The Weddell Gyre is one of the dominant features of the Southern Ocean circulation and its dynamics have been linked to processes of climatic relevance. Variability in the strength of the gyre's horizontal transport has been linked to heat transport towards the Antarctic margins and changes in the properties and rates of export of bottom waters from the Weddell Sea region to the abyssal global ocean. However, the precise physical mechanisms that force variability in the Weddell's lateral circulation across different timescales remain unknown. In this study, we use a barotropic vorticity budget from a high-resolution model simulation to attribute changes in gyre strength to variability in possible driving processes. We find that the Weddell Gyre's circulation is sensitive to bottom friction associated with the overflowing dense waters at its western boundary. In particular, an increase in the production of dense waters at the southwestern continental shelf strengthens the bottom flow at the gyre's western boundary, yet this drives a weakening of the depth-integrated barotropic circulation via increased bottom friction. Strengthening surface winds initially accelerates the gyre, but within a few years the response reverses once dense water production and export increases. These results reveal that the gyre can weaken in response to stronger surface winds, putting into question the traditional assumption of a direct relationship between surface stress and gyre strength in regions where overflowing dense water forms part of the depth-integrated flow.

The Weddell Gyre’s variability on seasonal and interannual timescales is investigated using an ocean-sea ice model at three different horizontal resolutions. The model is evaluated against available observations to demonstrate that the highest resolution configuration (0.1$^{\circ}$ in the horizontal) best reproduces observed features of the region. The simulations suggest that the gyre is subject to large variability in its circulation that is not captured by summer-biased or short-term observations. The Weddell Gyre’s seasonal cycle consists of a summer minimum and a winter maximum and accounts for changes that are between one third and a half of its mean transport . On interannual time scales we find that the gyre’s strength is correlated with the local Antarctic easterlies and that extreme events of gyre circulation are associated with changes in sea ice concentration and the characteristics of warm inflow at the eastern boundary.

Surface winds around the Antarctic continent control coupled ocean-ice processes that influence the climate system, including bottom water production, heat transport onto the continental shelf and sea ice coverage. Few studies have examined projected changes in these winds, even though it would aid in the interpretation and understanding of the ocean’s response to climate change. In this work we examine historical changes in the near-Antarctic surface winds using Coupled Model Intercomparison Project Phase 6 models and reanalysis data, and quantify projected changes to the end of the 21st Century. These changes include a significant reduction in both the easterly and meridional wind components, which under the high emission scenario amounts to 23% and 7% respectively, most of which occurs during the summer season. The projected weakening of surface winds are coherent with a trend towards a positive Southern Annular Mode and a reduction of the pole-to-coast meridional pressure gradient.