Abstract
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.
Highlights
The climate warms due to anthropogenic emissions of greenhouse gasses and, as a consequence, the ice sheets on Greenland and Antarctica are expected to loose mass (e.g. Joughin and Alley 2011)
The response to ice cap melting in the climate system is not well understood, since most climate models used for projections do not incorporate the complex interactions that lead to increased
We found that the Northern Hemisphere (NH) forcing leads to a positive feedback on the freshwater budget, amplifying the freshening in the Arctic–Atlantic basin
Summary
The climate warms due to anthropogenic emissions of greenhouse gasses and, as a consequence, the ice sheets on Greenland and Antarctica are expected to loose mass (e.g. Joughin and Alley 2011). Mass losses may increase further due to non-linear effects associated with ice sheet interactions with the atmosphere and ocean (see Hanna et al (2013) for an overview). In principle, coupled climate models (CCMs) could simulate such mass loss by including ice sheet (see Vizcaino 2014) and iceberg modules that simulate calving and iceberg drift in response to changing atmospheric and ocean temperatures. The current generation of CCMs used in the CMIP5 ensemble, and likely used in the CMIP6 ensemble (Eyring et al 2016), is not equipped with such modules and cannot simulate the mass loss of ice sheets to the ocean interactively with the other components of the climate system
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