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Precipitation changes projected for the end of the twenty-first century show an increase of more than 50 per cent in the Arctic regions. This marked increase, which is among the highest globally, has previously been attributed primarily to enhanced poleward moisture transport from lower latitudes. Here we use state-of-the-art global climate models to show that the projected increases in Arctic precipitation over the twenty-first century, which peak in late autumn and winter, are instead due mainly to strongly intensified local surface evaporation (maximum in winter), and only to a lesser degree due to enhanced moisture inflow from lower latitudes (maximum in late summer and autumn). Moreover, we show that the enhanced surface evaporation results mainly from retreating winter sea ice, signalling an amplified Arctic hydrological cycle. This demonstrates that increases in Arctic precipitation are firmly linked to Arctic warming and sea-ice decline. As a result, the Arctic mean precipitation sensitivity (4.5 per cent increase per degree of temperature warming) is much larger than the global value (1.6 to 1.9 per cent per kelvin). The associated seasonally varying increase in Arctic precipitation is likely to increase river discharge and snowfall over ice sheets (thereby affecting global sea level), and could even affect global climate through freshening of the Arctic Ocean and subsequent modulations of the Atlantic meridional overturning circulation. [ABSTRACT FROM AUTHOR]

The processes leading to the formation of a large anomaly of sea-ice volume integrated over the Northern Hemisphere have been investigated in a coarse-resolution three-dimensional atmosphere-ocean-sea-ice model. This anomaly lasts for about 20 years and reaches 8.6 ⊗ 10³ km³, i.e. 6 standard deviations. It is associated with a maximum surface cooling of more than 5 °C in the annual mean close to Spitzbergen. The trigger of the large event in the model is a sequence of years presenting an atmospheric circulation characterised by negative geopotential height anomalies over Greenland and positive ones in the Barents and Norwegian seas. This atmospheric anomaly induces an increase in the ice volume as well as of the ice extent. Sea ice survives in the area located close to Spitzbergen where deep convection occurs during normal years. This is associated with a shut down of convection in that area and thus by a strong reduction of the upward heat flux from the ocean to the atmosphere that strongly amplifies the initial cooling. The long sequence of following years presenting the same type of anomaly in the atmosphere is not just a rare realisation of the intrinsic atmospheric variability. A positive feedback between the modification of surface conditions and the atmospheric circulation reinforces the initial atmospheric anomaly. Finally, the large event stops when convection increases again as a result of an increase in ocean surface density close to Spitzbergen. This density increase is due to both the advection of a positive salinity anomaly created in the Arctic because of brine rejection and to local ice formation southward of Spitzbergen during the cold event. Sensitivity experiments performed with the model have shown that the frequency of the events is very sensitive to the mean climate simulated by the model, the frequency being higher in a colder climate. [ABSTRACT FROM AUTHOR]

A long-term simulation performed with a coarse-resolution, global, atmosphere–ocean–sea-ice model displays strong decadal variability of the sea-ice volume in the Northern Hemisphere with a significant peak at about 15–18 years. This model results from the coupling of ECBILT, a spectral T21, 3-level quasi-geostrophic atmospheric model, and CLIO, a sea-ice–ocean general circulation model. First, the mechanism underlying the variability of ice volume in the model was studied by performing correlation analyses between the simulated variables. In a second step, a series of additional sensitivity experiments was performed in order to illustrate the role of specific physical processes. This has allowed us to identify a feedback loop in the ice–ocean system, which proceeds as follows: an increase in Arctic sea-ice volume induces an increase in the salinity there. This salinity anomaly is transported to the Greenland Sea where it promotes convective activity. This warms up the surface oceanic layer and the atmosphere in winter and induces a decrease of the ice volume, completing half a cycle. The changes in ice volume are driven by a geopotential height pattern characterised by centres of action of opposite signs over Greenland and the Barents–Kara–Central Arctic area. Thermodynamic feedback between the ice and the atmosphere appear also to be very important for the persistence of the oscillation. The dynamical response of the atmosphere to sea-ice and temperature anomalies at surface plays a smaller role. [ABSTRACT FROM AUTHOR]