Dichotomy Between Freshwater and Heat Flux Effects on Oceanic Conveyor Belt Stability and Global Climate
Either freshwater or GHG-induced heat flux changes can independently affect the oceanic stratification in the subpolar North Atlantic where the deep convection occurs; therefore, it is important to isolate how these two factors may have influenced the AMOC stability in the past and to project their influence under present and future climate conditions. To do this, we have conducted a series of simulations under different climate conditions using the Community Climate System Model versions 3 and 4 (CCSM3 and CCSM4, see method section for details). These simulations represent mid-glacial (15 thousand years before the present day, 15ka), modern (1990AD), and future climate conditions (the representative concentration pathway 8.5 or RCP8.5 scenario). Three paired experiments are used to test primarily how freshwater flux alone affects the AMOC hysteresis under different background mean states (mid-glacial vs. modern) and different geographic setups (open vs. closed Bering Strait), while the FutureGHG experiment is used to test primarily how GHG-induced heating will influence the AMOC hysteresis. Here, the changes in temperature and other climate variables in the three paired experiments are viewed as responses to the AMOC changes due to freshwater forcing. The changes in the hydrological cycle in the FutureGHG experiment are considered as responses to AMOC changes due to GHGs. Therefore, we define the AMOC stability in the first three paired experiments as freshwater-induced AMOC hysteresis and that in the FutureGHG experiment as GHG-induced AMOC hysteresis.
It has always been in hot debate whether AMOC has hysteresis behavior under modern conditions. This work unequivocally demonstrates that with freshwater forcing as the primary external forcing, the existence of AMOC hysteresis depends only on whether the Bering Strat is open or not. With an open Bering Strait, AMOC hysteresis does not exist, but it does with a closed Bering Strait. This work also shows that AMOC hysteresis can exist in an open Bering Strait if the primary external forcing is greenhouse gas which changes the hydrological cycle and modulates the effects of Bering Strait transport on AMOC stability.
In summary, the existence of AMOC hysteresis due to freshwater forcing alone does not depend on the background mean climate but rather on the status of the Bering Strait. AMOC hysteresis exists with a closed Bering Strait (true for glacial conditions), but is absent when the Bering Strait is open (true for modern conditions) because throughflow across the Bering Strait plays a negative feedback role that prevents the AMOC from collapsing suddenly. However, with GHG forcing alone under modern geographic conditions, AMOC hysteresis can exist since the GHG forcing has modulated the negative feedback role for Bering Strait throughflow via changes in the hydrological cycle. Although the total volume transport across the Bering Strait is reduced as AMOC weakens in FutureGHG simulation, the freshwater transport via this strait from the Pacific into the subpolar North Atlantic increases as the subpolar North Pacific freshens due to increased precipitation. Because this fresher upper ocean due to the increased freshwater transport via Bering Strait is compounded by the GHG-induced surface warming, the upper ocean stratification in the subpolar North Atlantic becomes much stronger, inducing a rapid collapse of AMOC. Our results further suggest that with a collapsed AMOC, the surface temperature, precipitation, evaporation, and cloudiness decrease in Northern Hemisphere, but increase in Southern Hemisphere. Under GHG forcing alone, the changes of these variables generally agree with those under freshwater forcing alone, and the differences show up mainly in the Polar Regions in association with the feedbacks due to the amplified warming there.