Impact of ocean dynamics on the simulation of the Neoproterozoic "snowballEarth"

A fully coupled ocean-atmosphere general circulation model (the Fast Ocean- Atmosphere Model) is used to simulate the Neoproterozoic climate with a red uced solar luminosity (95% of present-day), low atmospheric CO2 (140 ppmv), and an idealized tropical supercontinent. Two coupled simulations were com pleted with present-day and cold initial ocean temperatures. These experime nts are compared with uncoupled (i.e., mixed-layer) model experiments to de termine the impact of a dynamical ocean on the Neoproterozoic simulations. In contrast to global sea-ice coverage in the uncoupled experiments, the se a-ice margin seasonally advances to 46 and 55 degrees latitude in the coupl ed experiments. The coupled simulations demonstrate that dynamic ocean proc esses can prevent a snowball solution and suggest that a reduced solar lumi nosity and low atmospheric CO2 are not by themselves sufficient conditions for a snowball solution. Heat exchange through vertical mixing in the mid-l atitudes, caused by static instability, is identified as the primary proces s halting the advance of the sea-ice margin.