North Atlantic Oscillation impact on the Atlantic Meridional Overturning Circulation shaped by the mean state

Abstract

Accurate representation of the Atlantic Meridional Overturning Circulation (AMOC) in global climate models is crucial for reliable future climate predictions and projections. In this study, we used 42 coupled atmosphere–ocean global climate models to analyze low-frequency variability of the AMOC driven by the North Atlantic Oscillation (NAO). Our results showed that the influence of the simulated NAO on the AMOC differs significantly between the models. We showed that the large intermodel diversity originates from the diverse oceanic mean state, especially over the subpolar North Atlantic (SPNA), where deep water formation of the AMOC occurs. For some models, the climatological sea ice extent covers a wide area of the SPNA and restrains efficient air–sea interactions, making the AMOC less sensitive to the NAO. In the models without the sea-ice-covered SPNA, the upper-ocean mean stratification critically affects the relationship between the NAO and AMOC by regulating the AMOC sensitivity to surface buoyancy forcing. Our results pinpoint the oceanic mean state as an aspect of climate model simulations that must be improved for an accurate understanding of the AMOC.

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

“…Note that the wave train is consistently shown in the boreal winter (DJF), when both the Rossby wave teleconnection and the SPNA deep convection are dominant (Figure S5 in Supporting Information ). The NAO is known to modulate the AMOC intensity through the surface buoyancy response in the deep convection regions, a relationship corroborated by many climate models, including the employed model (e.g., Delworth & Zeng, 2016; Kim et al., 2023; Medhaug et al., 2012). In alignment with this, the presence of an anticyclone over Greenland corresponds to a weakening of the surface westerly winds over the SPNA, resulting in a significant reduction of regional evaporation (Figure 3b).…”

“…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 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).…”