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Do Coupled Climate Models Correctly Simulate the Upward Branch of the Deep Ocean Global Conveyor

Funding Program Area(s)
Project Type
University Grant
Project Term
to
Project Team

Principal Investigator

In the current generation of global coupled climate models, the upwelling branch of the global meridional overturning circulation (MOC) largely occurs within the Southern Ocean to the south of 30S. This means that the Southern Ocean is disproportionately important as a location where the deep ocean is brought into contact with the surface. Furthermore, it implies that Southern Ocean winds and eddies are critical processes that must be simulated correctly to understand the response to climate change. However, a number of recent observational syntheses suggest that there is substantial upwelling, at least to intermediate depths, in the tropics. This would imply that tidally-driven internal wave mixing could play a critical role in determining the large-scale circulation. Since this mixing is often spatially and temporally constant in these models but is certainly not spatially constant in the real world, this would imply that the current generation of coupled climate models fail to simulate a key process determining the location where old waters, poor in anthropogenic carbon but rich in nutrients, come into the surface layers. Previous work done in our group suggests that the upwelling branch cannot be broadly distributed over the tropical oceans, as this would produce unrealistic distributions of radiocarbon. However, a number of new ideas about how localized mixing may be generated have recently been published. Additionally, the WOCE hydrographic program has made available new measurements of mantle 3He. Emanating from the mid-ocean ridges, mantle 3He is an almost ideal tracer for determining whether deep water has recently come into contact with the atmosphere (in contrast to radiocarbon, which because of its long equilibration time may come to the surface and be reinjected to depth without being reset). We propose to develop a suite of simulations that use localized mixing to close the global overturning, and then to use mantle 3He, radiocarbon and nutrient tracers to examine whether these simulations produce tracer distributions that are more realistic or less realistic than those associated with the Southern Ocean-dominated circulations produced by the IPCC AR4 simulation. This will involve a combination of initial scoping studies done at low resolution to evaluate how large enhanced localized mixing needs to be in order to alter the circulation and higher resolution studies in the oceanic components of global coupled climate models as well as within the global coupled models themselves. These runs will focus on locations where high levels of mixing have been postulated, including the Mid-Atlantic Ridge, the northwest Pacific, and the Indian Ocean, and regions where parametric subharmonic instability is thought to occur. Additionally a new 3He dataset will be synthesized by combining WOCE data with observations obtained from non-WOCE programs and hydrographic campaigns. This dataset will be made available to the community and used to estimate pathways of upwelling and the extent to which deep waters serve as source waters for water lying at the base of the pycnocline. The protocol for running mantle 3He, developed during the Ocean Carbon Model Intercomparison Project (OCMIP2) will be updated to reflect the new dataset.