Hugues Goosse

Several mechanisms have been proposed to explain natural climate variability in the Arctic. These include processes related to the influence of the North Atlantic Oscillation/Arctic Oscillation (NAO/AO), anticyclonic/cyclonic regimes, changes in the oceanic and atmospheric North Atlantic–Arctic exchange, and changes in the Atlantic meridional overturning circulation. After a brief critical review, the influence and interrelation of the above processes in a long climate integration of the Community Climate System Model, version 2 (CCSM2) are examined. The analysis is based on the time series of surface air temperature integrated northward of 70°N, which serves as a useful proxy for general Arctic climate conditions. This gives a large-scale view of the evolution of Arctic climate. It is found that changes in oceanic exchange and heat transport in the Barents Sea dominate in forcing the Arctic surface air temperature variability in CCSM2. Changes in atmospheric circulation are consistent with a wind forcing of this variability, while changes in the deep overturning circulation in the Atlantic are more weakly related in CCSM2. Over some time periods, the NAO/AO is significantly related to these changes in Arctic climate conditions. However, this is not robust over longer time scales.

An ensemble of simulations performed with a coarse resolution 3‐D climate model driven by various combinations of external forcing is used to investigate possible causes for differences noticed in two recent simulations of the climate of the past millennium using General Circulation Models (GCMs). Our results strongly suggest that differences in sensitivity (equilibrium and transient climate response) could be responsible for temperature changes that differ by more than a factor of two between two models. In addition, the spin‐up procedure could explain some differences between the simulations during the first centuries of the second millennium. The choice of the forcing reconstruction is found to play a smaller role for the differences in the simulated climate, in the model configurations analyzed here. Furthermore, at decadal scale, internal climate variability can mask the differences associated with different forcing reconstructions.

On the basis of long simulations performed with a three‐dimensional climate model, we propose an interhemispheric climate lag mechanism, involving the long‐term memory of deepwater masses. Warm anomalies, formed in the North Atlantic when warm conditions prevail at surface, are transported by the deep ocean circulation towards the Southern Ocean. There, the heat is released because of large scale upwelling, maintaining warm conditions and inducing a lagged response of about 150 years compared to the Northern Hemisphere. Model results and observations covering the first half of the second millenium suggest a delay between the temperature evolution in the Northern Hemisphere and in the Southern Ocean. The mechanism described here provides a reasonable hypothesis to explain such an interhemipsheric lag.

In order to investigate the causes of sea ice volume variations, two simulations have been performed using two different sea ice–ocean models driven by NCEP–NCAR reanalysis data over the period 1955–97. In both models, the largest changes in ice thickness occur in the central Arctic. The anomalous exchanges of sea ice, air, and water between the North Atlantic and the Arctic play a large role in explaining ice volume variations. In particular, the anomaly of oceanic heat transport between Iceland and Norway could alone explain a large part of the simulated ice volume changes. The time evolution of the ice volume appears well correlated with the low-frequency variations of the North Atlantic Oscillation (NAO). Nevertheless, the link between NAO and ice volume changes could operate through different mechanisms between the first and second half of the simulation. The influence of the NAO on the meridional heat transport in the North Atlantic and in the Nordic Seas hold during the whole simulation. B... Abstract In order to investigate the causes of sea ice volume variations, two simulations have been performed using two different sea ice–ocean models driven by NCEP–NCAR reanalysis data over the period 1955–97. In both models, the largest changes in ice thickness occur in the central Arctic. The anomalous exchanges of sea ice, air, and water between the North Atlantic and the Arctic play a large role in explaining ice volume variations. In particular, the anomaly of oceanic heat transport between Iceland and Norway could alone explain a large part of the simulated ice volume changes. The time evolution of the ice volume appears well correlated with the low-frequency variations of the North Atlantic Oscillation (NAO). Nevertheless, the link between NAO and ice volume changes could operate through different mechanisms between the first and second half of the simulation. The influence of the NAO on the meridional heat transport in the North Atlantic and in the Nordic Seas hold during the whole simulation. B...