Interannual variations in meltwater input to the Southern Ocean from Antarctic ice shelves

Abstract

Ocean-driven basal melting of Antarctica's floating ice shelves accounts for about half of their mass loss in steady-state, where gains in ice shelf mass are balanced by losses. Ice shelf thickness changes driven by varying basal melt rates modulate mass loss from the grounded ice sheet and its contribution to sea level, and the changing meltwater fluxes influence climate processes in the Southern Ocean. Existing continent-wide melt rate datasets have no temporal variability, introducing uncertainties in sea level and climate projections. Here, we combine surface height data from satellite radar altimeters with satellite-derived ice velocities and a new model of firn-layer evolution to generate a high-resolution map of time-averaged (2010-2018) basal melt rates, and time series (1994-2018) of meltwater fluxes for most ice shelves. Total basal meltwater flux in 1994 (1090±150 Gt/yr) was not significantly different from the steady-state value (1100±60 Gt/yr), but increased to 1570±140 Gt/yr in 2009, followed by a decline to 1160±150 Gt/yr in 2018. For the four largest "cold-water" ice shelves we partition meltwater fluxes into deep and shallow sources to reveal distinct signatures of temporal variability, providing insights into climate forcing of basal melting and the impact of this melting on the Southern Ocean.

Figure 1:. Basal melt rates of Antarctic ice shelves using CryoSat-2 altimetry.

Rates are averaged over 2010–2018 and shown at 500 m posting. The units are m of ice equivalent, assuming an ice density of 917 kg/m³. The thermal forcing, defined as the temperature above the in situ freezing point of seawater, is mapped for water depths <1500 m. For water depths less than 200 m the seafloor thermal forcing is shown, and for water depths >200 m, the maximum thermal forcing between 200 m and 800 m is shown (Methods Section S2).

Figure 2:. Vertical structure of melting and refreezing rates for selected ice shelves.

Depth-dependence of area-integrated meltwater flux (2010–2018) per m of ice shelf draft (depth of the ice shelf base below sea level) for six major ice shelves (locations shown in Figure 1). The scale for the horizontal axis is shown by the solid black line within the figure. The shaded regions in red and blue represent the mean values, and the dashed lines represent 95% confidence intervals. The purple lines are hypothetical steady-state meltwater fluxes (i.e., the meltwater fluxes required to maintain constant ice shelf mass). Warm-water ice shelves are distinguished from cold-water ice shelves by their higher average rates of meltwater production driven by intrusions of warm Circumpolar Deep Water (CDW) or modified CDW into the ice shelf cavity.

Figure 3:. Variations in Antarctic ice shelf mass between 1994 and 2018.

(a) Cumulative ice shelf mass change between 1994 and 2018 for the Pacific (red), Atlantic (blue), and Indian (orange) ocean sectors of Antarctica, with shading showing 95% confidence intervals. The region definitions are shown on the map, and the combined total for all ice shelves is shown in black. (b) Meltwater fluxes for 1994–2018 from ocean-driven ice shelf basal melting for the same regions. Dashed lines represent meltwater fluxes in steady-state, where the mass of the ice shelves is constant through time. Total meltwater flux estimates for the ICESat era are averaged between two studies^,.

Figure 4:. Time-dependent basal melt rates for different modes of melting.

(a-d) Area-averaged basal melt rates for selected regions within the four largest Antarctic ice shelves. Regions shown in red experience melting predominantly from cold, High Salinity Shelf Water inflows at deep ice drafts (Mode 1), while regions shown in blue typically experience melting from intrusions of Antarctic Surface Water at shallow ice drafts (Mode 3). (e) Basal melt rates for Amundsen Sea ice shelves, which experience melting from inflows of warm Circumpolar Deep Water (Mode 2). Gaps in the spatial coverage reflect the sampling of the altimeters prior to CryoSat-2.