Arctic climate and sea-ice changes over the last millennium in reconstructions and PMIP3 simulations

Arctic climate is characterized by substantial changes in surface temperature and sea-ice cover throughout the last millennium and high-latitude variations appear to be amplified compared to those at mid-latitudes. Model simulations demonstrate that both internal variability and the response to external forcing show aspects of Arctic amplification. Using recent reconstructions of pan-Arctic temperature and sea-ice changes we assess the ability of model simulations, carried out in the framework of the Paleo Model Intercomparison Project, to reproduce observed changes and to identify mechanisms driving variations on interdecadal to centennial time-scales. The combination of long control integrations and an ensemble of simulations with external (natural and anthropogenic) allows for a thorough assessment of internal and externally-forced variations. The energy budget of the Arctic domain, both for the atmosphere and the ocean is calculated and its sensitivity to changes in the external forcings and/or to changing atmospheric variability pattern is investigated. We study the relative role of lateral changes in atmospheric and oceanic heat transports. Both variations in ocean heat transport in connection with sea-ice changes in the Barents Sea and shifts in the atmospheric circulation regimes contribute significantly to events of particular strong surface air temperature anomalies. Internal variations are likely also responsible for large variations in sea-ice cover, such as the pronounced sea-ice minimum that has been reported for the generally cold period of the Little Ice Age. Changes forced by external drivers have also pronounced expressions at high-latitudes through the modification of atmospheric circulation patterns, such as the Arctic Oscillation.