameter of about 30 km and clear signatures at the sea surface and in currents measured near the bottom. They have a remnant of Arctic Intermediate Water (AIW) in their core. The authors' hypothesis is that these cyclones are formed by stretching of the high potential vorticity (PV) water column that outflows through Denmark Strait. The light, upper layer of the outflow, the East Greenland Current, remains on the surface in the Irminger Sea, while the dense overflow water descends the east Greenland continental slope. The midlevel waters, mostly AIW, could thus be stretched by more than 100%, which would induce very strong cyclonic relative vorticity. The main test of this new hypothesis is by way of numerical experiments carried out with an isopycnal coordinate ocean model configured to have a marginal sea connected to a deep ocean basin by a shallow strait. An outflow is produced by imposing buoyancy forcing over the marginal sea. If the buoyancy forcing is such as to produce a single overflow layer (analogous to the overflows through the Strait of Gibraltar and the Faroe Bank Channel), then the resulting overflow is slightly time dependent. If the buoyancy forcing is such as to produce both a deep overflow and a midlevel outflow (analogous to the AIW), then the resulting outflow is highly time dependent and develops intense midlevel cyclones just south of the sill where the dense overflow water begins to descend the continental slope. The cyclones found in the numerical solutions have time and space scales set by the midlevel outflow transport, the bottom slope, and the deep stratification. Their scales and structure are roughly consistent with the cyclones observed south of the sill in Denmark Strait. High PV outflow through Denmark Strait is a result of the large-scale wind and buoyancy forcing over the Norwegian–Greenland Sea and Denmark Strait's location on a western boundary. So far as we know, this configuration and this specific form of mesoscale variability are unique to Denmark Strait.
Abstract Benthic inputs of nutrients help support primary production in the Chukchi Sea and produce nutrient‐rich water masses that ventilate the halocline of the western Arctic Ocean. However, the complex biological and redox cycling of nutrients and trace metals make it difficult to directly monitor their benthic fluxes. In this study, we use radium‐228, which is a soluble radionuclide produced in sediments, and a numerical model of an inert, generic sediment‐derived tracer to study variability in sediment inputs to the Chukchi Sea. The 228 Ra observations and modeling results are in general agreement and provide evidence of strong benthic inputs to the southern Chukchi Sea during the winter, while the northern shelf receives higher concentrations of sediment‐sourced materials in the spring and summer due to continued sediment‐water exchange as the water mass traverses the shelf. The highest tracer concentrations are observed near the shelfbreak and southeast of Hanna Shoal, a region known for high biological productivity and enhanced benthic biomass.
The Community Climate System Model (CCSM) has been created to represent the principal components of the climate system and their interactions. Development and applications of the model are carried out by the U.S. climate research community, thus taking advantage of both wide intellectual participation and computing capabilities beyond those available to most individual U.S. institutions. This article outlines the history of the CCSM, its current capabilities, and plans for its future development and applications, with the goal of providing a summary useful to present and future users. The initial version of the CCSM included atmosphere and ocean general circulation models, a land surface model that was grafted onto the atmosphere model, a sea-ice model, and a "flux coupler" that facilitates information exchanges among the component models with their differing grids. This version of the model produced a successful 300-yr simulation of the current climate without artificial flux adjustments. The model was then used to perform a coupled simulation in which the atmospheric CO2 concentration increased by 1 % per year. In this version of the coupled model, the ocean salinity and deep-ocean temperature slowly drifted away from observed values. A subsequent correction to the roughness length used for sea ice significantly reduced these errors. An updated version of the CCSM was used to perform three simulations of the twentieth century's climate, and several projections of the climate of the twenty-first century. The CCSM's simulation of the tropical ocean circulation has been significantly improved by reducing the background vertical diffusivity and incorporating an anisotropic horizontal viscosity tensor. The meridional resolution of the ocean model was also refined near the equator. These changes have resulted in a greatly improved simulation of both the Pacific equatorial undercurrent and the surface countercurrents. The interannual variability of the sea surface temperature in the central and eastern tropical Pacific is also more realistic in simulations with the updated model. Scientific challenges to be addressed with future versions of the CCSM include realistic simulation of the whole atmosphere, including the middle and upper atmosphere, as well as the troposphere; simulation of changes in the chemical composition of the atmosphere through the incorporation of an integrated chemistry model; inclusion of global, prognostic biogeochemical components for land, ocean, and atmosphere; simulations of past climates, including times of extensive continental glaciation as well as times with little or no ice; studies of natural climate variability on seasonal-to-centennial timescales; and investigations of anthropogenic climate change. In order to make such studies possible, work is under way to improve all components of the model. Plans call for a new version of the CCSM to be released in 2002. Planned studies with the CCSM will require much more computer power than is currently available.
Abstract The factors that determine the heat transport and overturning circulation in marginal seas subject to wind forcing and heat loss to the atmosphere are explored using a combination of a high-resolution ocean circulation model and a simple conceptual model. The study is motivated by the exchange between the subpolar North Atlantic Ocean and the Nordic Seas, a region that is of central importance to the oceanic thermohaline circulation. It is shown that mesoscale eddies formed in the marginal sea play a major role in determining the mean meridional heat transport and meridional overturning circulation across the sill. The balance between the oceanic eddy heat flux and atmospheric cooling, as characterized by a nondimensional number, is shown to be the primary factor in determining the properties of the exchange. Results from a series of eddy-resolving primitive equation model calculations for the meridional heat transport, overturning circulation, density of convective waters, and density of exported waters compare well with predictions from the conceptual model over a wide range of parameter space. Scaling and model results indicate that wind effects are small and the mean exchange is primarily buoyancy forced. These results imply that one must accurately resolve or parameterize eddy fluxes in order to properly represent the mean exchange between the North Atlantic and the Nordic Seas, and thus between the Nordic Seas and the atmosphere, in climate models.
Abstract : LONG-TERM GOAL. The long-term goal of this project is to understand the physical processes that control the low-frequency variability of and exchange of water masses, properties, and materials across shelf-break fronts. OBJECTIVE. The near-term objectives are to gain fundamental insights into the role of mixed baroclinic / barotropic instabilities and the bottom boundary layer in the evolution of the shelf-break front density structure, eddy formations, and cross-front exchange.
