Circulation adjustment in the Arctic and Atlantic in response to Greenland and Antarctic mass loss

Abstract

AbstractFollowing a high-end projection for mass loss from the Greenland and Antarctic ice-sheets, a freshwater forcing was applied to the ocean surface in the coupled climate model EC-Earthv2.2 to study the response to meltwater release assuming an RCP8.5 emission scenario. The meltwater forcing results in an overall freshening of the Atlantic that is dominated by advective changes, strongly enhancing the freshening due to dilution by Greenland meltwater release. The strongest circulation change occurs in the western North Atlantic subpolar gyre and in the gyre in the Nordic Seas, leaving the North Atlantic subpolar gyre the region where most advective salt export occurs. Associated with counteracting changes in both gyre systems, the response of the Atlantic Meridional Overturning Circulation is rather weak over the 190 years of the experiment; it reduces with only 1 Sv ($$= 10^6$$ = 10 6 m $$^3$$ 3 s $$^{-1}$$ - 1 ), compared to changes in the subpolar gyre of 5 Sv. This relative insensitivity of the AMOC to the forcing is attributed to enhanced convection in the Nordic Seas and stronger overflows that compensate reduced convection in the Labrador and Irminger Seas, and lead to higher sea surface temperatures (SSTs) in the former and lower SSTs in the latter region. The weakened subpolar gyre in the west also shifts the North Atlantic Current and the subpolar-subtropical gyre boundary; with the subtropical gyre expanding, and the western subpolar gyre contracting. The SST changes are associated with obduction of Atlantic waters in the Nordic Seas that would otherwise obduct in the western subpolar gyre. The anomalous SSTs also induce a coupled ocean-atmosphere feedback that further strengthens the Nordic Seas circulation and weakens the western subpolar gyre. This occurs because the anomalous SST-gradient enhances the westerlies, especially between 65$$^{\circ }$$ ∘ N and 70$$^{\circ }$$ ∘ N; the associated increase in windstress curl consequently enhances the gyre in the Nordic Seas. This feedback is driven by the Greenland mass loss; Antarctic meltwater discharge causes a weaker, opposite response and more particularly affects the South Atlantic salinity budget through northward advection of low-salinity waters from the Southern Ocean. This effect, however, becomes visible only hundred years after the onset of Antarctic mass loss. We conclude that the response to freshwater forcing from both ice caps can lead to a complex response in the Atlantic circulation systems with opposing effects in different subbasins. The relative strength of the response is time-dependent and largely governed by internal feedbacks; the forcing acts mainly as a trigger and is decoupled from the response.