Two Decades of Ross and Weddell Gyre Variability from Observations

The circulation of the Weddell and Ross Gyres has a direct influence on processes such as poleward heat transport and upwelling of nutrients and natural carbon. However, both gyres are seasonally covered by sea ice, making them some of the most sparsely observed regions of the global ocean, limiting long-term tracking of changes in their circulation. Here we present observational estimates of gyre strength and area over 2003-2023 using satellite altimetry and four gridded hydrographic products. Our estimates show that the Weddell Gyre has been strengthening and broadening since the 2000s, and while no significant trends are observed for the Ross, this gyre displays large interannual variability that dominates month-to-month changes. Using an ocean-sea ice model, we quantify the skill of the estimates considering the sparsity of deep hydrographic observations and show that they likely capture 90\% (66\%) of the variability in strength of the Weddell (Ross) Gyre.