No abstract

The Department of Energy (DOE) supported Parallel Climate Model (PCM) makes use of the NCAR Community Climate Model (CCM3) and Land Surface Model (LSM) for the atmospheric and land surface components, respectively, the DOE Los Alamos National Laboratory Parallel Ocean Program (POP) for the ocean component, and the Naval Postgraduate School sea-ice model. The PCM executes on several distributed and shared memory computer systems. The coupling method is similar to that used in the NCAR Climate System Model (CSM) in that a¯ux coupler ties the components together, with interpolations between the dierent grids of the component models. Flux adjustments are not used in the PCM. The ocean component has 2/3°average horizontal grid spacing with 32 vertical levels and a free surface that allows calculation of sea level changes. Near the equator, the grid spacing is approximately 1/2°in latitude to better capture the ocean equatorial dynamics. The North Pole is rotated over northern North America thus producing resolution smaller than 2/3°in the North Atlantic where the sinking part of the world conveyor circulation largely takes place. Because this ocean model component does not have a computational point at the North Pole, the Arctic Ocean circulation systems are more realistic and similar to the observed. The elastic viscous plastic sea ice model has a grid spacing of 27 km to represent small-scale features such as ice transport through the Canadian Archipelago and the East Greenland current region. Results from a 300 year present-day coupled climate control simulation are presented, as well as for a transient 1% per year compound CO 2 increase experiment which shows a global warming of 1.27°C for a 10 year average at the doubling point of CO 2 and 2.89°C at the quadrupling point. There is a gradual warming beyond the doubling and quadrupling points with CO 2 held constant. Globally averaged sea level rise at the time of CO 2 doubling is approximately 7 cm and at the time of quadrupling it is 23 cm. Some of the regional sea level changes are larger and re¯ect the adjustments in the temperature, salinity, internal ocean dynamics, surface heat¯ux, and wind stress on the ocean. A 0.5% per year CO 2 increase experiment also was performed showing a global warming of 1.5°C around the time of CO 2 doubling and a similar warming pattern to the 1% CO 2 per year increase experiment. El NinÄ o and La NinÄ a events in the tropical Paci®c show approximately the observed frequency distribution and amplitude, which leads to near observed levels of variability on interannual time scales.

A concerted effort has been made to simulate the global ocean circulation with resolved eddies, using a highly optimized model on the best available supercomputer. An earlier 20-year spin-up has been extended for 12.5 additional years: the first 2.5 with continued annual mean forcing and the final 10.0 with climatological monthly forcing. Model output archived at 3-day intervals has been analyzed into mean fields, standard deviations, products, and covariances on monthly, annual, and multiyear time scales. The multiyear results are examined here in order to give insight into the general circulation of the world ocean. The three-dimensional How fields of the model are quite realistic, even though resolution of eddies in high latitudes is marginal with a 0.5°, 20-level grid. The use of seasonal forcing improves the simulation, especially in the tropics and high northern latitudes. Mid-latitude gyre circulations, western boundary currents, zonal equatorial Hows, and the Antarctic Circumpolar Current (ACC) all show mean and eddy characteristics similar to those observed. There is also some indication of eddy intensification of the mean How of the ACC and of separated boundary jets. A global thermohaline circulation of North Atlantic Deep Water is identified in deep western boundary currents connected by the ACC. This deep circulation rises mainly in the equatorial Pacific. Several zonal jets are an integral part of this circulation near the equator. The deep How rises toward the surface in a series of switchbacks. Much of the thermohaline return How then follows an eddy-rich warm-water route through the Indonesian archipelago and around the southern tip of Africa. However, some intermediate level portions of the thermohaline circulation return south into the ACC and follow a cold water route through the Drake Passage. The representation of a global "conveyor belt" circulation with narrow and relatively high-speed currents along most of its path may be the most important result of this modeling study. Statistics of scalar fields such as transport stream function and surface height are exhibited, as are time series and frequency spectra of certain variables at selected points. These provide a baseline for comparison both with observations and with other model studies at higher resolution. Mean and eddy characteristics of the near-surface temperature and salinity fields are discussed, and surface forcing fields are examined. In particular, combined thermal and hydrological forcing effects are found to drive a conveyor belt circulation between the tropical Pacific and the high-latitude North Atlantic. The effect of weak restoring terms to observed temperature and salinity at great depth and in polar latitudes is found mainly to augment the model's convective processes, which are poorly resolved with 0.5° grid spacing. However, the deep restoring terms are insignificant in both the tropics and the mid-latitudes. The geographical distributions of eddy heat and salt transport are discussed. The eddies transport heat and sa...

[1] The northward flow of Atlantic Water via the Barents Sea and Fram Strait is modeled, and climatological volume, heat, and salt fluxes into the Arctic Ocean are investigated. We argue that understanding of climate change in the region requires the knowledge of the mean circulation before its variability can be determined. Since estimates of long-term mean fluxes in the region are not available from observations, we present a modeling approach to quantify the climatological circulation and northward transports from the Norwegian Sea into the Arctic Ocean. A coupled ice-ocean model of the pan-Arctic region is configured at a 1/12°and 45-level grid and is integrated for 7 decades using a combination of daily-averaged 1979-2001 European Centre for Medium-Range Weather Forecasts data. Simulated water mass characteristics are compared with climatological atlas and selected observational data. The separation of the Norwegian Atlantic Current into Barents Sea and Fram Strait branches and their relative contributions to the total mass and property input into the Arctic Ocean are quantified. We emphasize the Barents Sea because fewer direct measurements of transports exist there and because water masses are significantly altered along this path by the seasonal ice melt/formation and the freshwater inputs. Under the given atmospheric forcing the Barents Sea outflow is shown to significantly contribute to the boundary flow continuing along the slopes of the Arctic Ocean. On the basis of model results, we argue that the contribution of the Barents Sea branch of Atlantic Water into the Arctic Ocean is equally, if not more, important than the Fram Strait branch.

Abstract.Numerical high-resolution ocean general circulation models have experienced a revolutionary development during the last decade. Today they are run globally in realistic configuration with realistic surface boundary forcing. R. ecent improvements in external surface forcing fields including daily wind-stress fields and sea surface heat fluxes lead to a significant improvement in the overall agreement of the simulated and observed large-scale mean circulation and its variability. However, simulated amplitudes of variability remain low by about a factor of 2 to 4 over a broad spectral range, including the long wavelengths and periods. Both the causes and consequences of this low variability remain obscure.