Dense shelf water production in the Adélie Depression, East Antarctica, 2004–2012: Impact of the Mertz Glacier calving

Abstract

Summer repeated hydrographic surveys and 4 years of mooring observations are used to characterize for the first time the interannual variability of the bottom water in the Mertz Glacier Polynya (MGP) on the East Antarctic shelf (142 E-146 E). Until 2010, large interannual variability was observed in the summer bottom salinity with year-to-year changes reaching 0.12 in Commonwealth Bay, the region with the highest sea ice production. The summer variability was shown to be linked to the efficiency of the convection during the preceding winter. The recent freshening of the bottom waters subsequent to the Mertz Glacier calving was well beyond the range of the precalving interannual variability. Within 2 years after the event, the bottom water of the shelf became too light to possibly contribute to renewal of the Antarctic Bottom Water. Rough estimates of the freshwater budget of the Ad elie Depression indicate that the freshening necessary to compensate for net sea ice production in the MGP did not change drastically after the Mertz calving. The year-to-year salinity changes appeared to respond to the MGP activity. Yet, prior to the calving, the convective system in the polynya was also partly controlled by the late winter bottom salinity through a mechanism leading to a sequence of alternatively fresher and more saline bottom waters over the period [2007][2008][2009][2010]. Exceptional events like the Mertz calving seem to be able to switch over the system into a less stratified state where convection responds more directly to changes in the surface forcing. IntroductionAntarctic Bottom Water (AABW), the densest water mass of the world ocean, occupies the abyssal layer of the Southern Ocean as a relatively well-oxygenated, cold and fresh product, which spreads north into the Atlantic, Indian, and Pacific oceans. Surface water transformation on the continental margins around Antarctica and subsequent sinking of the dense shelf waters along the slope supply the bottom limb of the ocean thermohaline circulation. It has been argued that changes in the properties or formation rate of the AABW could affect the strength of the thermohaline circulation, and therefore the global climate [Stouffer et al., 2007;Purkey and Johnson, 2012].The production of AABW is considered to originate on the Antarctic shelf in coastal polynyas where nearfreezing water (the so-called High-Salinity Shelf Water, HSSW) is formed through ocean surface cooling and brine drainage from growing sea ice. Upon mixing with ambient waters on the shelf and subsequent sinking and entrainment of Circumpolar Deep Water down the continental slope, these waters eventually acquire final AABW properties when reaching the abyssal layer of the Southern Ocean. While this process is suspected to occur at many locations around Antarctica, observations have unambiguously identified some primary formation sites: the Weddell Sea, the Ross Sea [Carmack, 1977], and the Ad elie-George V Land [Gordon and Tchernia, 1972;Rintoul, 1998]. Estimates of the relative im...

Glacial ice and atmospheric forcing on the Mertz Glacier Polynya over the past 250 years

“…1e) over the CB and MGP areas have experienced similar and synchronous pre-and postcalving changes over the recent years 9 . In addition, recent observations highlighting the presence of the saltiest waters of the Adélie depression in CB suggest that large production rates of dense shelf water occur in the bay 9,12 . Based on past expeditions, it has been suggested that the MG previously underwent similar calving events in the early part of the twentieth century 13 .…”

“…Indeed, mooring observations and satellite data showed that oceanic and sea ice conditions (Fig. 1e) over the CB and MGP areas have experienced similar and synchronous pre-and postcalving changes over the recent years 9 . In addition, recent observations highlighting the presence of the saltiest waters of the Adélie depression in CB suggest that large production rates of dense shelf water occur in the bay 9,12 .…”

“…The generation of an 80-km-long iceberg and subsequent retreat of the MG front had a profound impact on both regional icescape and ocean conditions [9][10][11] . Satellite passive microwave data show that SIC increased by 50% (relative to the 1979-2009 period) in the MGP area after the calving (Fig.…”

“…1c,d) led to a large reduction in sea ice production in the MGP, which in turn affected dense water formation [9][10][11] . Within 2 years following the calving, bottom water salinity decreased by over 0.30 psu in Commonwealth Bay (CB) 9 .…”

“…Located in the southern part of the polynya, CB is ideal for studying past dynamics of the MGP and dense water formation 9 . Indeed, mooring observations and satellite data showed that oceanic and sea ice conditions (Fig.…”