Research Catalog: CGD's Oceanography

The CCSM3 Contribution to IPCC ARF has been analyzed as a coupled climate system, and a number of new ocean and sea-ice science results can be found in the CCSM Special Issue of the Journal of Climate. Danabasoglu et al. (2006) have found that the Equatorial Pacific Cold Bias is reduced by far more than expected from rectification, though an atmospheric feedback when the daily-mean solar radiation received each daily coupling interval is distributed realistically over 12 daylight hours. The ocean's role in the Hydrological Cycle of the couple climate system of CCSM3 has been analyzed in Hack et al (2006).

Gent et al. (2000) show that the Uptake of Chlorofluorocarbon-11 (CFC-11) is simulated very well compared with ocean observations taken between 1980 and 2000 but there are regional biases. They also compare the ocean heat uptake between 1957 and 1996 in the ensemble of CCSM3 20th Century runs to the recent observational estimates of the secular trend and find that the ensemble average takes up 25% more heat. There are two changes in Ocean Heat Uptake that occur near sea-ice cover in response to increasing CO2 (Bitz et al., 2006). One is a warming in the Arctic Ocean at about 200m depth that is caused by enhanced heat transport in the North Atlantic. The second is a warming of the ocean below 500m in the Southern Ocean that is caused by weaker deep convection in this region. This is due to a freshening of the surface ocean that makes the water column more stable.

The ocean Thermohaline Circulation is important in the oceanic meridional heat transport and uptake of anthropogenic CO2, and it may be susceptible to abrupt changes. In CCSM3 there is a decrease in the overturning mass transport of approximately 25% of the unperturbed mean at the time of CO2 quadrupling (Bryan, et al., 2006). Ideal age changes, a direct measure of the change in ocean ventilation rates, indicate significant increases in age in the subpolar North Atlantic and Southern ocean, but decreases in age in the deep thermocline of the tropics and subtropics in response to the slowing of the thermohaline circulation (Bryan, et al., 2006, in press).

The Sea-Ice Cover in CCSM3 control integrations compares reasonably well to observations, although the winter ice cover in both hemisphere is too extensive. The influence of the simulated sub-grid scale ice thickness distribution, which is a parameterization new to CCSM, has been assessed and found to modify the mean climate and polar feedbacks (Holland et al., 2006). In particular, it enhances the surface albedo feedback and the ice thickness-ice growth rate feedback.

Coupled Analysis

This coupled analysis continues along several other fronts, including Arctic Climate Change. An analysis of the future trajectory of the Arctic sea ice cover has been performed using a number of IPCC-AR4 model results. CCSM3 exhibits abrupt reductions in the future summer ice cover that are driven by an increased efficiency of open water formation as the ice thins, rapid increases in ocean heat transport that appear to lead and trigger the events, and the albedo feedback that accelerates the ice retreat. Additional studies have examined projected changes in the Arctic Ocean freshwater budgets and their influence on the northern North Atlantic (Holland et al., 2006, in press) and are investigating the climate under seasonally ice-free Arctic conditions.

Antarctic Sea-Ice conditions in 20th century simulations have also been assessed (Holland and Raphael, 2006). In particular, the mean conditions and interannual variability from six IPCC-AR4 models, including CCSM3, have been found to simulate a reasonable Antarctic dipole in the ice cover with anomalies of opposite sign in the Atlantic and Pacific sectors. This variability is influenced to different degrees in different models by the simulated El Niño-Southern Oscillation and the Southern Annular Mode.

There is an ongoing search for the physical mechanism(s) responsible for the multidecadal oscillations present in the coupled ocean solutions. This variability is particularly prominent in the ocean's Meridional Overturning Circulation. Some other relevant questions are: How does this oscillation affect our assessment of the 20th century and future scenario climates? What are the effects on predictability? How do we initialize our ensemble integrations in the presence of this oscillation?

The future scenario integrations are being analyzed to investigate the role of the Ocean Thermostat in regulating the maximum ocean temperature in regions of coral reef. The ocean thermostat can play an important role in how these ecosystems fare within an increasing Green-House-Gas environment.

Ocean Observations

Ocean Observations are an essential element of modeling studies and model development. The Working Group on Ocean Modeling Development has adopted a series of Coordinated Ocean Research Experiments (CORE), and the air-sea forcing for these has been updated through 2006 and modified to include a wind direction adjustment and a different surface humidity. The companion global air-sea flux fields have been computed and analyzed. Sea surface salinity is much less well observed than temperature, and efforts on behalf of the CLIVAR Salinity Working Group and the NASA AQUARIUS Satellite Science team are aimed at narrowing this gap.

Physics of the Ocean

The Physics of the Ocean as a forced, rather than coupled, system is also a primary research goal. The mechanisms of interannual and Decadal Ocean Variability are being studied using ocean model simulations run in hindcast mode. The above CORE forcing is used to drive ocean and ocean-ice coupled simulations over the latter half of the 20th century. The relative importance of terms governing variability in upper ocean temperature and heat content has been diagnosed (Doney et al 2006), as have the origins of interannual spice variations (T and S anomalies on density surfaces) in the ocean interior. Recent work on ocean spice has focused on demonstrating the remarkable extent to which the ocean model behavior is corroborated by the latest ocean observations.

The concept of water mass age as a measure of Ocean Ventilation time scales described above in relation to the thermohaline circulation can be generalized to an age spectrum, or transit time distribution, describing the probability density function of ages in a fluid subject to turbulent mixing in addition to mean advection. Efforts are underway to compute the transit time distribution for the global ocean using an eddy-resolving global ocean general circulation model.

Ocean Biogeochemistry

Ocean Biogeochemistry is an important component of the coupled carbon cycle modeling effort with CCSM. Extensive experiments have been performed with the BEC ecosystem model at low resolution in the fully coupled CCSM3. Biases in the coupled physics project onto the BEC model. Improvements in the physical solution at higher ocean and atmosphere resolution are leading to improved BEC simulations.

CCSM3 Biases

Improving CCSM3 BIASES is a priority of the CCSM Advisory Board, the CGD Advisory Committee, and CCSM scientists. The major errors in ocean surface temperature and salinity have been exposed, and their sources inferred (Large and Danabasoglu, 2000). The largest biases are found along the subtropical eastern ocean boundaries with North America, South America, and Southern Africa. Efforts to resolve the processes responsible are focused on the coastal air-sea coupling in the Nested Regional Climate Modeling work with MMM. A major goal is to improve on the coupled representation of ENSO. The important factors have been found to include: strengthening the equatorial zonal windstress, improving the viscosity used in the FV atmosphere, reducing the ocean viscosity to permit tropical instability waves that remove the cold bias in the equatorial cold tongue, improving the representation of the Indonesian throughflow, and hence the SST around the maritime continent and the thermocline structure in the Indian Ocean. Many of these issues are being investigated as part of a national Tropical Biases Workshop.

Ocean Model Development

Continual effort on Ocean Model Development is needed to maintain a state-of-the-art ocean model for coupled climate studies, as well as ocean physics research. A low resolution version of CCSM3 (Yeager et al., 2006) has been developed for use in studies of paleoclimate, long term climate impacts, coupled sensitivities, and other applications where model computational cost and throughput are an issue. An off-line tracer transport model is being formulated for paleoclimate problems. Major ocean model developments are proceeding under the auspices of the CLIVAR Climate Process Teams (CPTs) on both gravity current entrainment and eddy mixed layer interaction. The former has resulted in a parameterization of the ocean exchange through the Strait of Gibraltar that is being adapted to the Denmark Strait and Faroe Bank Channel. For the latter, implementation of a near-surface Eddy Flux Parameterization (Danabasoglu et al. 2006) and a new prescription for the surface intensification and abyssal reduction of the tracer diffusivities (Danabasoglu and Marshall 2006) result in improved model solutions. These improvements include the elimination of strong, near-surface, eddy-induced circulations, and potential temperature distributions that compare more favorably with observations.

A new Tracer Advection scheme has been implemented and evaluated in POP. Compared to the previously used scheme, the new scheme greatly reduces advective false extrema, while not causing excessive numerical diffusion. The new scheme is being used for active and passive tracers.

While the fidelity of High-Resolution Ocean Simulations has increased considerably over the last several years, there remain significant biases in aspects of the simulated circulation and substantial uncertainties in the robustness of the results with respect to parameterization choices. A suite of North Atlantic model simulations at resolutions between 0.1° and 0.4° have been analyzed to investigate the convergence properties of the solutions (Bryan, et al., 2006, in press). The most realistic simulations are not found at the lowest levels of dissipation, and acceptable solutions are found in only a very limited range of parameter space. The lessons learned in these experiments are now being applied to configuration of global eddy-resolving ocean models for applications in climate and ocean transport studies.

Sea-Ice Modeling

To maintain the leading edge in Sea-Ice Modeling, there has been a transition to the Community Ice CodE (CICE) from the Los Alamos National Laboratory, which includes improved mechanical redistribution, generalized snow-layer treatment, and numerous computational enhancements. There are also two new local developments that are being evaluated in concert. First, to better represent the snow and sea-ice albedo feedback, a new solar radiation parameterization for sea-ice has been developed (Briegleb and Light, 2006). It will afford more consistent tuning for present climate, more accurate simulation of control climate annual cycle and variability, and provide increased confidence in simulations of future climate change. Second, a new physical parameterization for sea-ice meltponds explicitly keeps track of meltwater volume and a derived area fraction of meltponds that is used to compute an albedo for the meltpond covered regions and area-averaged to obtain an overall gridcell sea-ice albedo.

Administration of CCSM

The Administration of CCSM has been a very significant burden during 2006. Positions held include the chair and a member of the CCSM Science Steering Committee, co-chair of the CCSM Polar Climate Working Group, and co-chair of the Ocean Working Group. Also, there were liaison responsibilities for the Biogeochemistry, Paleoclimate, Polar Climate, and Ocean working groups.

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