Contribution of ocean processes to the drops in Antarctic sea ice extent at the end of the 1970s and after 2016

The Antarctic sea ice extent has displayed two large drops over the past 65 years, a small one at the end of the 1970s and a more substantial one after 2016. The atmospheric forcing provided a dominant contribution to those drops. Wind changes strongly control the spatial pattern of sea ice reduction, especially in the Pacific sector of the Southern Ocean and in the western Weddell Sea. The relationship between the winds and sea ice seems less direct in the east Antarctic sector (i.e., mainly in the eastern Weddell Sea and in the Indian sectors), where oceanic processes are expected to play a larger role. This contribution of oceanic processes in the sea ice reduction in the east Antarctic sector is estimated here using simulations performed with the ocean-sea-ice  model NEMO, substantiated by observations. The simulations cover the period 1958-2023, driven by both the ERA5 reanalysis and a forcing derived from a recent reconstruction that displays more homogenous time series than ERA5 over the whole period. Observations are used at first to evaluate the model behaviour over the past decades when the data network is denser. The simulations, then, allow the analysis to be extended over the past 65 years, to estimate the changes in oceanic heat transport, heat content and oceanic heat flux towards the sea ice. Several simulations with NEMO are compared, using different initial conditions and parameters influencing mixing to estimate how they influence the sea ice extent variations and thus to disentangle the role of different oceanic processes in the observed changes.