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.

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