Regionally-contrasted melting of Antarctic ice shelves in response to Southern Annular Mode variations

The Antarctic ice sheet represents the largest reservoir of potential sea-level rise (SLR) on Earth. Its contribution to SLR is largely controlled by the basal melting of ice shelves. Understanding the physical drivers behind this phenomenon may thus provide key insight for apprehending future SLR.
Regionally-contrasted melting of Antarctic ice shelves in response to Southern Annular Mode variations

In this study, we assess the impact of one of the most prominent climate variations during the last century in the high latitudes of the Southern Hemisphere, namely the increase in the Southern Annular Mode (SAM) index, on the basal melting of ice shelves (the floating extensions of the ice sheet) around Antarctica and its related mechanisms. The SAM index is a measure of the difference in atmospheric pressure between the mid- and the high latitudes of the Southern Hemisphere. To this end, we performed idealized numerical experiments at the pan-Antarctic scale with an ocean—sea-ice model taking into account the ocean circulation within ice-shelf cavities for different phases of the SAM (positive phase, negative phase and positive phase only manifested through changes in the wind variables).

We show that positive phases of the SAM (characterized by an increased intensity and southward shift of the westerly wind belt) lead to increased advection of warm and saline waters (referred to as "upwelling") towards the ice shelves, and increasing subsurface ocean temperature and salinity close to the base of ice shelves, while the opposite occurs for negative phases. A one-standard-deviation increase of the SAM leads to a net basal mass loss of 40 Gt yr−1 (i.e., about half the Antarctic ice-sheet mass change over the period 1992–2011) with strong regional contrasts around Antarctica. We observe increased ice-shelf basal melt in the Bellingshausen sector (a very sensitive region located on the western side of the Antarctic Peninsula) and the Western Pacific sector (harboring mainly small ice-shelf cavities that may be little resilient to climate variations) and the opposite response in the Amundsen sector (located between the Bellingshausen sector and the Ross ice shelf).

Changes in ice shelf basal melt (meters per year) around Antarctica due to positive phases of the Southern Annular Mode.

The decrease in basal melt observed in the Amundsen region could be due to two different mechanisms. The first one is the increase in northward winds that we observe during positive phases of the SAM, that directly implies a cooling of surface waters in the eastern Ross Sea due to colder air advected from the continent, which is transferred to the subsurface through intense vertical mixing of the ocean. The second one is related to the so-called "slope current", a strong oceanic current flowing westward along the continental shelf of Antarctica. During positive phases of the SAM, the slope current intensity increases in the Amundsen region and the eastern Ross Sea, thereby acting as a barrier preventing the warm and saline waters brought by the upwelling from reaching the base of ice shelves in these regions.

Changes in the Antarctic slope current intensity (meters per second) due to positive phases of the Southern Annular Mode.

Estimates of ice-shelf basal melt changes due to the SAM for the periods 1000–1200 and 2090–2100 are −86.6 Gt yr−1 and 55.0 to 164.9 Gt yr−1 (depending on the scenario), respectively, compared to the present, indicating the potentially important impact of the SAM on the Antarctic mass balance and its subsequent contribution to sea-level rise. Our idealized experimental design enabled us to clearly identify and isolate SAM-related effects on ice shelf-ocean interactions, but the contrasted (both sector and process-wise) response of the Antarctic cryosphere suggests that the physics at play cannot be interpreted uniformly and unequivocally. In particular, investigating the joint impact of SAM- and El Niño Southern Oscillation-related processes on ice-shelf cavities remains a considerable challenge, as the response of the cryosphere to such climate fluctuations may be more complex than the superposition of the responses to each of them taken separately.

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