The Role Of External Forcing In The Interdecadal Variability Of SST In The North Atlantic

Earth's climate fluctuates significantly on decadal to multidecadal time scales,which can not only provide an important background for interannual changes,but also significantly adjust the long-term changes(such as global warming).The decadal to multidecadal climate changes often causes large damages to our society and environment.The Atlantic Multidecadal Variability(AMV)and Interdecadal Pacific Oscillation(IPO)are two important oceanic interdecadal signals that have significant impacts on global climate.Determining the source and physical mechanism of interdecadal variability in sea surface temperature is therefore essential for understanding and predicting climate change on multiple time scales.Here we use the observations,CMIP5/6 simulations and large ensemble simulations together to separate externally-forced and internally-generated variations in observations and model simulations.We attempt to explore the role of external forcing in causing the decadal to multidecadal sea surface temperature(SST)variations over North Atlantic.The main conclusions are as follows:1.By examining the decadal to multidecadal SST variations over the global ocean,it is found that the different AMV indices are quite different after the 1970s,which confirms that the AMV is affected by external forcing.The role of external forcing such as human activities in influencing the interdecadal variations of global(especially North Atlantic)SST cannot be ignored.The IPO indices,which are characterized by different methods,are similar,indicating that the IPO is mainly dominated by internal climate variability.2.We investigate the influence of external forcing on the decadal to multidecadal SST variations over the global ocean.We show that variations in aerosols have caused in-phase,multidecadal SST variations across all ocean basins.These forced variations resemble the Atlantic Multidecadal Oscillation in time.Such conclusion confirms the important role of external forcing in influencing the decadal variations of global(especially North Atlantic)SST.Unlike the North Atlantic,where indirect and direct aerosol effects on surface solar radiation drive the multidecadal SST variations,over the tropical central and western Pacific atmospheric circulation response to aerosol forcing plays an important role.3.The relative contribution of external forcing and internal variability to AMV is quantified.It is found that AMV is mainly affected by the internal variability(IV),decadal variations in aerosol forcing happen to be in phase with the IV-induced AMV since 1920s and thus enlarged its amplitudes.Our new findings not only reconcile ongoing controversial debate on the role of IV and external forcing in causing the AMV,but also provide new convincing evidence on the notion that the recent AMV resulted from both internal climate variability and decadal changes in aerosols.4.The influence of anthropogenic warming in the 21st century on the internally-generated(AMV_IV)and externally-forced(AMV_EX)components of the AMV and their impacts on global climate are discussed.It is found that the variance of AMV_IV(AMV_EX)tends to decrease(increase)in the 21st century.The AMV-related SST variations would also change in the future:the subpolar(tropical)North Atlantic warming under the AMV_IV warm phase weakens(intensifies),while the subpolar North Atlantic cooling under the AMV_EX warm phase intensifies(switch to warm anomaly)under the RCP2.6/4.5(RCP8.5)scenario.Under the background of global warming,the positive correlation between AMV_IV and temperature variations over the United States and the precipitation around Mexico will be stronger in the future.