Contribution of giant icebergs to the Southern Ocean freshwater flux

In the period 1979–2003 the mass of “giant” icebergs (icebergs larger than 18.5 km in length) calving from Antarctica averaged 1089 ± 300 Gt yr−1 of ice, under half the snow accumulation over the continent given by the Intergovernmental Panel on Climate Change (2246 ± 86 Gt yr−1). Here we combine a database of iceberg tracks from the National Ice Center and a model of iceberg thermodynamics in order to estimate the amount and distribution of meltwater attributable to giant icebergs. By comparing with published modeled meltwater distribution for smaller bergs we show that giant icebergs have a different melting pattern: An estimated 35% of giant icebergs' mass is exported north of 63°S versus 3% for smaller bergs, although giant bergs spend more of the earlier part of their history nearer to the coast. We combine both estimates to produce the first iceberg meltwater map that takes into account giant icebergs. The average meltwater input is shown to exceed precipitation minus evaporation (P − E) in certain areas and is a nonnegligible term in the balance of freshwater fluxes in the Southern Ocean. The calving of giant icebergs is, however, episodic; this might have implications for their impact on the freshwater budget of the ocean. It is estimated that over the period 1987–2003 the meltwater flux in the Weddell and Ross seas has varied by at least 15,000 m3 s−1 over a month. Because of the potential sensitivity of the production of deep waters to abrupt changes in the freshwater budget, variations in iceberg melt rates of this magnitude might be climatologically significant

Calving fluxes and basal melt rates of Antarctic ice shelves

Iceberg calving has been assumed to be the dominant cause of mass loss for the Antarctic ice sheet, with previous estimates of the calving flux exceeding 2,000 gigatonnes per year1, 2. More recently, the importance of melting by the ocean has been demonstrated close to the grounding line and near the calving front3, 4, 5. So far, however, no study has reliably quantified the calving flux and the basal mass balance (the balance between accretion and ablation at the ice-shelf base) for the whole of Antarctica. The distribution of fresh water in the Southern Ocean and its partitioning between the liquid and solid phases is therefore poorly constrained. Here we estimate the mass balance components for all ice shelves in Antarctica, using satellite measurements of calving flux and grounding-line flux, modelled ice-shelf snow accumulation rates6 and a regional scaling that accounts for unsurveyed areas. We obtain a total calving flux of 1,321 ± 144 gigatonnes per year and a total basal mass balance of −1,454 ± 174 gigatonnes per year. This means that about half of the ice-sheet surface mass gain is lost through oceanic erosion before reaching the ice front, and the calving flux is about 34 per cent less than previous estimates derived from iceberg tracking1, 2, 7. In addition, the fraction of mass loss due to basal processes varies from about 10 to 90 per cent between ice shelves. We find a significant positive correlation between basal mass loss and surface elevation change for ice shelves experiencing surface lowering8 and enhanced discharge9. We suggest that basal mass loss is a valuable metric for predicting future ice-shelf vulnerability to oceanic forcing

Iceberg trajectory modeling and meltwater injection in the Southern Ocean

This is the first large-scale modeling study of iceberg trajectories and melt rates in the Southern Ocean. An iceberg model was seeded with climatological iceberg calving rates based on a calculation of the net surface accumulation from each snow catchment area on the Antarctic continent. In most areas, modeled trajectories show good agreement with observed patterns of iceberg motion, though discrepencies in the Weddell Sea have highlighted problems in the ocean general circulation model output used to force the iceberg model. The Coriolis force is found to be important in keeping bergs entrained in the coastal current around Antarctica, and topographic features are important in causing bergs to depart from the coastal regions. The modeled geographic distribution of iceberg meltwater joining the ocean has been calculated and is found in many near-coastal regions to be comparable in magnitude to the excess of precipitation over evaporation (P-E)