Energetics of the global ocean: The role of mesoscale eddies

This article reviews the energy cycle of the global ocean circulation, focusing on the role of baroclinic mesoscale eddies. Two of the important effects of mesoscale eddies are: (i) the flattening of the slope of large-scale isopycnal surfaces by the eddy-induced overturning circulation, the basis for the Gent–McWilliams parametrization; and (ii) the vertical redistribution of the momentum of basic geostrophic currents by the eddy-induced form stress (the residual effect of pressure perturbations), the basis for the Greatbatch–Lamb parametrization. While only point (i) can be explained using the classical Lorenz energy diagram, both (i) and (ii) can be explained using the modified energy diagram of Bleck as in the following energy cycle. Wind forcing provides an input to the mean KE, which is then transferred to the available potential energy (APE) of the large-scale field by the wind-induced Ekman flow. Subsequently, the APE is extracted by the eddy-induced overturning circulation to feed the mean KE, indicating the enhancement of the vertical shear of the basic current. Meanwhile, the vertical shear of the basic current is relaxed by the eddy-induced form stress, taking the mean KE to endow the eddy field with an energy cascade. The above energy cycle is useful for understanding the dynamics of the Antarctic Circumpolar Current. On the other hand, while the source of the eddy field energy has become clearer, identifying the sink and flux of the eddy field energy in both physical and spectral space remains major challenges of present-day oceanography. A recent study using a combination of models, satellite altimetry, and climatological hydrographic data shows that the western boundary acts as a “graveyard” for the westward-propagating eddies

A model for the inertial recirculation of a gyre

This paper considers the time-mean circulation of wind-driven ocean gyres in the limit referred to as inertial or almost-free. In this limit, potential vorticity is conserved following the flow with sources and sinks of potential vorticity balancing in an integral sense around the gyre. Approximate analytic solutions are obtained for a continuously stratified quasi-geostrophic ocean by neglecting the relative vorticity in the gyre interiors. The solutions have features similar to those found in the western part of ocean basins both in eddy-resolving numerical models and in observations. In particular, a deep westward recirculation, such as proposed by Worthington (1976) for the Gulf Stream system, arises naturally from the analysis as an enhanced barotropic flow inside the region where the bowl containing the circulation has intersected the ocean floor. This flow, which is driven by eddies and dissipated by bottom friction, leads to a sudden increase in westward velocity similar to that found between 35N and 36N in the long-term current records along 55W discussed by Schmitz (1977, 1978, 1980)

Barotropic variability in the presence of an ocean gyre

We describe results from some idealized numerical calculations in which we examine the influence of a barotropically stable mean flow on wind-driven variability in a flat-bottomed barotropic vorticity equation model. The mean flow has features in common with the vertically integrated time-mean circulation found in eddy-resolving models with a Sverdrup interior, western boundary current and inertial recirculation region. We integrate the equations of motion linearized about this flow and driven by oscillating wind forcing and compare the results with those obtained when the mean state is one of rest. We find that the presence of a mean flow leads to significant distortion of the model response particularly near the western boundary and in the inertial recirculation region. This distortion is characterized by downstream amplification and phase lag compared to the rest mean state cases. It is related, on the one hand, to the distortion of the mean potential vorticity contours from lines of latitude and, on the other, to advection by the mean flow. Possible applications of these results are discussed to explain features of observed variability in the Gulf Stream system and also the anomalous southward intrusion of the Oyashio Current along the coast of Japan

A decadal oscillation due to the coupling between an ocean circulation model and a thermodynamic sea-ice model

3-dimensional, planetary-geostrophic, ocean general circulation model is coupled to a thermodynamic sea-ice model. The thermal coupling takes account of the insulating effect of the ice. A simple approach is taken in the case of the freshwater flux by allowing this to pass through the ice, except that some is used for snow accumulation. It is then modified by salinity rejection/dilution due to freezing/melting. The model has idealized box geometry extending 60° in both latitude and longitude, with a horizontal resolution of 2° and 14 vertical levels. Annual mean surface forcings are used. The coupled system is first spun up using restoring conditions on both surface temperature and surface salinity to reach a steady state which includes ice in the high latitudes. A switch of the surface forcing to mixed boundary conditions (restoring on temperature and flux on salinity) leads to an oscillation of period 17 years in the magnitude of the thermohaline circulation and the ice extent. The oscillation is due to a feedback between ice cover and ocean temperature. Since ice forms only in regions where the ocean loses heat to the atmosphere, the thermal insulation of an increased ice cover makes the ocean warmer. The thermohaline circulation plays a role in transporting this heat polewards, which in turn melts the ice. The heat loss over open water at high latitudes then leads to ice formation and the process repeats itself. Salinity rejection/dilution associated with ice formation/melting is shown to be of secondary importance in this oscillation. Rather, changes in surface salinity are dominated by changes in deep convection and the associated vertical mixing, which are themselves associated with the reduction in surface heat loss due to the insulating effect of the ice. As a consequence the model exhibits the negative correlation between surface salinity and ice extent that is observed in the high latitude North Atlantic

On the Interpentadal Variability of the North Atlantic Ocean: Model Simulated Changes in Transport, Meridional Heat Flux and Coastal Sea Level Between 1955-1959 and 1970-1974

Previous studies by Greatbatch et al. (1991) indicate significant changes in the North Atlantic thermohaline structure and circulation between the pentads 1955–1959 and 1970–1974, using data analyzed by Levitus (1989a,b,c) and a simple diagnostic model by Mellor et al. (1982). In this paper these changes are modeled using a three-dimensional, free surface, coastal ocean model. Diagnostic and short-term prognostic calculations are used to infer the dynamically adjusted fields corresponding to the observed hydrographic and wind stress climatology of each pentad. While the results agree with earlier studies indicating that the Gulf Stream was considerably weaker (by about 30 Sv) during the 1970s compared to the 1950s, they also indicate some changes in the poleward heat transport, although the statistical significance of these changes relative to sampling errors in the data is not clear. The change of wind pattern between the two pentads, associated with changes in sea surface temperature, resulted in changes in the Ekman contribution to the poleward heat flux transport. The modeled sea level along the North American coast shows a sea level rise of about 5–10 cm between 1955–1959 and 1970–1974; a comparison with observed sea level at 15 tide gage stations shows good agreement. Most of the coastal sea level change is attributed to changes in thermohaline ocean circulation and wind stress; thermal expansion seems to play a lesser role. The methodology tested here demonstrates an effective way to estimate climate changes in ocean circulation and sea level from observed hydrographic data and winds using ocean models to enhance and analyze the data

Application of a barotropic model to North Atlantic synoptic sea level variability

A barotropic, shallow-water model of the North Atlantic is used to investigate variability in adjusted sea level on time scales of a few days to a few months (by “adjusted,” we mean that the inverse barometer is removed from both the model-computed sea level and the observations). The model has 1/3° × 0.4° resolution in latitude and longitude, respectively, and is forced using atmospheric pressure and wind stress data derived from European Centre for Medium Range Weather Forecasts (ECMWF, 1994) analyses. The model results are compared with coastal tide gauge data. Along the western boundary, from St. John\u27s, Newfoundland, to Fernandina Beach, Florida, coherence squared between model and data is greater than 0.5 in the period range 3 to 10 days. South of Cape Hatteras, the model underestimates the amplitude seen in the data, with much better agreement north of the Cape. Model performance on the eastern boundary is generally poor. We suggest this is because on the eastern boundary, the shelf width is much narrower, compared to the internal radius of deformation, than on the western boundary. In addition, the model resolution is insufficient to adequately represent the shelf on the eastern boundary. The poorer agreement south of Cape Hatteras may be due Gulf Stream effects not accounted for by the model dynamics. Finally, we discuss the model-computed variability in the ocean interior

A thermocline model for ocean-climate studies

A 3-dimensional planetary geostrophic (PG) ocean general circulation model in spherical coordinates is formulated to examine the thermocline structure and thermohaline circulation of an idealized ocean basin. The model equations consist of full prognostic temperature and salinity equations and diagnostic momentum equations. A simple linear friction is used to close the barotropic circulation at the western boundary. An extensive sensitivity study is conducted with different model parameters and processes. The results are also compared with those obtained using the Bryan-Cox primitive equation model. For the steady state case, the PG model can reproduce the primitive equation model results, and displays a similar sensitivity for a variety of model parameters, but with much lower computational cost. With higher vertical diffusivity and lower horizontal resolution than primitive equation models, the PG model simulates comparable currents and thermocline depth. This difference is attributed to the large horizontal eddy viscosity used in primitive equation models, but absent in the PG model formulation. The model also illustrates the crucial role of convective overturning in providing a source of cold, dense water at depth. Implications of these results to 2-dimensional zonally averaged models are discussed. In particular, we show that parameterizations used in zonally averaged models to relate the east-west pressure difference to the north-south pressure gradient are not valid when convective overturning is turned off. Finally, the model can be used to efficiently investigate time-dependent transient problems of interest in climate studies, such as the effect of seasonally varying surface forcings, without the need to use asynchronous time-stepping techniques

Interdecadal variability and oceanic thermohaline adjustment

Changes in the strength of the thermohaline overturning circulation are associated, by geostrophy, with changes in the east-west pressure difference across an ocean basin. The tropical-polar density contrast and the east-west pressure difference are connected by an adjustment process. In flat-bottomed ocean models the adjustment is associated with viscous, baroclinic Kelvin wave propagation. Weak-high latitude stratification leads to the adjustment having an interdecadal timescale. We reexamine model interdecadal oscillations in the context of the adjustment process, for both constant flux and mixed surface boundary conditions. Under constant surface flux, interdecadal oscillations are associated with the passage of a viscous Kelvin wave around the model domain. Our results suggest the oscillations can be self-sustained by perturbations to the western boundary current arising from the southward boundary wave propagation. Mixed boundary condition oscillations are characterized by the eastward, cross-basin movement of salinity-dominated density anomalies, and the westward return of these anomalies along the northern boundary. We suggest the latter is associated with viscous Kelvin wave propagation. Under both types of boundary conditions, the strength of the thermohaline overturning and the tropical-polar density contrast vary out of phase. We show how the phase relationship is related to the boundary wave propagation. The importance of boundary regions indicates an urgent need to examine the robustness of interdecadal variability in models as the resolution is increased, and as the representation of the coastal, shelf/slope wave guide is improved. (Abriged abstract)Comment: 20 pages, AGU LaTeX, 12 figures included using epsfig, to appear in JGR, complete manuscript also available at ftp://crosby.physics.mun.ca/pub/drew/papers/gp1.ps.g

Atmospheric Response to the North Pacific Enabled by Daily Sea Surface Temperature Variability

Ocean–atmosphere interactions play a key role in climate variability on a wide range of time scales from seasonal to decadal and longer. The extratropical oceans are thought to exert noticeable feedbacks on the atmosphere especially on decadal and longer time scales, yet the large-scale atmospheric response to anomalous extratropical sea surface temperature (SST) is still under debate. Here we show, by means of dedicated high-resolution atmospheric model experiments, that sufficient daily variability in the extratropical background SST needs to be resolved to force a statistically significant large-scale atmospheric response to decadal North Pacific SST anomalies associated with the Pacific Decadal Oscillation (PDO), which is consistent with observations. The large-scale response is mediated by atmospheric eddies. This implies that daily extratropical SST fluctuations must be simulated by the ocean components and resolved by the atmospheric components of global climate models to enable realistic simulation of decadal North Pacific sector climate variability