Oceanic Heat Delivery to the Antarctic Continental Shelf: Large-Scale, Low-Frequency Variability
The Antarctic continental slope (ACS) stands between the warmer, deep-ocean waters transported by the Antarctic Circumpolar Current and the coastal, floating ice-shelves around the Antarctic coastline. Ice shelves help keep the massive continental ice sheets from increasing the sea level by sliding into the ocean but can melt faster when in contact with oceanic waters that are warmer than the near-freezing coastal waters. The study examines the variability of this oceanic heat transport by mean flows and eddies across the ACS toward the Antarctic coast, using a global coupled ocean-sea ice model with realistic atmospheric forcing. This is the first time that the realistic seasonal and interannual variability of cross-ACS heat transport (HT) in all Antarctic marginal seas is analyzed in a global model. Important conclusions arise, particularly regarding the role of HT by eddies, the potential importance of the along-slope HT convergence and the use of averaged in situ measurements as metrics for model evaluation.
This study gives insight on the mechanisms governing heat transport to the Antarctic continental shelf and introduces useful control baseline values (in time and space) for the representation of ocean-shelf heat exchange around Antarctica in next-generation Earth System Models, such as the Energy Exascale Earth System Model (E3SM). Comparison of this study's results with independent simulations featuring ice-shelves, more accurate topography, tides, and fully resolved eddies will help inform which of these time-varying processes should have higher priority in the development of parameterizations for climate models unable to directly represent them. In addition, the study uses temperature/salinity observations to evaluate the simulated water mass structure along the Antarctic continental slope, illustrating the usefulness of this approach as a metric for model evaluation, even in under-sampled regions of the World Ocean. This knowledge may contribute to the reduction of uncertainties in predictions of the rate at which continental ice sheets are melting, thereby helping to improve projections of global mean sea-level rise and informing the development of mitigation and adaptation policies to better address the societal impacts of these global changes.
This study examines the seasonal to multi-decadal variability of ocean-Antarctic Continental Slope (ACS) heat transport (HT) with potential to melt ice shelves in a global, ocean-sea ice coupled model forced by a realistic atmosphere along the full circum-Antarctic extent of the ACS, as well as its time-averaged spatial structure (in the along- and cross-slope directions). On-shelf HT due to advection/stirring by eddies, mean flow-topographic interactions and surface Ekman flow are important in different regions of the ACS. Most of the temporal variability in the total HT is contained in the HT component due to the slowly-varying part of the flow (motions with period of 1 month and longer), and is more strongly controlled by the mechanical forcing of the large-scale wind patterns than by the ocean's density changes caused by the sea ice formation/melting cycle and the ocean-atmosphere density fluxes. The interannual HT variability has some weak and statistically significant correlation with climate indices (SAM and Niño 3.4).