Slow and soft passage through tipping point of the Atlantic Meridional Overturning Circulation in a changing climate

Abstract

Paleo-proxy records suggest that the Atlantic Meridional Overturning Circulation (AMOC) exhibits a threshold for an abrupt change, a so-called tipping point. A classical bifurcation theory, a basis of the tipping dynamics of AMOC implicitly assumes that the tipping point is fixed. However, when a system is subjected to time-varying forcing (e.g., AMOC exposed to ice meltwater) an actual tipping point can be overshot due to delayed tipping, referred to as the slow passage effect. Here, using an Earth system model of intermediate complexity and a low-order model with freshwater forcing, we show that the tipping point of AMOC is largely delayed by the slow passage effect. It causes a large tipping lag of up to 1300 years, and strongly relaxes the abruptness of tipping as well. We further demonstrate that the tipping modulation can actively occur in past, present, and future climates by quantifying the effect during Dansgaard-Oeschger events, meltwater pulse 1A (MWP-1A), and current Greenland ice sheet melting. The suggested slow passage effect may explain the observed lagged AMOC collapse to MWP-1A of about 1000 years and provides implications tipping risk in the future.

Fast and Slow Responses of Atlantic Meridional Overturning Circulation to Antarctic Meltwater Forcing

“…For example, the AMOC is known as notorious spread among current climate models (e.g., Gong et al., 2022). Thus, even if the atmospheric teleconnection rapidly adjusts the AMOC, the magnitude of the adjustment would be highly variable across models (e.g., Kim et al., 2023). In addition, although the tropical response is similar to that shown in the studies imposing the Antarctic meltwater (Bronselaer et al., 2018; Oh et al., 2020), some models project a similar weakened convection in response to the Antarctic sea‐ice loss, which accompanies with surface warming (e.g., Ayres et al., 2022; England, Polvani, et al., 2020).…”

The Influence of Increased CO2 Concentrations on AMOC Interdecadal Variability Under the LGM Background

“…This confirms the driving role of NAO in AMOC interdecadal variability. Actually, numerous model simulations have demonstrated that NAO variability exerts a significant impact on AMOC variability across interannual to interdecadal timescales (Jackson et al., 2022; Kim et al., 2023; Olsen et al., 2012; X. Wang et al., 2017). This influence is primarily realized through the NAO's effect on the surface wind field and air‐sea buoyancy fluxes (Buckley & Marshall, 2016; Delworth & Zeng, 2016; Visbeck et al., 1998; Zhang et al., 2019).…”

“…This leads us to revisit the behavior of AMOC interdecadal variability to CO 2 concentrations at different backgrounds. Moreover, given that the AMOC interdecadal variability involves a variety of processes, these mechanisms include, but are not limited to, the exchange of freshwater between the North Atlantic and the Arctic Ocean (Delworth et al., 1997; Hawkins & Sutton, 2007; W. Liu & Fedorov, 2022; W. Liu et al., 2019), the westward propagation of large‐scale baroclinic Rossby waves (Buckley et al., 2012; Frankcombe et al., 2008), the westerly winds over the Southern Ocean (Delworth & Zeng, 2008), and the North Atlantic Oscillation (NAO) (Kim et al., 2023; Megann et al., 2021; Ruprich‐Robert & Cassou, 2015; Timmermann et al., 1998). The aforementioned future warming studies all emphasize the oceanic process in modulating the AMOC variability (Cheng et al., 2016; Ma et al., 2021).…”

Evaluation of the mechanisms acting on the Atlantic Meridional Overturning Circulation in CESM2 for the 1pctCO2 experiment

“…The North Atlantic Ocean's total surface freshwater flux (FW) was calculated using Equations (1) and ( 2) to assess the region's freshwater changes. The green-shaded area in Figure 1 shows the region of interest for this analysis, which has also been used in previous studies [17,45,46] . Understanding freshwater inputs to the North Atlantic Ocean is important because they can impact ocean circulation and climate patterns.…”