Future climate change shaped by inter-model differences in Atlantic meridional overturning circulation response

Abstract

In climate model simulations of future climate change, the Atlantic Meridional Overturning Circulation (AMOC) is projected to decline. However, the impacts of this decline, relative to other changes, remain to be identified. Here we address this problem by analyzing 30 idealized abrupt-4xCO₂ climate model simulations. We find that in models with larger AMOC decline, there is a minimum warming in the North Atlantic, a southward displacement of the Inter-tropical Convergence Zone, and a poleward shift of the mid-latitude jet. The changes in the models with smaller AMOC decline are drastically different: there is a relatively larger warming in the North Atlantic, the precipitation response exhibits a wet-get-wetter, dry-get-drier pattern, and there are smaller displacements of the mid-latitude jet. Our study indicates that the AMOC is a major source of inter-model uncertainty, and continued observational efforts are needed to constrain the AMOC response in future climate change.

Fig. 1. Inter-model differences in the AMOC response.

Panels a, b show the annual mean AMOC anomalies in the abrupt-4xCO₂ simulations at 26.5 °N for a CMIP5 and b CMIP6 models with respect to the mean AMOC strength computed from the preindustrial control. A 10-year running average smoothing has been applied to all curves for better visualization. Solid lines represent the models in the small AMOC decline group, while thick dashed lines represent the models in the large AMOC decline group. Models that do not belong to any groups, are represented by thin dashed lines. Panels c, d show transects of the ocean overturning stream-function in the North Atlantic for c the average of the large AMOC decline group, and d the average of the small AMOC decline group. Superimposed contours show the climatology computed from the preindustrial control of c the ten large AMOC decline models and d the 10 small AMOC decline models.

Fig. 2. Surface temperature change associated with AMOC.

Panels a, b show the normalized annual mean surface temperature change in the abrupt-4xCO₂ with respect to the preindustrial control for a the average of the large AMOC decline group and b the average of the small AMOC decline group. Panel c is their difference (a minus b). For each model, surface temperature change is divided by the respective ΔGSAT, hence units are °C per degree of global warming. In panels (a) and (b), superimposed contours show the climatological mean surface temperature computed from all models. In panel c, superimposed contours show the numerical values associated with the surface temperature differences of the colored contours. Panel d shows the change in subpolar North Atlantic SST (ΔSPNA) divided by ΔGSAT, against AMOC change (ΔAMOC) in units of Sv. Circles represent CMIP5 models, while diamonds represent CMIP6 models. The dashed black line is the linear regression ($y=0.13x+1.8$) with R²: 0.63. The blue and red vertical lines represent the lower and upper terciles of the ΔAMOC distribution. Models to the left of the blue line belong to the large AMOC decline group, while models to the right of the red line belong to the small AMOC decline group.

Fig. 3. Precipitation change associated with AMOC.

Panels a, b show the normalized annual mean precipitation change in the abrupt-4xCO₂ with respect to the preindustrial control for a the average of the large AMOC decline group and b the average of the small AMOC decline group. Panel c is their difference (a minus b). For each model, precipitation change (ΔP) is divided by the respective ΔGSAT, hence the units are mm/day per degree of global warming. In panels a–c, superimposed contours show the climatological mean precipitation computed from all models. Panel d shows the zonal mean precipitation change in the North Atlantic sector in the (blue) large AMOC decline group and (red) small AMOC decline group. Thick lines are the group averages, while thin lines show each group member. Round markers indicate where the Student’s t-test between the means of the two groups is significant at the 90% level. The black line is the zonal mean precipitation climatology computed from the preindustrial control of all models: units are of mm/day and the corresponding scale is located on right-side y-axis.

Fig. 4. Winter wind speed change associated with AMOC.

Panels a, b show the normalized zonal mean wind speed change in DJF in the abrupt-4xCO₂ with respect to the preindustrial control for a the average of the large AMOC decline group and b the average of the small AMOC decline group. Panel c is their difference (a minus b). For each model, the zonal mean wind speed change is divided by the respective ΔGSAT, hence units are of m/s per degree of global warming. In panels a–c, superimposed contours show the climatological zonal mean wind speed in DJF computed from all models. Panel d shows the change in the latitude of the maximum westerly wind speed at 250 hPa in the northern hemisphere (ΔLAT) divided by ΔGSAT, against AMOC change (ΔAMOC). Circles represent CMIP5 models, while diamonds represent CMIP6 models. The dashed black line is the linear regression including all models ($y=-0.42x-2.84$ with R²: 0.33), while the dashed magenta line is the linear regression excluding the models GISS-E2-1-G and SAM0-UNICON ($y=-0.68x-4.54$ with R²: 0.48). The blue and red vertical lines represent the lower and upper terciles of the ΔAMOC distribution. Models to the left of the blue line belong to the large AMOC decline group, while models to the right of the red line belong to the small AMOC decline group.