Lagrangian perspective on the drivers of future AMOC decline: reduced upper limb water availability and weaker diapycnal transformation efficiency

Speaker: Soumaïa Tajouri (BAS, University of Southampton
Series: British Antarctic Survey - Polar Oceans seminar series
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The formation of dense waters in the Subpolar North Atlantic (SPNA) and Nordic seas is an essential component of the Atlantic Meridional Overturning Circulation (AMOC). Here, we explore the mechanisms responsible for projected AMOC-weakening at 49N by comparing two Lagrangian water-parcel trajectory experiments representing strong (1972) and weak (2050) AMOC states in an eddy-permitting coupled climate model. AMOC-weakening is associated with a decrease in the volume transport of the subpolar gyre boundary current (-6.6 Sv), which is almost entirely compensated by an increase (+5.7 Sv) in the southward North Atlantic Current (NAC) recirculation south of 55N. The volume transport of dense water formed north of 49N nearly halves between 1972 and 2050 (-8.16 Sv). This reduction is dominated by the subpolar gyre boundary current pathway (85.5%), particularly via the western (28.9%) and central (49.8%) branches of the NAC. We find a regime shift in subpolar North Atlantic overturning under climate change: from a historical regime largely independent of initial thermohaline properties, with a steady along-stream diapycnal transformation rate, where the availability of upper limb water is the main constraint, to a future one in which overturning is both limited by the reduced efficiency of along-stream diapycnal transformation and the decreased availability (inflow) of upper-limb water. The reduced densification-efficiency is due to both the inflow of fresher upper-limb waters via the branches of the NAC, and enhanced along-stream freshening, preventing parcels from becoming dense enough to enter the lower-limb. Our findings emphasize the importance of pathway-dependent water-mass transformation and the emerging role of the salinity of upper-limb waters in modulating densification and future AMOC-decline.