Abstract
Following 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 m ^3s ^{-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.
Highlights
In a warming climate, the increase in atmospheric temperature will affect the ocean and cryosphere (Pörtner et al (2019) provides a synthesis of current research on this topic)
The overturning circulation further salinifies the subpolar gyre and freshens the subtropical gyre through changes in velocity (Fig. 5, blue dashed lines). At first sight this is surprising because the Atlantic Meridional Overturning Circulation (AMOC) transports salt from the STG to the SPG, a weakening of the AMOC in response to freshwater hosing is expected to result in subpolar freshening and subtropical salinification
The overturning increase in the eastern North Atlantic is coupled to an increased gyre circulation in the Nordic Seas, with a decrease in the western subpolar gyre (Fig. 6), which is consistent with the results described in Zhang et al (2011)
Summary
The increase in atmospheric temperature will affect the ocean and cryosphere (Pörtner et al (2019) provides a synthesis of current research on this topic). Deep convection in the Labrador, Irminger and Nordic Seas is tightly coupled to the AMOC, as shown by model experiments in which the AMOC weakens when deep convection declines in response to additional freshwater release (Stouffer et al 2006; Brodeau and Koenigk 2016; Drijfhout 2015) This coupling is corroborated by analysis of the pathways of the lower branch of the AMOC (Lavender et al 2000; Gary et al 2011; Rhein et al 2017) which shows that an interior pathway from the subpolar to the subtropical North Atlantic gyres carries a significant export of deep water.
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