Decadal fingerprints of freshwater discharge around Greenland in a multi-model ensemble

Abstract

International audienceThe recent increase in the rate of the Greenland ice sheet melting has raised with urgency the question of the impact of such a melting on the climate. As former model projections, based on a coarse representation of the melting, show very different sensitivity to this melting, it seems necessary to consider a multi-model ensemble to tackle this question. Here we use five coupled climate models and one ocean-only model to evaluate the impact of 0.1 Sv (1 Sv = 10⁶ m³/s) of freshwater equally distributed around the coast of Greenland during the historical era 1965-2004. The ocean-only model helps to discriminate between oceanic and coupled responses. In this idealized framework, we find similar fingerprints in the fourth decade of hosing among the models, with a general weakening of the Atlantic Meridional Overturning Circulation (AMOC). Initially, the additional freshwater spreads along the main currents of the subpolar gyre. Part of the anomaly crosses the Atlantic eastward and enters into the Canary Current constituting a freshwater leakage tapping the subpolar gyre system. As a consequence, we show that the AMOC weakening is smaller if the leakage is larger. We argue that the magnitude of the freshwater leakage is related to the asymmetry between the subpolar-subtropical gyres in the control simulations, which may ultimately be a primary cause for the diversity of AMOC responses to the hosing in the multi-model ensemble. Another important fingerprint concerns a warming in the Nordic Seas in response to the re-emergence of Atlantic subsurface waters capped by the freshwater in the subpolar gyre. This subsurface heat anomaly reaches the Arctic where it emerges and induces a positive upper ocean salinity anomaly by introducing more Atlantic waters. We found similar climatic impacts in all the coupled ocean-atmosphere models with an atmospheric cooling of the North Atlantic except in the region around the Nordic Seas and a slight warming south of the equator in the Atlantic. This meridional gradient of temperature is associated with a southward shift of the tropical rains. The free surface models also show similar sea-level fingerprints notably with a comma-shape of high sea-level rise following the Canary Current

The Holocene thermal maximum in the Nordic Seas: the impact of Greenland Ice Sheet melt and other forcings in a coupled atmosphere–sea-ice–ocean model

“…Therefore, our model experiments do not support the idea that higher summer SSTs were caused by increased inflow of warm Atlantic waters as proposed by Risebrobakken et al (2011). However, in other modelling studies -for instance Swingedouw et al (2012), Kleinen et al (2009) andStouffer et al (2006) -such increase in warm inflow is simulated in response to surface ocean freshening.…”

Recent Progress in Understanding and Projecting Regional and Global Mean Sea Level Change

“…The enhanced RSL rise in the North Atlantic is also a part of dipole feature, with reduced RSL rise to the south, though its geographical detail is model dependent [145,146]. This feature has previously been associated with weakening of the Atlantic Meridional Overturning Circulation (AMOC) due to buoyancy forcing, and is caused by a change in surface heat flux (a reduction of heat loss in the North Atlantic in a warming climate) and increase of surface freshwater flux in CMIP5 models (Bouttes and Gregory [139]), as in earlier models.…”

“…This feature has previously been associated with weakening of the Atlantic Meridional Overturning Circulation (AMOC) due to buoyancy forcing, and is caused by a change in surface heat flux (a reduction of heat loss in the North Atlantic in a warming climate) and increase of surface freshwater flux in CMIP5 models (Bouttes and Gregory [139]), as in earlier models. RSL rise in the north of the North Atlantic, whether due to heat or freshwater input, is mainly the direct result of the added buoyancy [145,146], and the consequent weakening of the AMOC brings about a redistribution of heat which opposes the effect of the surface flux change on RSL in most of the affected area, but reinforces it along the east coast of North America [145], where the projected RSL rise is consequently particularly large [147]. Howard et al [148] assessed the magnitude of the effects of increased freshwater inflow from ice sheet mass loss on ocean dynamic sea level change and the AMOC to be small compared with the effects of changing surface fluxes from the atmosphere.…”

The land-ice contribution to 21st-century dynamic sea level rise

“…Gregory et al, 2005;Meehl et al, 2007;Pardaens et al, 2011). Some DSL studies impose an increase in surface freshwater to the northern North Atlantic ("hosing"), and in some cases this additional water is confined around Greenland to represent additional ice sheet melt (Swingedouw et al, 2013). Many features of DSL change in the North Atlantic, both in projections under greenhouse gas warming and in hosing experiments, are related to a weakening of the meridional overturning circulation (MOC) (e.g.…”

“…For example Swingedouw et al (2013) show DSL impacts of a 0.1 Sv hosing around Greenland averaged over the fourth decade of hosing for four different models (their Fig. 16).…”