Seasonal response of sea surface salinity to sea ice retreat and polynya opening in the Ross Sea

Antarctic sea ice extent increased gradually over recent decades but has declined rapidly since 2016, including in the Ross Sea. Because sea ice melt alters the freshwater budget and sea surface salinity (SSS), understanding SSS variability is essential for interpreting recent hydrographic changes in the Ross Sea. Here, we reconstruct SSS for 1999–2023 using in-situ observations and a Random Forest (RF) model. The RF-based SSS (SSS RF ) shows high accuracy (R 2 = 0.80, RMSE = 0.10 psu) and captures spatial variability more consistently with in-situ observations than existing satellite- and model-based SSS products. Regime shift analysis of sea ice concentration anomalies using the Sequential t-Test Analysis of Regime Shifts (STARS) identifies three distinct periods: Regime 1 (R1; 1999–2004), Regime 2 (R2; 2004–2016), and Regime 3 (R3; 2016–2023), corresponding to high, moderate, and low sea ice conditions, respectively. During the recent low-sea-ice regime (R3), SSS RF initially decreases as sea ice retreats, then rises after the opening of the Ross Sea Polynya, as exchange with relatively saline waters intensifies. Cross-correlation analysis further shows that changes in open-water area precede SSS variations by approximately one month. These results demonstrate that the recent decline in sea ice does not simply drive surface freshening. Instead, the observed seasonal evolution of SSS may be associated with the timing of polynya opening and sea ice-ocean connectivity, potentially reflecting changes in the relative contributions of freshwater input and ocean mixing. These findings highlight a timing-dependent relationship between sea-ice retreat, polynya evolution, and surface salinity in the Ross Sea.