On freshwater fluxes and the Atlantic meridional overturning circulation

Abstract

We address the role of freshwater forcing in the modern day ocean. Specifically, we ask the question of whether an amplification of the global freshwater forcing pattern leads to a strengthening or weakening of the steady‐state Atlantic Meridional Overturning Circulation (AMOC). While the role of freshwater forcing in the AMOC has received much attention, this question remains unresolved, in part because past studies have primarily investigated idealized models, large regime shifts away from the modern ocean state, or coupled atmosphere–ocean simulations on shorter timescales than required for the deep ocean to equilibrate. Here we study the AMOC's sensitivity at equilibrium to small perturbations in the magnitude of the global freshwater fluxes in simulations performed with a realistically configured ocean circulation model. Our results robustly suggest that for the equilibrium state of the modern ocean, freshwater fluxes strengthen the AMOC, in the sense that an amplification of the existing freshwater flux‐forcing pattern leads to a strengthening of the AMOC and vice versa. A simple physical argument explains these results: the North Atlantic is anomalously salty at depth and increased freshwater fluxes act to amplify that salinity pattern, resulting in enhanced AMOC transport.

Greenhouse-gas forced changes in the Atlantic meridional overturning circulation and related worldwide sea-level change

“…2c). Cael and Jansen (2020) argued that, all other things being equal, the equilibrium response of the AMOC to an intensification of the freshwater cycle will be a strengthening. The reason they gave was that intensified freshwater fluxes amplify the salinity contrasts that drive the AMOC.…”

“…Previous studies have examined the sensitivity of the AMOC to these different forcings in individual models (e.g. Bouttes and Gregory 2014;Garuba and Klinger 2018;Dias et al 2020b;Cael and Jansen 2020), the small original FAFMIP ensemble (Gregory et al 2016) and pairs of models (Todd et al 2020;Jin et al 2021a). Such studies suggest that the anomalous heat flux is the main driver of AMOC change under greenhouse gas forcing (in the absence of large freshwater additions via glacial melt, which is beyond the scope of this study).…”

“…Some studies find that freshwater cycle amplification strengthens the AMOC (e.g. Cael and Jansen 2020;Todd et al 2020). While Garuba and Klinger (2018) found evidence of weakening in an ocean-only model study, subsequent work using coupled simulations found that heat fluxes rather than freshwater fluxes drive AMOC weakening (Garuba and Rasch 2020).…”

Is the Surface Salinity Difference between the Atlantic and Indo-Pacific a Signature of the Atlantic Meridional Overturning Circulation?

“…Modeling studies indicate that if the present-day surface freshwater forcing pattern is amplified, the salinity difference between the North Atlantic and the North Pacific increases, and so does the AMOC (Cael and Jansen 2020). Some studies on the role of the net evaporation have emphasized local differences in subpolar regions (Warren 1983;Emile-Geay et al 2003), whereas others have emphasized basin-integrated differences (Weyl 1968;Rahmstorf 1996).…”

“…This may indicate that the geographical features assumed to favor Atlantic sinking are rather weak or that their impacts are inadequately represented in some climate models. The AMOC ''problem'' is usually addressed by tuning of model parameters and drainage pathways until a realistic AMOC is obtained: an approach that may yield a model AMOC with incorrect stability features and sensitivity to global warming (Stouffer et al 2006;Cimatoribus et al 2012;Weijer et al 2019;Cael and Jansen 2020). Second, the locations of the deep sinking and associated MOC pathways in past epochs of Earth can have a strong influence on carbon cycling and climate (DeConto and Pollard 2003;Ferrari et al 2014;Galbraith and de Lavergne 2019).…”