Warm surface waters increase Antarctic ice shelf melt and delay dense water formation

The lowest recorded Antarctic sea-ice cover in 2016/2017 had many impacts on the Antarctic climate system, including an unprecedented amount of surface heat storage in front of ice shelves that are already experiencing enhanced melting from warm circumpolar deep water below. Our study demonstrates that the additional surface heat increased melt of Amery Ice Shelf by 30% and also delayed the onset of dense shelf water formation, precursor of the Antarctic Bottom Water production, in the Cape Darnley Polynya.
Warm surface waters increase Antarctic ice shelf melt and delay dense water formation

The Amery Ice Shelf is the largest glacier system in East Antarctica, flowing into Prydz Bay in the Indian Ocean sector of the Antarctic margin bounding the Southern Ocean. While floating ice shelves do not directly impact sea-level, their suppression of the upstream glacial flow does. Additionally, the ocean melting of ice shelves significantly freshens the waters of the continental shelf. To the west, the Cape Darnley Polynya is the second largest polynya around the Antarctic coast with intense wind-driven sea-ice production. Such active sea-ice production leads to the formation of high-density seawater due to the rejection of salt into the underlying water column during the freezing process. This high-density water that forms on the continental shelf beneath large polynyas is the source of Antarctic Bottom Water, which departs the shelf and mixes down the continental slope filling the global abyss and driving the lower limb of the global overturning circulation. The coastal area from the Amery Ice Shelf to Cape Darnley is one of the few places around Antarctica where the melting of ice shelves and the formation of high-density water are mutually linked and have a global impact. 

Recently, it has been revealed that the melting of ice shelves in West Antarctica has accelerated and the outflow of the ice sheet behind them has increased, leading to an increase in the global sea level rise. It has been thought that this melting of ice shelves is bottom-intensified, caused by the access of deep warm water from the Southern Ocean that is ubiquitous across a bottom layer of several hundred meters. In comparison, it was thought that any melting at the upper layers of an ice shelf would be limited, due to the  ephemeral nature of ocean surface warming in summertime. However, there were almost no direct field observations of the relationship between available surface heat, ice shelf melting, and their subsequent impact on polynya processes and dense water formation. 

This novel study of the Amery Ice Shelf and Cape Darnley Polynya region utilizes several years of state-of-the-art oceanographic observations, together with numerical modelling, to investigate the impact of surface warming on ice shelf melt and sea-ice growth. We conducted in-situ summer observations using ship-borne hydrographic instruments (six summers during 2010s) and year-round observations (two years of 2017 and 2019) using mooring observation equipment, and investigated structural changes in the ocean and their effects on ice shelf melting. In particular, the amount of glacial melt present in the local waters, known as the glacial melt water fraction, was investigated using the oxygen isotope ratio of seawater samples obtained by ships and moored water samplers. Furthermore, satellite observations were used to map the spatial pattern of sea ice coverage and sea surface temperature, together with the results of numerical experiments using an ocean-sea ice general circulation model. By combining field observations, satellite data, and numerical experiments, some unique behavior of the system around the anomalously warm summer of 2016/17 emerged. 

In the summer of 2016/17, there was little sea ice in Prydz Bay, and the surface water temperature became very high across the region from Cape Darnley Polynya to the front of the Amery Ice Shelf. The ice shelf melt water component of the ocean surface layer in the Cape Darnley Polynya was about 30% higher than usual. Data from the time-series sampler indicated that this high melt water fraction continued through to winter. Analysis of the satellite data and numerical model output indicated a positive feedback between decreasing sea ice and warming ocean was at work. When there is less sea ice in a summer, the sea surface absorbs more heat, which then further melts the ice and absorb more heat. This positive feedback in 2016 promoted increased heat absorption near the ocean surface and the prevailing easterly wind transported this warm surface water under the ice shelf and caused abnormally higher melt.

 In the subsequent autumn of 2017, the mooring observations revealed warm surface water with low salinity flowing from the Amery Ice Shelf that delayed the onset of sea-ice formation at Cape Darnley Polynya, relative to 2019. Accordingly, delayed/reduced sea-ice growth reduced local dense shelf water formation, and by extension Antarctic Bottom Water production. This study shows that in addition to the well-known role of deep warm water in melting ice shelves from below, subduction of warm summer surface can also contribute to ice shelf melt and subsequently impact dense water formation.

 The low sea ice in the Amery Ice Shelf/Cape Darnley Polynya region during the summer of 2016/17 formed part of a record-breaking circum-Antarctic minimum sea-ice cover, that was dramatically broken again in this year’s minimum sea-ice cover for 2021/22.  It is soon to know if these two record lows are statistical anomalies or a shift towards a new normal in the future under global warming. If it is the latter, the previously overlooked process of summer surface ocean heating will be increasingly important when considering total ice shelf melting. Reduced summertime sea-ice and increased surface ocean heating will place increasing pressure on globally significant processes that affect sea-level rise and the meridional overturning circulation.  

Article link:  https://doi.org/10.1038/s43247-022-00456-z 

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