J.-M. Campin

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.

The 8.2 kyr BP event simulated by a Global Atmosphere—Sea-Ice—Ocean Model

Seven freshwater perturbation experiments were performed with a global atmosphere-sea-ice-ocean model to study the mechanism behind the 8.2 kyr BP Holocene cooling event. These experiments differed in initial state and duration of the applied freshwater pulse, while the amount of freshwater was kept constant (4.67x10(14) m(3)). One of the scenarios, with freshwater added to the Labrador Sea at a rate of 0.75 Sv during 20 years, resulted in weakening of the North Atlantic thermohaline circulation during 320 years and surface cooling varying from 1 to 5 degreesC over adjacent continents. This result is consistent with proxy data, suggesting that a meltwater-induced weakening of the thermohaline circulation caused the event. Moreover, our results indicate that the time-scale of the meltwater release and the initial state are important, as both have a strong effect on the magnitude and duration of the produced model response.

The effects of the water flow through the Canadian Archipelago in a global ice-ocean model

Numerical experiments are conducted with a global ice-ocean model in order to evaluate the influence of the water flow from the Arctic Ocean to Baffin Bay through the Canadian Archipelago on the water-mass properties of the Arctic Ocean and adjacent seas and, more generally, on the global ocean circulation. The results indicate that this flow plays a significant role in controlling the freshwater budget of the Arctic Ocean. When the Canadian Archipelago passage is open in the model, the Arctic pycnocline experiences a noticeable increase in salinity. Furthermore, the flow of relatively fresh Arctic waters through the passage yields a pronounced decrease of surface salinity and density in the Labrador Sea, which leads to a diminution of convective activity there. As a result, the North Atlantic Deep Water outflow in the model is reduced by about 5%. Deep convection in the Norwegian Sea exhibits almost no change, and this despite a weakening of the inflow of relatively fresh Arctic waters through Fram Strait.

Realistic representation of the surface freshwater flux in an ice–ocean general circulation model

Ocean general circulation models usually use an equivalent salt-flux in order to represent the freshwater surface inflow/outflow. This unphysical approach has numerous shortcomings, especially for climate studies. A more physical representation has been originally proposed by R.X. Huang [Journal of Physical Oceanography 23 (1993) 2428-2446] for ocean models. It consists in taking into account the vertical velocity at the sea surface. Here this formulation is introduced in a coupled ice-ocean general circulation model designed for climate studies. The treatment of the ice-ocean exchanges needs special care in order to conserve salt and freshwater masses, and to correctly represent the physics involved. This formulation allows to simulate the Goldsbrough-Stommel circulation and the meridional pathway of the freshwater at the ocean surface. Furthermore, the meridional freshwater transport diagnosed using such an approach is more directly comparable to the atmospheric water-vapor transport. Nevertheless, it produces only small changes in the ocean general circulation

Another reason why simple discretizations of rotated diffusion operators cause problems in ocean models: comments on "Isoneutral diffusion in a z-coordinate ocean model"

Another reason why simple discretizations of rotated diffusion operators cause problems in ocean models : Comments on Isoneutral diffusion in a z-coordinate ocean model