2011: Climate determinism revisited: multiple equilibria in a complex climate model
Multiple equilibria in a coupled ocean-atmosphere-sea ice general circulation model (GCM) of an Aquaplanet with many degrees of freedom are studied. Three different stable states are found for exactly the same set of parameters and external forcings: a cold state in which a polar sea-ice cap extends...
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| Format: | Text |
| Language: | English |
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.419.8673 http://www.seas.harvard.edu/climate/seminars/pdfs/MultipleEquilibria_FMR_revised.pdf |
| Summary: | Multiple equilibria in a coupled ocean-atmosphere-sea ice general circulation model (GCM) of an Aquaplanet with many degrees of freedom are studied. Three different stable states are found for exactly the same set of parameters and external forcings: a cold state in which a polar sea-ice cap extends into midlatitudes, a warm state which is ice-free and a completely sea ice covered ‘snowball ’ state. Although low-order energy balance models of the climate are known to exhibit intransitivity, i.e. more than one climate state for a given set of governing equations, the results reported here are the first to demonstrate that this is a property of a complex coupled climate model with a consistent set of equations representing the 3d dynamics of the ocean and atmosphere. The coupled model notably includes atmospheric synoptic systems, large-scale circulation of the ocean, a fully active hydrological cycle, sea-ice and a seasonal cycle. There are no flux adjustments, the system being solely forced by incoming solar radiation at the top of the atmosphere. We demonstrate that the multiple equilibria owe their existence to the presence of meridional structure in ocean heat transport, namely a large heat transport out of the tropics and a relatively weak high-latitudes transport. The associated large midlatitude ocean heat transport convergence leads to a preferred latitude at which the sea ice-edge can rest. The mechanism operates in two very different ocean circulation regimes, suggesting that the stabilization of the large ice cap could be a robust feature of the climate system. The role of ocean heat convergence in permitting multiple equilibria is further explored in simpler models, an atmospheric GCM coupled to a slab mixed-layer ocean and an energy balance model. 1 1 |
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