Abstract
Abstract. The variability of the Atlantic meridional overturing circulation (AMOC) strength is investigated in control experiments and in transient simulations of up to the last millennium using the low-resolution Community Climate System Model version 3. In the transient simulations the AMOC exhibits enhanced low-frequency variability that is mainly caused by infrequent transitions between two semi-stable circulation states which amount to a 10 percent change of the maximum overturning. One transition is also found in a control experiment, but the time-varying external forcing significantly increases the probability of the occurrence of such events though not having a direct, linear impact on the AMOC. The transition from a high to a low AMOC state starts with a reduction of the convection in the Labrador and Irminger Seas and goes along with a changed barotropic circulation of both gyres in the North Atlantic and a gradual strengthening of the convection in the Greenland-Iceland-Norwegian (GIN) Seas. In contrast, the transition from a weak to a strong overturning is induced by decreased mixing in the GIN Seas. As a consequence of the transition, regional sea surface temperature (SST) anomalies are found in the midlatitude North Atlantic and in the convection regions with an amplitude of up to 3 K. The atmospheric response to the SST forcing associated with the transition indicates a significant impact on the Scandinavian surface air temperature (SAT) in the order of 1 K. Thus, the changes of the ocean circulation make a major contribution to the Scandinavian SAT variability in the last millennium.
Highlights
The Atlantic meridional overturning circulation (AMOC) is an important pacemaker for the climate in the North Atlantic region
Comparing these results with the Type 3 transitions, we find that the changes in MIXwest and in the velocity indices are a general feature of any low-frequency AMOC variation, even though they are exceptionally strong for the Type 3 transitions
The AMOC variability is investigated in several control and transient simulations that are performed using the lowresolution version of the Community Climate System Model version 3 (CCSM3)
Summary
The Atlantic meridional overturning circulation (AMOC) is an important pacemaker for the climate in the North Atlantic region. In D’Orgeville and Peltier (2009) the focus is set on the impact of different external forcing levels and different warming scenarios on the AMOC variability in the CCSM3 They show that the mechanism for AMOC variations is similar in the control simulations and in a moderate warming scenario (historical 1870–2000 simulation), namely a delayed response to salinity anomalies at the DWF sites of the subpolar gyre. The model results suggest a significant response of the AMOC to strong forcing changes (e.g., IPCC, 2007; Goosse and Renssen, 2006; Stenchikov et al, 2009), whereas the impact of moderate forcing variations is less clear (e.g., Zorita et al, 2004; Meehl et al, 2006).
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