Abstract
Anthropogenic climate change is projected to lead to a weakening of the Atlantic meridional overturning circulation (AMOC). One of the mechanisms contributing to this is ice melt leading to a freshening of the North Atlantic Ocean. We use two global climate models to investigate the role of temperature and salinity in the weakening of the AMOC resulting from freshwater forcing. This study finds that freshwater hosing reduces the strength of the AMOC, but in some situations it is not through reduced density from freshening, but a reduction in density from subsurface warming. When the freshwater is mixed down it directly reduces the density of the North Atlantic, weakening the strength of the AMOC. As the AMOC weakens, the mixed layer depth reduces and surface properties are less effectively mixed down. A buoyant surface cap forms, blocking atmospheric fluxes. This leads to the development of a warm anomaly beneath the surface cap, which becomes the primary driver of AMOC weakening. We found that the mean North Atlantic salinity anomaly can be used as a proxy for AMOC weakening because it describes the extent of this surface cap.
Highlights
Climate change projections suggest that global warming will cause ice melt and freshening of the North Atlantic Ocean
Atlantic meridional overturning circulation (AMOC) weakening in response to freshwater hosing in FAMOUS is invariably driven by warming at 50°N, Fig. 3
In all of our simulations, the temperature contribution eventually dominates over salinity in driving AMOC decline in response to freshwater hosing
Summary
Bryan 1987; Butler et al 2016; Sévellec and Huck 2016) indicate that density changes that are confined to the surface have a negligible effect on the AMOC, because thermohaline-led changes to the large scale circulation require perturbations to horizontal pressure gradients over a substantial depth range (Oliver et al 2005). It is necessary for the buoyant anomaly to propagate downwards through the water column; which is assumed to occur through the mixing down of freshwater. We use two general circulation models (GCMs) to understand the timescales and forcing rates at which different ocean mechanisms become the primary driver of AMOC weakening
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