Abstract

The National Center for Atmospheric Research’s Climate System Model is a comprehensive model of the physical climate system. A 300-yr integration of the model has been carried out without flux correction. The solution shows very little drift in the surface temperature distribution, sea-ice extent, or atmospheric circulation. The lack of drift in the surface climate is attributed to relatively good agreement in the estimates of meridional heat transport in the uncoupled ocean model and that implied by the uncoupled atmospheric model. On the other hand, there is significant drift in the temperature and salinity distributions of the deep ocean. The ocean loses heat at an area-averaged rate of 0.35 W m⁻², the upper ocean becomes fresher, and the deep ocean becomes colder and saltier than in the uncoupled ocean model equilibrium or in observations. The cause of this drift is an unreasonably large meridional transport of freshwater in the sea ice model, resulting in the production of excessively cold and salty Antarctic Bottom Water. There is also significant drift in the Arctic basin, with the complete erosion of the surface halocline early in the coupled integration.

Abstract

For the first time estimates of divergent eddy heat flux (DEHF) from a high-resolution (0.1°) simulation of the Parallel Ocean Program (POP) are compared with estimates made during the Kuroshio Extension System Study (KESS). The results from POP are in good agreement with KESS observations. POP captures the lateral and vertical structure of mean-to-eddy energy conversion rates, which range from 2 to 10 cm² s⁻³. The dynamical mechanism of vertical coupling between the deep and upper ocean is the process responsible for DEHFs in POP and is in accordance with baroclinic instability observed in the Gulf Stream and Kuroshio Extension. Meridional eddy heat transport values are ~14% larger in POP at its maximum value. This is likely due to the more zonal path configuration in POP. The results from this study suggest that HR POP is a useful tool for estimating eddy statistics in the Kuroshio Extension region, and thereby provide guidance in the formulation and testing of eddy mixing parameterization schemes.

Abstract

Present-day control and 1% yr⁻¹ increasing carbon dioxide runs have been made using two versions of the Community Climate System Model, version 3.5. One uses the standard versions of the ocean and sea ice components where the horizontal resolution is 1° and the effects of mesoscale eddies are parameterized, and the second uses a resolution of 1/10° where the eddies are resolved. This is the first time the parameterization has been tested in a climate change run compared to an eddy-resolving run. The comparison is made not straightforward by the fact that the two control run climates are not the same, especially in their sea ice distributions. The focus is on the Antarctic Circumpolar Current region, where the effects of eddies are of leading order. The conclusions are that many of the differences in the two carbon dioxide transient forcing runs can be explained by the different control run sea ice distributions around Antarctica, but there are some quantitative differences in the meridional overturning circulation, poleward heat transport, and zonally averaged heat uptake when the eddies are parameterized rather than resolved.

Abstract

The global distributions of the air–sea fluxes of heat and freshwater and water mass transformation rates from a control integration of the coupled National Center for Atmospheric Research (NCAR) Climate System Model (CSM) are compared with similar fields from an uncoupled ocean model equilibrium spinup and a new surface climatology. The climatology and uncoupled model use the same bulk-flux forcing scheme and are forced with National Centers for Environmental Predicition (formerly the National Meteorological Center) atmospheric reanalysis data and satellite-based cloud cover, solar flux, and precipitation estimates. The climatological fluxes for the open ocean are adjusted to give a global net balance and are in broad general agreement with standard ship-based estimates. An exception is the ice-free Southern Ocean, where the net heat and evaporative fluxes appear to be too weak but where the observational coverage underlying the reanalyis is quite poor. Major differences are observed between the climatology and the NCAR CSM coupled solution, namely, enhanced tropical and subtropic solar insolation, stronger energy and hydrologic cycles, and excessive high-latitude ice formation/melt producing a several-fold increase in Arctic and Antarctic deep water formation through brine rejection. The anomalous fluxes and corresponding water-mass transformations are closely tied to the coupled ocean model drift, characterized by a reorganization of the vertical salinity distribution. Some error features in the heat flux and sea surface temperature fields are common to both the coupled and uncoupled solutions, primarily in the western boundary currents and the Antarctic circumpolar current, and are thus likely due to the poor representation of the circulation field in the coarse-resolution NCAR ocean model. Other problems particular to the uncoupled spinup are related to the bulk-flux forcing scheme, an example being excess freshwater deposition in the western boundary currents arising from the inclusion of a weak open ocean surface salinity restoring term. The effective thermal restoring coefficent, which relates the change in nonsolar surface heat flux to sea surface temperature changes, is on average 14.6 W m⁻² K⁻¹ for the coupled solution or about a third of the range from the bulk flux forcing scheme, 40–60 W m⁻² K⁻¹.

Abstract

The National Center for Atmospheric Research (NCAR) Ocean Model has been developed for use in NCAR’s Climate System Modeling project, a comprehensive development of a coupled ocean–atmosphere–sea ice–land surface model of the global climate system. As part of this development, new parameterizations of diffusive mixing by unresolved processes have been implemented for the tracer equations in the model. Because the strength of the mixing depends upon the density structure under these parameterizations, it is possible that local explicit mixing may be quite small in selected locations, in contrast to the constant diffusivity model generally used. When a spatially centered advection scheme is used in the standard model configuration, local overshooting of tracer values occurs, leading to unphysical maxima and minima in the fields. While the immediate problem is a local Gibbs’s phenomenon, there is the possibility that such local tracer anomalies might propagate by advection and diffusion far from the source, causing inaccuracies in the tracer fields globally.

Because of these issues, a third-order upwind scheme was implemented for the advection of tracers. Numerical experiments show that this scheme is computationally efficient compared to alternatives (such as the flux-corrected transport scheme) and that it works well with other aspects of the model, such as acceleration (important for spinup efficiency) and the new mixing parameterizations. The scheme mimimizes overshooting effects while keeping the dissipative aspect of the advective operator reasonably small. The net effect is to produce solutions in which the large-scale fields are affected very little while local extrema are nearly (but not completely) removed, leading to physically much more realistic tracer patterns.

Abstract

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.

Abstract

The water mass transformation (WMT) framework describes how water of one class, such as a discrete interval of density, is converted into another class via air–sea fluxes or interior mixing processes. This paper investigates how this process is modified at the surface when mesoscale ocean eddies are present, using a state-of-the-art high-resolution climate model with reasonable fidelity in the Southern Ocean. The method employed is to coarse-grain the high-resolution model fields to remove eddy signatures, and compare the results with those from the full model fields. This method shows that eddies reduced the WMT by 2–4 Sv (10%–20%; 1 Sv ≡ 10⁶ m³ s⁻¹) over a wide range of densities, from typical values of 20 Sv in the smoothed case. The corresponding water mass formation was reduced by 40% at one particular density increment, namely, between 1026.4 and 1026.5 kg m⁻³, which corresponds to the lighter end of the range of Indian Ocean Mode Water in the model. The effect of eddies on surface WMT is decomposed into three terms: direct modulation of the density outcrops, then indirectly, by modifying the air–sea density flux, and the combined effect of the two, akin to a covariance. It is found that the first and third terms dominate, i.e., smoothing the outcrops alone has a significant effect, as does the combination of smoothing both outcrops and density flux distributions, but smoothing density flux fields alone has little effect. Results from the coarse-graining method are compared to an alternative approach of temporally averaging the data. Implications for climate model resolution are also discussed.

Abstract

Observational and model evidence has been mounting that mesoscale eddies play an important role in air–sea interaction in the vicinity of western boundary currents and can affect the jet stream storm track. What is less clear is the interplay between oceanic and atmospheric meridional heat transport in the vicinity of western boundary currents. It is first shown that variability in the North Pacific, particularly in the Kuroshio Extension region, simulated by a high-resolution fully coupled version of the Community Earth System Model matches observations with similar mechanisms and phase relationships involved in the variability. The Pacific decadal oscillation (PDO) is correlated with sea surface height anomalies generated in the central Pacific that propagate west preceding Kuroshio Extension variability with a ~3–4-yr lag. It is then shown that there is a near compensation of O(0.1) PW (PW ≡ 10¹⁵ W) between wintertime atmospheric and oceanic meridional heat transport on decadal time scales in the North Pacific. This compensation has characteristics of Bjerknes compensation and is tied to the mesoscale eddy activity in the Kuroshio Extension region.

Abstract

A key question in climate modeling is to what extent sea surface temperature and upper-ocean heat content are driven passively by air–sea heat fluxes, as opposed to forcing by ocean dynamics. This paper investigates the question using a climate model at different resolutions, and observations, for monthly variability. At the grid scale in a high-resolution climate model with resolved mesoscale ocean eddies, ocean dynamics (i.e., ocean heat flux convergence) dominates upper 50 m heat content variability over most of the globe. For deeper depths of integration to 400 m, the heat content variability at the grid scale is almost totally controlled by ocean heat flux convergence. However, a strong dependence on spatial scale is found—for the upper 50 m of ocean, after smoothing the data to around 7°, air–sea heat fluxes, augmented by Ekman heat transports, dominate. For deeper depths of integration to 400 m, the transition scale becomes larger and is above 10° in western boundary currents. Comparison of climate model results with observations show that the small-scale influence of ocean intrinsic variability is well captured by the high-resolution model but is missing from a comparable model with parameterized ocean-eddy effects. In the deep tropics, ocean dynamics dominates in all cases and all scales. In the subtropical gyres at large scales, air–sea heat fluxes play the biggest role. In the midlatitudes, at large scales >10°, atmosphere-driven air–sea heat fluxes and Ekman heat transport variability are the dominant processes except in the western boundary currents for the 400 m heat content.

Abstract

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.