Non‐Monotonic Feedback Dependence Under Abrupt CO<sub>2</sub> Forcing Due To a North Atlantic Pattern Effect
Ivan Mitevski, Yue Dong, Lorenzo M. Polvani, Maria Rugenstein, Clara Orbe
Abstract
Abstract Effective climate sensitivity (EffCS), commonly estimated from model simulations with abrupt 4×CO 2 for 150 years, has been shown to depend on the CO 2 forcing level. To understand this dependency systematically, we performed a series of simulations with a range of abrupt CO 2 forcing in two climate models. Our results indicate that normalized EffCS values in these simulations are a non‐monotonic function of the CO 2 forcing, decreasing between 3× and 4×CO 2 in CESM1‐LE (2× and 3×CO 2 in GISS‐E2.1‐G) and increasing at higher CO 2 levels. The minimum EffCS value, caused by anomalously negative radiative feedbacks, arises mainly from sea‐surface temperature (SST) relative cooling in the tropical and subtropical North Atlantic. This cooling is associated with the formation of the North Atlantic Warming Hole and Atlantic Meridional Overturning Circulation collapse under CO 2 forcing. Our findings imply that understanding changes in North Atlantic SST patterns is important for constraining near‐future and equilibrium global warming.