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Abstract Climate models vary widely in their projections of twenty-first century global warming and changes in the Atlantic meridional overturning circulation (AMOC). However, the extent to which this uncertainty in AMOC contributes to the uncertainty in global warming has not yet been quantified. To investigate this, we perform climate model experiments in which we increase CO2concentrations while imposing the range of AMOC declines found in the Coupled Model Intercomparison Project phase 6 (CMIP6). We find that intermodel spread in the AMOC decline, imposed within a single model, reproduces 20% of the total intermodel spread in global warming. An idealized energy-balance model indicates that changes in ocean heat uptake and climate feedbacks contribute approximately equally to enhanced warming in our experiment with a smaller AMOC decline. A smaller AMOC decline produces greater ocean-to-atmosphere heating in the North Atlantic, which increases near-surface warming and reduces the lower-tropospheric stability to produce less-negative lapse-rate and shortwave cloud feedbacks. In the northern tropics, surface warming is enhanced by the wind–evaporation–SST feedback and by more-positive longwave cloud and water vapor feedbacks due to a northward shift of the intertropical convergence zone. A Green’s function analysis confirms that a less-negative global feedback with a smaller AMOC decline is predominantly driven by sea surface warming in the extratropical North Atlantic. Spread in the AMOC decline is correlated with similar regional changes in ocean heat uptake and climate feedbacks across CMIP6 models as in our experiments. These results suggest that constraining projections of AMOC may substantially impact projections of near-future warming.
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CMIP6 Intermodel Spread in Interhemispheric Asymmetry of Tropical Climate Response to Greenhouse Warming: Extratropical Ocean Effects
Abstract Tropical climate response to greenhouse warming is to first order symmetric about the equator but climate models disagree on the degree of latitudinal asymmetry of the tropical change. Intermodel spread in equatorial asymmetry of tropical climate response is investigated by using 37 models from phase 6 of the Coupled Model Intercomparison Project (CMIP6). In the simple simulation with CO 2 increase at 1% per year but without aerosol forcing, this study finds that intermodel spread in tropical asymmetry is tied to that in the extratropical surface heat flux change related to the Atlantic meridional overturning circulation (AMOC) and Southern Ocean sea ice concentration (SIC). AMOC or Southern Ocean SIC change alters net energy flux at the top of the atmosphere and sea surface in one hemisphere and may induce interhemispheric atmospheric energy transport. The negative feedback of the shallow meridional overturning circulation in the tropics and the positive low cloud feedback in the subtropics are also identified. Our results suggest that reducing the intermodel spread in extratropical change can improve the reliability of tropical climate projections.
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- PAR ID:
- 10337371
- Date Published:
- Journal Name:
- Journal of Climate
- Volume:
- 35
- Issue:
- 14
- ISSN:
- 0894-8755
- Page Range / eLocation ID:
- 4869 to 4882
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1175/JCLI-D-21-0541.1
