An hierarchy of intermediate coupled climate models will be used to investigate multiple equilibrium states of the climate system, the role of ocean heat transport in mediating the latitudinal range over which stable ice caps can extend, and the stability of the equilibrium states. The modeling framework is a coupled atmosphere, ocean and sea-ice model of an aquaplanet where the geometrical constraints on ocean circulation and extent of land are represented by a sequence of barriers. Key components of the ocean circulation, such as subtropical gyres, zonal jets, and meridional overturning circulation will be controlled by altering the geometrical barriers in the ocean. Their impacts on the mean state of the coupled system and its ability to sustain polar ice caps and multiple equilibrium states will then be examined. The proposal addresses fundamental questions about the energy balance of the planet, the processes that sustain polar sea-ice, interaction with the ocean and multiple equilibrium of the coupled climate system.
While experimenting with ocean-atmosphere models of an idealized Earth, NSF-funded researchers discovered multiple stable climate states for the same external forcing. The finding suggests the possibility of dramatic and possibly abrupt shifts in climate due to small variations in the Earth's orbit or changes in atmospheric gas concentrations. The researchers also found that transitions between stable states were asymmetric. For example, transitions from a stable cold climate to a stable warm climate were much quicker than the reverse transitions. What does all of this mean? If high levels of greenhouse gases in the atmosphere initiate a dramatic climate shift, significantly reducing those gases will not guarantee a return to the previous climate state. To arrive at their conclusion, Massachusetts Institute of Technology researchers John Marshall, David Ferreira (now at the Universiity of Reading, UK) and Brian Rose (now at the University of Albany, NY) experimented with ocean-atmosphere models using model planets with idealized configurations of land masses and ocean basins. The model planets are much like Earth, but the continents have taken on general shapes and primarily serve as barriers to divide the ocean basins. The idealized Earth models, with names such as Double Drake World, provide realistic comparisons to the actual Earth's ocean-atmosphere interactions. Double Drake World, named after the Drake Passage, has two strips of land that originate from the North Pole and extend to 35 degrees south, dividing the world's oceans into three major basins. Applying a single set of external forces to the model planet yielded three stable states: A cold state with ice coverage extending to the mid-latitudes, a warm state with minimal ice coverage, and a snowball state with total ice coverage. While the model planet has a simple geometry, it includes complex dynamics such as large-scale ocean currents, large-scale atmospheric systems, a fully active hydrologic cycle and seasonal variations. John Marshall with help from Susan M. Reiss, Science Writer
