North Atlantic and Arctic Ocean Climate Change in Pliocene Simulations Using the GISS ModelE2-R GCM

As part of the Pliocene Model Intercomparison Project (PlioMIP) Experiment 2, twelve research groups simulated the middle Pliocene climate using fully coupled versions of their ocean-atmosphere GCMs. Under the conditions prescribed by PlioMIP Experiment 2 (especially 25 meters of sea level rise, 405 ppm CO2, and reduced ice sheets) most coupled GCMs still underestimate ocean temperatures in the North Atlantic and Arctic Ocean regions. The GISS ModelE (AR5-version) originally produced the coolest results in these regions out of all the Pliocene simulations, with a greatly decreased AMOC and colder temperatures than modern in a large portion of the North Atlantic. However, improvements in the formulation of mesoscale mixing in the GISS ModelE, which have been incorporated in a more recent model update, led to significant changes in the simulation of the Pliocene (Chandler et al., 2013), including a warmer North Atlantic ocean, decreased Arctic sea ice, increased Atlantic meridional overturning circulation (AMOC) relative to the control run, and generally a more favorable comparison to proxy data. Despite these results, the relative role of the various forcings and the numerous boundary condition changes was not analyzed. Zhang et al. (2013) did show that the increase in ocean heat transport is small compared to the change in the AMOC and was not likely to be the direct cause of the North Atlantic warming. Furthermore, using a subset of the PlioMIP models they showed that the role of ocean heat flux in the models, in general, is not strongly correlated to either the strengthening of the Pliocene AMOC or the warming of the North Atlantic. We have now run a series of sensitivity tests with the newer version of the GISS model and will discuss the relative effects of Pliocene CO2, ice sheets (Greenland and Antarctica separately), orbit, vegetation and the change in the mesoscale mixing parameterization as a means of better understanding the role of various factors that made the Pliocene North Atlantic a region so sensitive to climate change.