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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 3 s - 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 ∘ N and 70 ∘ 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. [ABSTRACT FROM AUTHOR]

The impact of an idealised scenario of future mass release of major ice sheets on the Atlantic ocean is studied. A freshwater forcing is applied to the ocean surface in a coupled climate model forced in accordance with a high-end future climate projection for mass loss from the Greenland and Antarctic ice-sheet, together with the RCP8.5 emission scenario. The added freshwater dilutes the entire ocean by increasing total volume, but changes in freshwater budget are non-linear in time, especially in the Atlantic Ocean. In the Atlantic the initial dilution mainly comes from Greenland freshwater, but the increase in mass is counteracted by the mass flux across the boundaries of the Atlantic, with the outflow into the Southern Ocean becoming larger than the inflow through Bering Strait. Associated with this mass divergence, salt is exported to the Southern Ocean by the barotropic flow. Further freshening is associated with more freshwater import by the Atlantic Meridional Overturning Circulation across the southern boundary of the Atlantic. Also, the subtropical gyre exports salt and imports freshwater across the Atlantic's southern boundary, especially when freshwater from the Antarctic Ice Sheet arrives at the boundary of the basin. It appears that the response to Northern Hemisphere (NH) sources (the Greenland Ice Sheet) and Southern Hemisphere (SH) sources (the Antarctic Ice Sheet) are opposite. In the case of NH-only freshwater forcing, sea surface height (SSH) increases in the Arctic, causing a reduction of the SSH gradient over the Bering Strait, and hence the barotropic throughflow across the Arctic–Atlantic basin reduces. In case of SH-only freshwater forcing, SSH increases in the Pacific, enhancing the barotropic throughflow in the Arctic–Atlantic. When both NH and SH freshwater forcings are present, the response in the Atlantic is dominated by NH forcing. Changes in overturning transport to either NH or SH forcing counteract the response to changes in barotropic transport. These changes are not due to volume transport but mainly due to salinity changes, in particular across the southern boundary of the Atlantic. Only when both SH and NH freshwater forcing are present changes in barotropic transport and overturning transport reinforce each other: the barotropic transport more strongly reacts to NH forcing, while the overturning transport reacts more strongly to SH forcing. [ABSTRACT FROM AUTHOR]

The mechanisms for Bjerknes compensation of heat transport variations through the atmosphere and ocean on decadal time scales are investigated, using data output from a preindustrial control run of the Third Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3). It has recently been shown that Bjerknes compensation occurs on decadal time scales in a long preindustrial control run of HadCM3. This result is elaborated on by performing lead/lag correlations of the atmospheric and oceanic heat transports. By using statistical analysis, Bjerknes compensation is observed on decadal time scales at latitudes between 50° and 80°N. A maximum compensation rate of ∼55% occurs at 70°N. At this latitude, the correlation rate peaks when the ocean leads the atmosphere by one year. The mechanisms by which Bjerknes compensation occurs at this latitude are investigated. Anomalies in oceanic heat transport appear to be associated with variations in the strength of the Atlantic meridional overturning circulation (MOC). The associated sea surface temperature (SST) anomalies are in general too weak to assert a significant impact on the atmosphere. At 70°N, however, such SST anomalies are a prelude to the transition from sea ice coverage to open water after which the associated changes in heat exchange with the atmosphere are strong enough to force an atmospheric response. Because of the presence of a strong MOC component in the Atlantic Ocean, this interaction is confined to the region where the northeast Atlantic and Arctic Oceans connect. The atmospheric response to increased (decreased) heating from below is a decreased (increased) poleward temperature gradient, leading to a decreased (increased) heat transport by baroclinic eddies. The anomalous thermal low that is set up by heating from the ocean is associated with anomalous advection of cold air from the Greenland landmass. [ABSTRACT FROM AUTHOR]