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Vertical ocean heat transport near Antarctic ice shelves: data and processing code

Friedrichs, Andrew1; Forrest, Alexander1

Research facility: University of California, Davis
Published Dec 20, 2024 on Dryad. https://doi.org/10.5061/dryad.msbcc2g7n

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Abstract

Antarctic ice shelves, or the floating extension of terrestrial glaciers, help limit Antarctica’s contribution to global sea level rise by slowing the export of grounded ice into the Southern Ocean. Identifying the distribution of meltwater from the ocean-driven melt of these ice shelves helps determine where and at what rate they are thinning, providing insight into how quickly their buttressing effect is diminishing. In regions with rapid ice shelf melt, the resulting freshwater flux into the coastal ocean contains enough buoyancy forcing to reshape coastal currents and alter ocean stratification up to thousands of kilometers from the meltwater’s source. Global circulation models are employed to help understand how ice shelf meltwater modifies the Southern Ocean and are extended forward in time to attempt to predict the consequences of a future with increased ice melt. However, because they are unable to efficiently resolve small scale turbulent mixing, these models make assumptions regarding the mechanisms that drive the mixing and use parameterizations to simplify the effects of turbulence, such as the diffusion rates heat and salinity. While common turbulence parameterizations have been found to work well throughout much of the Earth’s oceans, the lack of in situ turbulence measurements near Antarctica have left parameterizations comparatively uniformed and untested near melting ice shelves. This dissertation works to improve the understanding of the downstream effects of ice shelf meltwater via the collection of rare in situ oceanographic measurements near Antarctic ice shelves.

Here, we present the identification and analysis of meltwater layers within the Southern Ocean near two Antarctic ice shelves. At Nansen Ice Shelf, traces of meltwater in the form of frigid Ice Shelf Water were observed at the opening of the ice shelf cavity, and vertical heat flux estimates derived from direct measurements of microstructure turbulence show approximately 10 W m-2 of heat transport into the meltwater from both above and below. The position of this layer within the water column was heavily modified by the presence of a coastal, submesoscale eddy, which lifted the meltwater toward the ice shelf-ocean interface, potentially resulting local variations in basal melt rate. On the other hand, at Dotson Ice Shelf, ice shelf melt in the form of warmer Glacial Meltwater was observed across two layers in front of the ice shelf cavity at quantities not observed for over a decade. These meltwater layers were determined to be largely derived from different sources, with the deeper Subglacial Meltwater layer originating from beneath the western corner of Dotson Ice Shelf, and the shallower Ventilated Meltwater layer originating from the previous melt of other upstream ice shelves. Here, additional but more limited measurements of turbulence were used to validate modeled energy dissipation rates, which then again indicate approximately 10 W m-2 of heat transport through the meltwater: largely upwards through the deeper layer and downwards through the shallower layer. Our results show that the meltwater layers resulted in the formation of step-like changes temperature stratification, which support particularly efficient mixing. Together, the observations from these two unique ice shelf systems show that melt from ice shelves is intrinsically tied to local oceanography, and that our ability to predict how future changes in the Antarctic cryosphere will affect global thermohaline circulation relies on the accuracy of well-informed ocean mixing parameterizations.