Basal melting of Ross Ice Shelf from solar heat absorption in an ice-front polynya

Abstract

Ice-ocean interactions at the base of Antarctic ice shelves are rarely observed, yet have a profound influence on ice sheet evolution and stability. Ice sheet models are highly sensitive to assumed ice shelf basal melt rates; however, there are few direct observations of basal melting or the oceanographic processes that drive this, and consequently our understanding of these interactions remains limited. Here we use new in-situ observations from the Ross Ice Shelf to examine the oceanographic processes that drive basal ablation of the world's largest ice shelf. We show that basal melt rates beneath a thin and structurally important part of the shelf are an order of magnitude higher than the shelf-wide average. This melting is strongly influenced by a seasonal inflow of solar-heated surface water from the adjacent Ross Sea Polynya that downwells into the ice shelf cavity, nearly tripling basal melt rates during summer. Melting driven by this frequently overlooked process is expected to increase with predicted surface warming. We infer that solar heat absorbed in ice front polynyas can make an important contribution to the present-day mass balance of ice shelves, and potentially impact their future stability.

Multidecadal Basal Melt Rates and Structure of the Ross Ice Shelf, Antarctica, Using Airborne Ice Penetrating Radar

“…The observed heightened melting along most of the ice front (Figure ) has been previously identified from analyses of satellite laser altimetry (Horgan et al, ; Moholdt et al, ). Direct measurements of basal melt rate near Ross Island (Stewart et al, ) support the hypothesis that long‐term melt rates at Site E are dominated by rapid melting during summers when Antarctic Surface Water (AASW) has been warmed locally by insolation after the sea ice near the ice front has disappeared (e.g., Assmann et al, ; Stern et al, ; Stewart et al, ; Tinto et al, ). The subsequent advection of warmed AASW under the outer ice shelf may be driven by a combination of numerous processes including tides (Arzeno et al, ; MacAyeal, ; Makinson & Nicholls, ), eddies and other instabilities of the westward flowing ice front current (Li et al, ), and isopycnal stirring under a wedge of buoyant meltwater along the ice front (Malyarenko et al, ).…”

“…However, w b varies spatially over a wide range, from close to 0 over much of the central portion of the ice shelf to over 10 m a −1 near the deep grounding line of Byrd Glacier (Kenneally & Hughes, ) and in a channel near Whillans Ice Plain (Marsh et al, ). Relatively high basal melt rates of about 1–2 m a −1 have been estimated near the ice shelf front from Lagrangian analyses of ICESat repeat‐track laser altimetry (Horgan et al, ; Moholdt et al, ) and from an autonomous phase‐sensitive radio‐echo sounder survey in a small region near Ross Island (Stewart et al, ). Satellite‐based estimates of melt rate identify extensive regions where ice appears to be accreting to the base of the ice shelf (i.e., w b < 0); however, the rates are usually smaller than the associated uncertainties (Moholdt et al, ).…”

“…Various methods have been used to measure ice shelf basal melt rates. In situ measurements have been obtained using ice penetrating radar (Catania et al, 2010), autonomous phase-sensitive radio-echo sounder radars (e.g., Marsh et al, 2016;Stewart et al, 2019), and moored turbulence sensors (Stanton et al, 2013) and fiber-optic cables (Kobs et al, 2014) deployed through boreholes in the ice shelf. These measurements are, however, spatially sparse and provide estimates only over short periods, generally less than 1 year.…”

Long‐term projections of sea‐level rise from ice sheets

“…A reduction in sea ice formation rates could prevent this overturning and the warmer, deeper water would instead spread laterally and enhance submarine ice shelf melt, thereby thinning and weakening the shelf (Bronselaer et al, ). Other studies have shown that the direct solar heating of surface waters in polynyas (areas permanently free of sea ice) adjacent to the ice shelf front can also exacerbate sub‐shelf melting, perhaps ultimately affecting the structural stability of the whole ice shelf (Porter et al, ; C. Stewart, Christofferson, Nicholls, Williams, & Dowdeswell, ). A final possibility is that the presence of annealed pack‐ice works to mechanically inhibit iceberg calving, promoting a stronger calving front whose along‐flow strain rates are reduced compared to situations where no pack‐ice exists.…”

Evolution of the Seasonal Surface Mixed Layer of the Ross Sea, Antarctica, Observed With Autonomous Profiling Floats

“…The seasonality of both HSSW production and circulation of Ice Shelf Water (ISW) from under the Ross Ice Shelf contributes to seasonal variability of Antarctic Bottom Water production near the continental shelf break (Budillon et al, 2011). Local warming of the upper ocean near the Ross Ice Shelf front, which contributes to the observed rapid melting in the ice shelf's frontal zone (Arzeno et al, 2014;Horgan et al, 2011;Malyarenko et al, 2019;Moholdt et al, 2014;Stewart et al, 2019;Tinto et al, 2019), only occurs during austral summer when there is minimal sea ice. The southward flux and hydrographic properties of the subsurface layer of modified Circumpolar Deep Water (mCDW) near Hayes Bank also vary seasonally (Pillsbury & Jacobs, 1985).…”