A Zonally Averaged, 3-basin Ocean Circulation Model for Climate Studies

A two-dimensional, three-basin ocean model suitable for long-term climate studies is developed. The model is based on the zonally averaged form of the primitive equations written in spherical coordinates. The east-west density difference which arises upon averaging the momentum equations is taken to be proportional to the meridional density gradient. Lateral exchanges of heat and salt between the basins are explicitly resolved. Moreover, the model includes bottom topography and has representations of the Arctic Ocean and of the Weddell and Ross seas. Under realistic restoring boundary conditions, the model reproduces the global conveyor belt: deep water is formed in the Atlantic between 60 and 70-degrees-N at a rate of about 17 Sv (1 Sv = 10(6) m3 s-1) and in the vicinity of the Antarctic continent, while the Indian and Pacific basins show broad upwelling. Superimposed on this thermohaline circulation are vigorous wind-driven cells in the upper thermocline. The simulated temperature and salinity fields and the computed meridional heat transport compare reasonably well with the observational estimates. When mixed boundary conditions (i.e., a restoring condition on sea-surface temperature and flux condition on sea-surface salinity) are applied, the model exhibits an irregular behavior before reaching a steady state characterized by self-sustained oscillations of 8.5-y period. The conveyor-belt circulation always result at this stage. A series of perturbation experiments illustrates the ability of the model to reproduce different steady-state circulations under mixed boundary conditions. Finally, the model sensitivity to various factors is examined. This sensitivity study reveals that the bottom topography and the presence of a submarine meridional ridge in the zone of the Drake Passage play a crucial role in determining the properties of the model bottom-water masses. The importance of the seasonality of the surface forcing is also stressed

The glacial ocean: a study with a zonally averaged, three-basin ocean circulation model

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A Model Study of the Atlantic Thermohaline Circulation During the Last Glacial Maximum

STABLE isotope measurements in deep-sea sediment cores have indicated that the Atlantic thermohaline circulation experienced significant changes during the last glacial maximum: the North Atlantic Deep Water (NADW) was shallower than today and the Antarctic Bottom Water (AABW) penetrated much farther north(1-6). Numerical ocean models have, so far, been unable to simulate these circulation changes realistically(7). Here we show that a zonally averaged, three-basin ocean model, driven by glacial boundary conditions(8-10), reproduces the main trends of the geochemically constrained glacial Atlantic circulation. In addition, we provide quantitative estimates of the meridional water transport during glacial times. Our results suggest that the glacial production of AABW was slightly higher than at present, whereas that of NADW was reduced by similar to 40%, resulting in an intermediate circulation cell which closed within the Atlantic basin. We also show that the strength of the Atlantic conveyor belt strongly depends on the surface density contrast between the high latitudes of the Northern and Southern hemispheres

High latitude deep water sources during the last glacial maximum and the intensity of the global oceanic circulation

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Long-term climate commitments projected with climate - carbon cycle models

Eight earth system models of intermediate complexity (EMICs) are used to project climate change commitments for the recent IPCC Fourth Assessment Report (IPCC AR4). Simulations are run until year 3000 AD and extend substantially further into the future than conceptually similar simulations with Atmosphere-Ocean General Circulation Models (AOGCMs) coupled to carbon cycle models. We investigate (1) the climate change commitment in response to stabilized greenhouse gases and radiative forcing, (2) the climate change commitment in response to earlier CO2 emissions, and (3) emission trajectories for profiles leading to stabilization of atmospheric CO2 and their uncertainties due to carbon cycle processes. Results over the 21st century compare reasonably well with results from AOGCMs and the suite of EMICs proves well suited to complement more complex models. We identify substantial climate change commitments for sea level rise and global mean surface temperature increase after a stabilization of atmospheric greenhouse gases and radiative forcing in year 2100. The additional warming by year 3000 is 0.6 to 1.6 K for the low-CO2 SRES B1 scenario and 1.3 to 2.2 K for the high-CO2 SRES A2 scenario. Correspondingly, the post-2100 thermal expansion commitment is 0.3 to 1.1 m for SRES B1 and 0.5 to 2.2 m for SRES A2. Sea level continues to rise due to thermal expansion for several centuries after CO2 stabilization.In contrast, surface temperature changes slow down after a century. The meridional overturning circulation is weakened in all EMICs, but recovers to nearly initial values in all but one of the models after centuries for the scenarios considered. Emissions during the 21st century continue to impact atmospheric CO2 andclimate even at year 3000. All models consistently find that most of the anthropogenic carbon emissions are eventually taken up by the ocean (49 to 62 %) in year 3000, and that a substantial fraction (15 to 28 %) is still airborne even after carbon emissions have ceased for 900 years. Future stabilization of atmospheric CO2 and climate change requires a substantial reduction of CO2 emissions below present levels in all EMICs. This reduction needs to be substantially larger if carbon cycle - climate feedbacks are accounted for or if terrestrial CO2 fertilization is not operating. We identify large differences among EMICs in both the response to increasing atmospheric CO2 and the response to climate change. This highlights the need for improved representations of carbon cycle processes in these models apart from the sensitivity to climate change. Sensitivity simulations with one single EMIC indicate that the impact of climate sensitivity related uncertainty on projected allowable emissions is substantially smaller than the uncertainty related to different carbon cycle settings