Multiple climate and sea ice states on a coupled Aquaplanet

Details Multiple climate and sea ice states on a coupled Aquaplanet Multiple climate and sea ice states on a coupled Aquaplanet

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmgc42c..03r&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #GC42C-03

Physics

[0750] Cryosphere / Sea Ice, [1620] Global Change / Climate Dynamics, [3339] Atmospheric Processes / Ocean/Atmosphere Interactions, [4500] Oceanography: Physical

Scientific paper

A fully coupled atmosphere-ocean-sea ice GCM is used to explore the climates of Earth-like planets with no continents and idealized ocean basin geometries. We find three qualitatively different stable equilibria under identical external forcing: an equable ice-free climate, a cold climate with ice caps extending into mid-latitudes, and a completely ice-covered "Snowball" state. These multiple states persist for millennia with no drift despite a full seasonal cycle and vigorous internal variability of the system on all time scales. The behavior of the coupled system is rationalized through an extension of the Budyko-Sellers model to include explicit ocean heat transport (OHT), and the insulation of the ice-covered sea surface. Sensitivity tests are also conducted with a slab ocean GCM with prescribed OHT. From these we conclude that albedo feedback and ocean circulation both play essential roles in the maintenance of the multiple states. OHT in the coupled system is dominated by a wind-driven subtropical cell carrying between 2 and 3 PW of thermal energy out of the deep tropics, most of which converges in the subtropics to lower mid-latitudes. This convergence pattern (similar to modern Earth) is robust to changes in the ocean basin geometry, and is directly responsible for the stabilization of the large ice cap. OHT also plays an essential but indirect role in the maintenance of the ice-free pole in the warm states, by driving an enhanced poleward atmospheric latent heat flux. The hysteresis loop for transitions between the warm and large ice cap states spans a much smaller range of parameter space (e.g. ±1.8% variations in solar constant) than the transitions in and out of the Snowball. Three qualitatively different climate states for the same external forcing in a coupled GCM: ice-free, large ice cap, and Snowball. SST and sea ice thickness are plotted. Similar results are found in a pure Aquaplanet (lower) and a "RidgeWorld" with a global-scale ocean basin (upper).

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