Contribution of meridional overturning circulation and sea ice changes to large-scale temperature asymmetries in CMIP6 overshoot scenarios

Abstract. Analysis of overshoot scenarios, characterized by a peak in radiative forcing levels followed by a decline, show that changes during the CO₂ increasing phase are not necessarily compensated during the CO₂ decreasing phase, particularly at regional level. Even if the global mean temperature may recover after the overshoot, at the regional level the situation post-overshoot may differ from the situation pre-overshoot, with spatial patterns characterized by large-scale temperature asymmetries. These asymmetries, found between Northern (NH) and Southern Hemisphere (SH), between high and mid-latitudes of the NH, and between western and eastern areas of the Southern Ocean, alter atmospheric dynamics and through it the hydroclimate. Changes in the sea ice, changes in the ocean circulation and heat transport, and thermal inertia of the ocean have been identified as potential sources of hysteresis, highlighting the impact of oceanic changes in the behavior of atmospheric variables in case of overshoot. This work analyzes SSP5-3.4OS and SSP1-1.9 overshoot experiments from the Coupled Model Intercomparison Project Phase 6 (CMIP6) to assess how well these mechanisms can explain the large-scale temperature asymmetries that characterize the difference between pre-overshoot and post-overshoot states. These analyses show that the relative contribution of each mechanism strongly depends on the model. Certain models like MRI-ESM2-0 are mainly impacted by changes in the Atlantic Meridional Overturning Circulation (AMOC), others like CNRM-ESM2-1 show a relevant impact of sea ice changes in high-latitude areas, and others like IPSL-CM6A-LR show also a relevant impact of changes in the Southern Meridional Overturning Circulation (SMOC). Inter-model differences in the contributions of the meridional overturning can be associated with different climatologies of Mixed Layer Depth (MLD) in the northern North Atlantic (NNA) and in certain areas of the Southern Ocean. Despite these differences across models, all the mechanisms contribute to shape the regional temperatures after overshoot, with the temperature asymmetries between NH and SH mainly explained by changes in the AMOC, those between high and mid-latitudes of the NH by sea ice changes, and those between western and eastern areas of the Southern Ocean by the SMOC. These results highlight the importance of model intercomparison and analysis of ocean dynamics to understand the regional impacts of an overshoot, and more generally the responses to forcing changes.