Modelling the sensitivity of dense shelf water formation in the Mertz Glacier region, East Antarctica
Given the importance of the overturning circulation to global climate, there is a need to improve our understanding of Antarctic Bottom Water (AABW) formation and its sensitivity to change. The offshore properties of AABW are changing. Within the Australian-Antarctic basin AABW has freshened and dec...
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| Format: | Thesis |
| Language: | English |
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2016
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| Online Access: | https://eprints.utas.edu.au/23427/ https://eprints.utas.edu.au/23427/1/Cougnan_whole_thesis.pdf |
| Summary: | Given the importance of the overturning circulation to global climate, there is a need to improve our understanding of Antarctic Bottom Water (AABW) formation and its sensitivity to change. The offshore properties of AABW are changing. Within the Australian-Antarctic basin AABW has freshened and decreased in volume by about 50% over the last few decades. Understanding what is driving these changes requires focusing on the key formation region along the Adélie and George V Land (AGVL) coast. Here, the intense production of sea ice in the Mertz Glacier Polynya system drives Dense Shelf Water (DSW) formation, the precursor to AABW. This thesis uses a version of the Regional Ocean Modeling System (ROMS) that has been adapted for ocean/ice-shelf interactions to explore the sensitivity of DSW formation to surface heat and salt fluxes and ice shelf basal melting. Interannual variability in surface heat and salt fluxes drives DSW export and ice shelf basal melting variability in the AGVL region. DSW export decreases by 86% during a sustained period (2000-2002) of weak polynya activity (sea ice production) before recovering during a sustained period of stronger polynya activity (2003-2005). Basal melting of the Mertz Glacier Tongue (MGT) doubles under weak polynya activity because more warm water reaches the base of the ice shelf. Idealised simulations highlight the importance of the air/sea fluxes on DSW formation and ice shelf basal melting. A mean to strong air/sea forcing drives convection of dense water to sink at the sea floor and drives melt near the deep grounding line. Weaker air/sea forcing limits the depth of the convection and allows greater intrusions of warm modified Circumpolar Deep Water within the ice shelf cavity and increases basal melting. The resultant input of glacial meltwater produces a buoyant plume that stratifies the water column. Two simulations are run to investigate the impact of the calving of the MGT in 2010. Weaker polynya activity after calving results in an 89% increase in area averaged ice shelf basal melting and an 80% decrease of DSW export from the Adélie depression. Most importantly a distinct warming of the exported DSW leads to a decrease in AABW production downstream. This thesis demonstrates the sensitivity of Antarctic ocean-cryosphere interactions to interannual variability and episodic changes to the local icescape (ice shelves, icebergs and sea ice), finding that ice shelf basal melting and DSW formation in the AGVL region is dramatically impacted by the MGT calving. |
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