Atlantic salinity budget in response to Northern and Southern Hemisphere ice sheet discharge

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.

Climate Response to Increasing Antarctic Iceberg and Ice Shelf Melt

“…S4 in the online supplemental material). The SSS response here and throughout the Northern Hemisphere agrees with that in van den Berk et al (2019), where increasing meltwater fluxes around Antarctica resulted in changes to ocean circulation that impacted on salt transport into (and out of) the Arctic Ocean over a similar time scale to our experiments, using a similar model. The mechanisms shown in that work to link an Antarctic meltwater perturbation to SSS changes in the North Atlantic are complex, and we refer the reader to van den Berk et al (2019) for a comprehensive description.…”

“…Further work using our results would be a useful verification of those studies but is beyond the scope of the present study. Nonetheless, our findings support the assertion that the effects of Antarctic meltwater fluxes on Northern Hemisphere ocean circulation are significant, possibly more so than Arctic meltwater effects since Antarctica represents a much larger freshwater source (van den Berk et al 2019), and because the surface salinity response does not remain local to the Antarctic meltwater perturbation location (in contrast to Arctic meltwater perturbations; Stouffer et al 2007).…”

“…The ocean surface cools and midlayer waters warm in response to increases in both ice shelf and iceberg melt, in agreement with Pauling et al (2016) and Bronselaer et al (2018); however, our results show that the warming response extends farther north and to greater depths for increasing ice shelf melt. We find near-surface density changes that agree with the response to Antarctic meltwater-induced circulation changes in van den Berk et al (2019), even far from the perturbation source, and we show that these changes are more sensitive to increased, concentrated meltwater anomalies at depth than to distributed melt at the surface. We demonstrate that, as a more concentrated meltwater source, increasing ice shelf melt has a greater impact on the meridional density gradient than increasing iceberg melt, and therefore drives a more severe decrease in ACC transport.…”

“…Neglecting the increasing melt rate of Antarctica could therefore have implications for projections of Northern Hemisphere climate. Some research into the mechanisms that drive these distal effects exists (Richardson et al 2005;Stouffer et al 2007;van den Berk et al 2019), but more is needed so that we can understand the implications of increasing Antarctic meltwater fluxes for the wider Northern Hemisphere climate system.…”

“…The AMOC is important to Northern Hemisphere climate (Buckley and Marshall 2016;Sévellec and Fedorov 2016) and is projected to decline as the climate warms (Rahmstorf et al 2015). Links between the two overturning cells have been found; for example, Weaver et al (2003) found a freshwater perturbation in the Southern Hemisphere resulted in reduced AABW production, which ''reactivated'' the AMOC from an ''off'' state, providing a mechanism by which changes to Antarctic sea ice could impact on Northern Hemisphere climate, and other studies have also found Northern Hemisphere changes to result from an Antarctic meltwater perturbation (Richardson et al 2005;van den Berk et al 2019).…”