Predicting a Decadal Shift in North Atlantic Climate Variability Using the GFDL Forecast System

Decadal prediction experiments were conducted as part of phase 5 of the Coupled Model Intercomparison Project (CMIP5) using the GFDL Climate Model, version 2.1 (CM2.1) forecast system. The abrupt warming of the North Atlantic Subpolar Gyre (SPG) that was observed in the mid-1990s is considered as a case study to evaluateforecastcapabilitiesandbetterunderstandthereasonsfortheobservedchanges.InitializingtheCM2.1 coupledsystemproduces highskillinretrospectivelypredictingthemid-1990s shift,whichisnotcapturedbythe uninitializedforecasts. Allthehindcasts initialized intheearly1990s show awarmingoftheSPG;however,only the ensemble-mean hindcasts initialized in 1995 and 1996 are able to reproduce the observed abrupt warming and the associated decrease and contraction of the SPG. Examination of the physical mechanisms responsible forthesuccessfulretrospectivepredictionsindicatesthatinitializingtheoceaniskeytopredictingthemid-1990s warming.The successful initialized forecasts showan increased Atlanticmeridional overturning circulation and North Atlantic Current transport, which drive an increased advection of warm saline subtropical waters northward, leading to a westward shift of the subpolar front and, subsequently, a warming and spindown of the SPG. Significant seasonal climate impacts are predicted as the SPG warms, including a reduced sea ice concentration over the Arctic, an enhanced warming over the central United States during summer and fall, and anorthwardshiftofthemeanITCZ.Theseclimateanomaliesaresimilartothoseobservedduringawarmphase of the Atlantic multidecadal oscillation, which is encouraging for future predictions of North Atlantic climate.