Climate response to the meltwater runoff from Greenland Ice Sheet: evolving sensitivity to discharging locations

Greenland Ice Sheet (GIS) might have lost a large amount of its volume during the last interglacial and may do so again in the future due to climate warming. In this study, we show that the climate response to the GIS meltwater is sensitive to its discharging location initially but become insensitive after two to three hundred years. Two fully coupled atmosphere-ocean general circulation models, CM2G and CM2M, are employed to do the test. They differ in only their ocean components, one with isopycnal coordinate and the other with z-coordinate. The ocean components of both model are run at the nominal 1° horizontal resolution. In each experiment, a prescribed freshwater flux of 0.1 Sv is discharged into a single gridbox near one of the four locations around Greenland - Petermann, 79 North, Jacobshavn and Helheim glaciers. The results from both models show that the climate impact during the first two to three hundred years, in terms of AMOC and sea ice extent, is 15% (CM2G) and 31% (CM2M) stronger when the freshwater is discharged from the northern GIS (Petermann and 79 North) than when it is discharged from the southern GIS (Jacobshavn and Helheim). This is due to easier access of the freshwater from northern GIS to the deepwater formation site in the Nordic Seas. In the long term (>300 year), however, the climate impacts become similar for freshwater discharged from all locations of the GIS. The East Greenland current accelerates with time and becomes significantly faster when the freshwater is discharged from the north than from the south. Therefore, freshwater from the north is transported efficiently towards the south first and then circulates back to the the Nordic Seas, making its impact to the deepwater formation there similar to the freshwater discharged from the south. Our study demonstrates that if freshwater is injected into the ocean in a very localized form as in the real world, its ability to impact the deepwater formation evolves with time. At equilibrium state, the impact of freshwater from upstream of deepwater formation site is not necessarily larger than that from other locations, as obtained by relatively low-resolution models. This may have implication on the deglacial phase of glacial cycles, during which freshwater discharge often lasts for many hundreds of years, and often studied with low-resolution models.