A weakening of the Atlantic Meridional Overturning Circulation (AMOC) in the next century is simu... more A weakening of the Atlantic Meridional Overturning Circulation (AMOC) in the next century is simulated by most state-of-the-art coupled models but none of them accounted for land-ice melting. Here we evaluate the impact of this melting on future climate projection using the IPSL-CM4 coupled ocean-atmosphere model. For this purpose we use two different versions of the model, one with a crude land-ice melting parameterization, and the other without. The analysis compares results of experiments where atmospheric CO 2 increases by 1%/yr, performed with the two versions of this model. The AMOC is reduced by 47% when the melting of land-ice is considered, and represents an extreme melting scenario. This reduction is of 21% without this melting. It is shown that this difference in AMOC impacts the northern hemisphere mostly through the sea-ice cover feedback.
Atlantic meridional overturning circulation (AMOC) variability is documented in the Community Cli... more Atlantic meridional overturning circulation (AMOC) variability is documented in the Community Climate System Model, version 4 (CCSM4) preindustrial control simulation that uses nominal 18 horizontal resolution in all its components. AMOC shows a broad spectrum of low-frequency variability covering the 50-200-yr range, contrasting sharply with the multidecadal variability seen in the T85 3 1 resolution CCSM3 presentday control simulation. Furthermore, the amplitude of variability is much reduced in CCSM4 compared to that of CCSM3. Similarities as well as differences in AMOC variability mechanisms between CCSM3 and CCSM4 are discussed. As in CCSM3, the CCSM4 AMOC variability is primarily driven by the positive density anomalies at the Labrador Sea (LS) deep-water formation site, peaking 2 yr prior to an AMOC maximum. All processes, including parameterized mesoscale and submesoscale eddies, play a role in the creation of salinity anomalies that dominate these density anomalies. High Nordic Sea densities do not necessarily lead to increased overflow transports because the overflow physics is governed by source and interior region density differences. Increased overflow transports do not lead to a higher AMOC either but instead appear to be a precursor to lower AMOC transports through enhanced stratification in LS. This has important implications for decadal prediction studies. The North Atlantic Oscillation (NAO) is significantly correlated with the positive boundary layer depth and density anomalies prior to an AMOC maximum. This suggests a role for NAO through setting the surface flux anomalies in LS and affecting the subpolar gyre circulation strength.
A weakening of the Atlantic Meridional Overturning Circulation (AMOC) in the next century is simu... more A weakening of the Atlantic Meridional Overturning Circulation (AMOC) in the next century is simulated by most state-of-the-art coupled models but none of them accounted for land-ice melting. Here we evaluate the impact of this melting on future climate projection using the IPSL-CM4 coupled ocean-atmosphere model. For this purpose we use two different versions of the model, one with a crude land-ice melting parameterization, and the other without. The analysis compares results of experiments where atmospheric CO 2 increases by 1%/yr, performed with the two versions of this model. The AMOC is reduced by 47% when the melting of land-ice is considered, and represents an extreme melting scenario. This reduction is of 21% without this melting. It is shown that this difference in AMOC impacts the northern hemisphere mostly through the sea-ice cover feedback.
Atlantic meridional overturning circulation (AMOC) variability is documented in the Community Cli... more Atlantic meridional overturning circulation (AMOC) variability is documented in the Community Climate System Model, version 4 (CCSM4) preindustrial control simulation that uses nominal 18 horizontal resolution in all its components. AMOC shows a broad spectrum of low-frequency variability covering the 50-200-yr range, contrasting sharply with the multidecadal variability seen in the T85 3 1 resolution CCSM3 presentday control simulation. Furthermore, the amplitude of variability is much reduced in CCSM4 compared to that of CCSM3. Similarities as well as differences in AMOC variability mechanisms between CCSM3 and CCSM4 are discussed. As in CCSM3, the CCSM4 AMOC variability is primarily driven by the positive density anomalies at the Labrador Sea (LS) deep-water formation site, peaking 2 yr prior to an AMOC maximum. All processes, including parameterized mesoscale and submesoscale eddies, play a role in the creation of salinity anomalies that dominate these density anomalies. High Nordic Sea densities do not necessarily lead to increased overflow transports because the overflow physics is governed by source and interior region density differences. Increased overflow transports do not lead to a higher AMOC either but instead appear to be a precursor to lower AMOC transports through enhanced stratification in LS. This has important implications for decadal prediction studies. The North Atlantic Oscillation (NAO) is significantly correlated with the positive boundary layer depth and density anomalies prior to an AMOC maximum. This suggests a role for NAO through setting the surface flux anomalies in LS and affecting the subpolar gyre circulation strength.
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Papers by Pablo Ortega