Irreducible Southern Ocean Climate Uncertainty due to Global Ocean Initial Conditions

Abstract:

Do ocean initial conditions impact the range of future climate projections in Earth system models? And if they do, where are these impacts the greatest? In this study, we answer these questions using a 50-member (historical and future; 1950 to 2100) large ensemble of a state-of-the-art Earth system model, the CanESM2, in which individual ensemble members are initialized using a strategic combination of varied ocean initial conditions and atmospheric surface air temperature perturbations. We show that global ocean heat content anomalies associated with ocean initial conditions persist for at least 150 years following model initialization, and that these anomalies most readily impact surface climate over the Southern Ocean. Ocean initial conditions affect Southern Ocean surface climate because persistent deep ocean temperature and salinity anomalies upwell along sloping isopycnal surfaces that delineate neighboring branches of the Upper and Lower Cells of the Global Meridional Overturning Circulation. As a result, some ensemble variance in Southern Ocean turbulent (sensible and latent) heat fluxes, heat uptake, and surface temperature trends is attributable to variance in the ocean initial state. This, in turn, impacts the Antarctic sea ice edge, the subpolar trough, and the position of the eddy-driven jet. Such a discernible impact of varying ocean initial conditions on ensemble variance over the Southern Ocean is evident throughout the full 150 years of the ensemble, even though upper ocean temperature anomalies due to varying ocean initial conditions rapidly dissipate over the first two decades of model integration over much of the rest of the globe. Insofar as there remain difficulties with reckoning the state of the deep ocean, and difficulties with initializing Earth system models with realistic observations, the ocean initial state will likely remain an irreducible uncertainty for future climate projections, especially over the Southern Ocean.