Simulated polar amplification and its causes on decadal to millennial timescales

Recent studies demonstrate that transient versus near-equilibrium responses to CO2 forcing can look very different, in cases even opposing each other. In this study we examine the responses to warming across timescales in a series of abrupt, perturbed-CO2 or perturbed-cloud-albedo coupled model simulations run for 3000 years, focusing on the meridional structure of surface temperature changes and atmospheric and oceanic energy transports. Except under forcings heavily concentrated in the tropics, surface temperature responds with a polar-amplified structure within decades. The degree of amplification then increases dramatically in subsequent centuries and millennia, but with the Arctic vs. Antarctic timescales depending on changes in sea ice driven in part by altered deep ocean heat transports. These high-latitude responses are much more similar across simulations with different forcings than are low-latitude thermal responses or atmospheric energy transport responses. Nevertheless, based on simulations with weak global-mean forcing but large meridional forcing gradients and vice-versa, we show the central role that perturbed energy transports play in surface temperature responses. In particular, simulations with zero imposed forcing in the high-latitudes but positive forcing equatorward thereof still exhibit polar warming, which can only occur due to altered energy transport convergence into the high-latitudes. These results imply that polar amplification cannot generally be understood solely in terms of local high-latitude forcings and feedbacks.