Matthew W. Hecht

Numerical Simulation of the North Atlantic Ocean at 1/10°

Abstract In this paper an initial analysis of an 0.1° simulation of the North Atlantic Ocean using a level-coordinate ocean general circulation model forced with realistic winds covering the period 1985–96 is presented. Results are compared to the North Atlantic sector of a global 0.28° simulation with similar surface forcing and to a variety of satellite and in situ observations. The simulation shows substantial improvements in both the eddy variability and the time-mean circulation compared to previous eddy-permitting simulations with resolutions in the range of 1/2°–1/6°. The resolution is finer than the zonal-mean first baroclinic mode Rossby radius at all latitudes, and the model appears to be capturing the bulk of the spectrum of mesoscale energy. The eddy kinetic energy constitutes 70% of the total basin-averaged kinetic energy. Model results agree well with observations of the magnitude and geographical distribution of eddy kinetic energy and sea-surface height variability, with the wavenumber–fre...

Evaluation of ocean model ventilation with CFC-11: comparison of 13 global ocean models

We compared the 13 models participating in the Ocean Carbon Model Intercomparison Project (OCMIP) with regards to their skill in matching observed distributions of CFC-11. This analysis characterizes the abilities of these models to ventilate the ocean on timescales relevant for anthropogenic CO2 uptake. We found a large range in the modeled global inventory (±30%), mainly due to differences in ventilation from the high latitudes. In the Southern Ocean, models differ particularly in the longitudinal distribution of the CFC uptake in the intermediate water, whereas the latitudinal distribution is mainly controlled by the subgrid-scale parameterization. Models with isopycnal diffusion and eddy-induced velocity parameterization produce more realistic intermediate water ventilation. Deep and bottom water ventilation also varies substantially between the models. Models coupled to a sea-ice model systematically provide more realistic AABW formation source region; however these same models also largely overestimate AABW ventilation if no specific parameterization of brine rejection during sea-ice formation is included. In the North Pacific Ocean, all models exhibit a systematic large underestimation of the CFC uptake in the thermocline of the subtropical gyre, while no systematic difference toward the observations is found in the subpolar gyre. In the North Atlantic Ocean, the CFC uptake is globally underestimated in subsurface. In the deep ocean, all but the adjoint model, failed to produce the two recently ventilated branches observed in the North Atlantic Deep Water (NADW). Furthermore, simulated transport in the Deep Western Boundary Current (DWBC) is too sluggish in all but the isopycnal model, where it is too rapid.

Three‐dimensional reconstruction of oceanic mesoscale currents from surface information

The ability to reconstruct the three-dimensional (3D) dynamics of the ocean by an effective version of Surface Quasi-Geostrophy (eSQG) is examined. Using the fact that surface density plays an analogous role as interior potential vorticity (PV), the eSQG method consists in inverting the QG PV generated by sea-surface density only. We also make the extra assumption that sea-surface temperature (SST) anomalies fully represent surface density anomalies. This approach requires a single snapshot of SST and the setup of two parameters: the mean Brunt-Vaisala frequency and a parameter that determines the energy level at the ocean surface. The validity of this approach is tested using an Ocean General Circulation Model simulation representing the North Atlantic in winter. It is shown that the method is quite successful in reconstructing the velocity field at the ocean surface for mesoscales (between 30 and 300 km). The eSQG framework can also be applied to reconstruct subsurface fields using surface information. Results show that the reconstruction of velocities and vorticity from surface fields is reasonably good for the upper 500 m and that the success of the method mainly depends on the quality of the SST as a proxy of the density anomaly at the base of the mixed layer. This situation happens after a mixed-layer deepening period. Therefore the ideal situation for the application of this method would be after strong wind events.

Upwind-weighted advection schemes for ocean tracer transport: An evaluation in a passive tracer context

Centered-in-space, centered-in-time integration has generally been used for the advection of scalars in ocean models. An assessment is made of the implications of centered leapfrog integration in the context of two-dimensional passive tracer advection within a Stommel (1948) gyre. Nonphysical ripples in the tracer field grow to alarming levels in purely advective integrations. Diffusive parameterizations of eddy mixing moderate these ripples, but it is found that Laplacian diffusion greatly reduces the peak amplitude of the tracer field, while biharmonic or weaker Laplacian diffusion allows ripples of large amplitude. Several forward-in-time, upwind-weighted schemes are found to provide better solutions. Smolarkiewiczs (1984) Multi-Dimensional Positive-Definite Advection and Transport Algorithm (MPDATA) scheme is slightly superior for an integration at moderate resolution within which the western boundary current is poorly resolved in typical fashion. Third-order, upwind-based schemes exhibit little sensitivity to the details of multidimensional treatment for this problem of passive tracer advection, with results nearly as good as for MPDATA.

Ocean chlorofluorocarbon and heat uptake during the twentieth century in the CCSM3

Abstract An ensemble of nine simulations for the climate of the twentieth century has been run using the Community Climate System Model version 3 (CCSM3). Three of these runs also simulate the uptake of chlorofluorocarbon-11 (CFC-11) into the ocean using the protocol from the Ocean Carbon Model Intercomparison Project (OCMIP). Comparison with ocean observations taken between 1980 and 2000 shows that the global CFC-11 uptake is simulated very well. However, there are regional biases, and these are used to identify where too much deep-water formation is occurring in the CCSM3. The differences between the three runs simulating CFC-11 uptake are also briefly documented. The variability in ocean heat content in the 1870 control runs is shown to be only a little smaller than estimates using ocean observations. The ocean heat uptake between 1957 and 1996 in the ensemble is compared to the recent observational estimates of the secular trend. The trend in ocean heat uptake is considerably larger than the natural v...

Antarctic Bottom Water Formation and Deep-Water Chlorofluorocarbon Distributions in a Global Ocean Climate Model

Abstract The ocean distributions of chlorofluorocarbons (CFCs) have been measured extensively in order to determine the mechanisms, rates, and pathways associated with thermohaline deep-water formation. Model temperature, salinity, and CFC-11 fields from the National Center for Atmospheric Research (NCAR) global ocean climate model are compared against observations with emphasis on the patterns of Antarctic Bottom Water (AABW) production, properties, and circulation in the Southern Ocean. The model control simulation forms deep water as observed in both the Weddell and Ross Seas, though not along other sectors of the Antarctic coast. Examination of the deep water CFC-11 distribution, total inventory, and profiles along individual observational sections demonstrates that the decadal-scale deep-water ventilation in the model Southern Ocean is both too weak and too restricted to the Ross and Weddell Sea source regions. A series of sensitivity experiments is conducted to determine the factors contributing to ...

The Southern Ocean and Its Climate in CCSM4

AbstractThe new Community Climate System Model, version 4 (CCSM4), provides a powerful tool to understand and predict the earth’s climate system. Several aspects of the Southern Ocean in the CCSM4 are explored, including the surface climatology and interannual variability, simulation of key climate water masses (Antarctic Bottom Water, Subantarctic Mode Water, and Antarctic Intermediate Water), the transport and structure of the Antarctic Circumpolar Current, and interbasin exchange via the Agulhas and Tasman leakages and at the Brazil–Malvinas Confluence. It is found that the CCSM4 has varying degrees of accuracy in the simulation of the climate of the Southern Ocean when compared with observations. This study has identified aspects of the model that warrant further analysis that will result in a more comprehensive understanding of ocean–atmosphere–ice dynamics and interactions that control the earth’s climate and its variability.

Adaptive Extraction and Quantification of Geophysical Vortices

We consider the problem of extracting discrete two-dimensional vortices from a turbulent flow. In our approach we use a reference model describing the expected physics and geometry of an idealized vortex. The model allows us to derive a novel correlation between the size of the vortex and its strength, measured as the square of its strain minus the square of its vorticity. For vortex detection in real models we use the strength parameter to locate potential vortex cores, then measure the similarity of our ideal analytical vortex and the real vortex core for different strength thresholds. This approach provides a metric for how well a vortex core is modeled by an ideal vortex. Moreover, this provides insight into the problem of choosing the thresholds that identify a vortex. By selecting a target coefficient of determination (i.e., statistical confidence), we determine on a per-vortex basis what threshold of the strength parameter would be required to extract that vortex at the chosen confidence. We validate our approach on real data from a global ocean simulation and derive from it a map of expected vortex strengths over the global ocean.

A consideration of tracer advection schemes in a primitive equation ocean model

We consider several tracer advection schemes in the context of a simple sector configuration of a Bryan-Cox-Semtner type ocean model. The schemes are centered-in-time centered-in-space, donor cell upwind differencing, flux corrected transport (FCT), QUICKEST and MPDATA. While we discuss the transport of temperature and salinity, and apply alternative schemes to this transport within our model, we concentrate primarily on a problem which involves passive tracer advection in which vertical gradients are high and concentrations asymptote to zero near the sea surface, as is the case in biogeochemical ocean modeling. We find that flux correction (available optionally with the MPDATA scheme, as well as the FCT scheme) is required in order to produce acceptable results on this challenging problem. A method of supercycling of passive tracers relative to dynamic variables, whereby the time step for passive tracers is larger that that used for the other prognostic variables of the model, is introduced in order to apply these more expensive flux limited schemes efficiently. We also present a general method for the second-order accurate application of one-dimensional forward-in-time advection schemes in three dimensions.

Response of a Strongly Eddying Global Ocean to North Atlantic Freshwater Perturbations

AbstractThe strongly eddying version of the Parallel Ocean Program (POP) is used in two 45-yr simulations to investigate the response of the Atlantic meridional overturning circulation (AMOC) to strongly enhanced freshwater input due to Greenland melting, with an integrated flux of 0.5 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1). For comparison, a similar set of experiments is performed using a noneddying version of POP. The aim is to identify the signature of the salt advection feedback in the two configurations. For this reason, surface salinity is not restored in these experiments. The freshwater input leads to a quantitatively comparable reduction of the overturning strength in the two models. To examine the importance of transient effects in the relation between AMOC strength and density distribution, the results of the eddy-resolving model are related to water mass transformation theory. The freshwater forcing leads to a reduction of the rate of light to dense water conversion in the North Atlantic, but there...