North Atlantic 20th century multidecadal variability in coupled climate models: sea surface temperature and ocean overturning circulation

摘要

Output from a total of 24 state-of-the-art Atmosphere-Ocean GeneralCirculation Models is analyzed. The models were integrated with observedforcing for the period 1850–2000 as part of the Intergovernmental Panel onClimate Change (IPCC) Fourth Assessment Report. All models show enhancedvariability at multi-decadal time scales in the North Atlantic sector similarto the observations, but with a large intermodel spread in amplitudes andfrequencies for both the Atlantic Multidecadal Oscillation (AMO) andthe Atlantic Meridional Overturning Circulation (AMOC). The models, ingeneral, are able to reproduce the observed geographical patterns of warm andcold episodes, but not the phasing such as the early warming (1930s–1950s) andthe following colder period (1960s–1980s). This indicates that the observed20th century extreme in temperatures are due to primarily a fortuitousphasing of intrinsic climate variability and not dominated by externalforcing. Most models show a realistic structure in the overturningcirculation, where more than half of the available models have a meanoverturning transport within the observed estimated range of 13–24 Sverdrup.Associated with a stronger than normal AMOC, the surface temperature isincreased and the sea ice extent slightly reduced in the North Atlantic.Individual models show potential for decadal prediction based on therelationship between the AMO and AMOC, but the models strongly disagree bothin phasing and strength of the covariability. This makes it difficult toidentify common mechanisms and to assess the applicability for predictions.