| 09 Feb 2026
Critical freshwater forcing for AMOC tipping in climate models – compensation matters
Abstract. Ocean and climate models of various complexity have shown that the Atlantic Meridional Overturning Circulation (AMOC) can undergo tipping, i.e., transition abruptly to a state without North Atlantic deep-water formation, as a function of freshwater forcing. Most of these model experiments compensate for the freshwater input to conserve global salinity, with salt being added either at the surface or throughout the ocean volume. However, these two different compensation methods have so far only been compared in a single, coarse-resolution climate model, and therefore little is known robustly about the effect of salinity compensation on the AMOC tipping point. Here, using an ocean model at 1° resolution and an intermediate-complexity coupled climate model, we systematically compare the effect of surface vs volume compensation on the tipping point of the AMOC as diagnosed from quasi-equilibrium experiments using a freshwater flux over the region 20° N–50° N. Salinity compensation at the surface consistently delays AMOC tipping compared to volume compensation. This is mainly because the compensation salinity added over the subpolar North Atlantic counteracts the weakening salinity gradient from freshwater forcing. In contrast, the compensation method does not strongly impact AMOC recovery when tracing the full hysteresis loop. Our results indicate that the distance of present-day climate to the AMOC tipping point with respect to freshwater forcing might have been overestimated in recent modeling studies, compounding the effect of model biases.
This is an interesting, thorough and well-written study that investigates the effects of how global salinity is compensated for in hosing experiments of larger climate models. In particular, they investigate the differences between surface and volume compensation, which shows a lower AMOC collapse threshold for volume compensation compared to surface compensation. They further find that the AMOC threshold for a collapse is similar for volume compensation and no compensation at all, concluding that volume compensation is probably providing more trustworthy estimates of AMOC stability compared to surface compensation. They then tease out the origin of these differences, concluding that surface compensating over the Pacific can be a pragmatic compromise that limits artificial stabilization of the AMOC, if volume compensation cannot be used for technical or scientific reasons. In all cases, the study shows the importance of any study clearly stating how salinity is compensated for, and that the quantitative outputs from studies should be interpreted in that light.
The study is an important contribution to better understanding of model outputs from large and intermediate complexity climate models, and is a substantial step forward for providing more robust estimates of the risk of an AMOC collapse.
Minor comments
Should Delta H be Delta F_H in the caption of figure 1? In Figure B2 it is called Delta H.
Figure B4: a, b, c, d indications are missing in the figure.