Zhaomin Wang

This is Part II of a study investigating the processes that regulate interannual salinity variability on the Ross Sea continental shelf using a coupled ocean–sea ice–ice shelf model. Part I identified the role of surface forcing. Here, we focus on the internal oceanic processes and quantify their contribution. We show that surface-generated salinity anomalies propagate into deeper layers with a lag of 1–2 years, a process strongly modulated by vertical stratification. We further identify a negative feedback: Surface salinization enhances the formation and export of High Salinity Shelf Water, which in turn promotes the inflow of fresher upper-layer water onto the continental shelf, thereby decreasing surface salinity. Thus, while southwesterly wind anomalies over the eastern continental shelf regulate sea ice production and associated salt rejection, which primarily drives salinity variability (Part I), the resulting overturning circulation, partly modulated by winds over the western continental shelf, acts to dampen this variability. Together with Part I, this study provides a comprehensive analysis of the potential mechanisms driving interannual salinity variability, shedding light on understanding the observed salinity variability on the Ross Sea continental shelf. Significance Statement: Interannual salinity variability in High Salinity Shelf Water on the Ross Sea continental shelf directly shapes the properties of Ross Sea Bottom Water, with surface processes initiating anomalies and oceanic processes controlling their eventual impacts. While Part I elucidated the role of surface forcing, this study quantifies the contribution of internal oceanic processes. We identify a key negative feedback: Enhanced salinity strengthens dense water export, which in turn drives fresher inflows and reinforces stratification, thereby limiting further salinification. By explicitly quantifying the oceanic processes that regulate salinity variability in High Salinity Shelf Water, this work provides critical insight into bottom formation and variability, with broad implications for understanding Antarctic and Southern Ocean climate change.

Using a pan-Arctic coupled sea ice–ocean model, this study examines how Arctic river runoff influences sea ice distribution through a combination of thermodynamic and dynamic processes. Our results show that while runoff has little effect on total ice volume, it significantly redistributes ice thickness across the Arctic, with local changes exceeding 0.1 m (nearly 10%) in typical regions. We attribute this to a three‐stage mechanism. Direct thermodynamic effect, i.e. coastal melt from riverine heat and offshore protection from riverine freshwater, triggers initial changes in ice conditions that subsequently alter kinetic energy transfer from winds to sea ice. This modifies large-scale ice drift patterns, weakening the anticyclonic Beaufort Gyre and accelerating westward transport from the northern coast of Alaska to the East Siberian Sea. The resulting dynamic redistribution then activates a negative feedback whereby thinner ice promotes faster thermodynamic growth (and vice versa), partially offsetting the dynamic change. This mechanism may explain the spatial heterogeneity of sea ice variability in the context of ongoing ice retreat.

The Ross Sea continental shelf plays a critical role in Antarctic Bottom Water formation through the production of High Salinity Shelf Water (HSSW), with implications for the global overturning circulation. The salinity of HSSW exhibits pronounced interannual variability, while its spatiotemporal characteristics and governing mechanisms remain poorly understood. Using a regional ocean–sea ice–ice shelf coupled model, we investigate the nature of this variability and the major driving processes. Here, in Part I, we focus on surface–driven processes, analyzing the factors governing surface salinity variability, including interannual changes in surface salt flux and the role of surface salt advection. Our results show that variability in surface salt flux is primarily driven by southwesterly winds anomalies over the continental shelf, which are influenced by the atmospheric circulation pattern across the Ross Sea. These wind anomalies control the net sea ice transport change across the shelf break, along with sea ice production mainly over the eastern continental shelf and within the Ross Ice Shelf Polynya. Advected westward by surface currents, salinity anomalies originating in the eastern continental shelf region thereby influence surface properties in the western Ross Ice Shelf Polynya and the Terra Nova Bay Polynya, the primary HSSW formation sites. Identifying these surface-driven mechanisms provides a foundation for assessing the contributions of internal oceanic processes to interannual salinity variability, which is investigated in Part II.

Complex interactions among the ocean, sea ice, and ice shelves in the Southern Ocean are critical for global climate, yet accurately simulating these processes remains challenging in climate models, such as those participating in the Coupled Model Intercomparison Project Phase 6, due to their coarse resolution and incomplete physical components. Therefore, the development of high-resolution circumpolar coupled ocean–sea ice–ice shelf models could improve our understanding of the evolution of the Southern Ocean. In this study, we use the c66m version of the Massachusetts Institute of Technology General Circulation Model, including a sea ice component and an ice shelf component, to configure the coupled Southern Ocean–Sea ice–Ice shelf Model (SOSIM v1.0). Adopting the Refined Topography dataset version 2 for the geometry of seafloor and ice draft, SOSIM features a horizontal resolution of ~5 km and 70 vertical layers. Forced by the European Centre for Medium-Range Weather Forecasts Reanalysis v5, a long-term integration of SOSIM is run forward from 1979 to 2022, with daily outputs for estimating the oceanic state, sea ice evolution, and basal mass balance of ice shelves. A comprehensive evaluation of the performance of SOSIM has been conducted against multiple observational and reanalysis datasets. Identified biases include an underestimated Antarctic Circumpolar Current transport, an overestimated Antarctic Slope Current, a warm drift in abyssal waters, an exaggerated seasonality of sea ice extent, and an underestimated total ice shelf mass loss. Despite these limitations, SOSIM still captures large-scale hydrographic structures, the annual variability of sea ice, and cross-slope exchanges over shelf seas. Furthermore, SOSIM is set to serve as the dynamical core for the next-generation Southern Ocean Ice Prediction System being developed in China.

Complex interactions among the ocean, sea ice, and ice shelves in the Southern Ocean are critical for global climate, yet accurately simulating these processes remains challenging in climate models, such as those participating in the Coupled Model Intercomparison Project Phase 6, due to their coarse resolution and incomplete physical components. Therefore, the development of high-resolution circumpolar coupled ocean–sea ice–ice shelf models could improve our understanding of the evolution of the Southern Ocean. In this study, we use the c66m version of the Massachusetts Institute of Technology General Circulation Model, including a sea ice component and an ice shelf component, to configure the coupled Southern Ocean–Sea ice–Ice shelf Model (SOSIM v1.0). Adopting the Refined Topography dataset version 2 for the geometry of seafloor and ice draft, SOSIM features a horizontal resolution of ~5 km and 70 vertical layers. Forced by the European Centre for Medium-Range Weather Forecasts Reanalysis v5, a long-term integration of SOSIM is run forward from 1979 to 2022, with daily outputs for estimating the oceanic state, sea ice evolution, and basal mass balance of ice shelves. A comprehensive evaluation of the performance of SOSIM has been conducted against multiple observational and reanalysis datasets. Identified biases include an underestimated Antarctic Circumpolar Current transport, an overestimated Antarctic Slope Current, a warm drift in abyssal waters, an exaggerated seasonality of sea ice extent, and an underestimated total ice shelf mass loss. Despite these limitations, SOSIM still captures large-scale hydrographic structures, the annual variability of sea ice, and cross-slope exchanges over shelf seas. Furthermore, SOSIM is set to serve as the dynamical core for the next-generation Southern Ocean Ice Prediction System being developed in China.

Complex interactions among the ocean, sea ice, and ice shelves in the Southern Ocean are critical for global climate, yet accurately simulating these processes remains challenging in climate models, such as those participating in the Coupled Model Intercomparison Project Phase 6, due to their coarse resolution and incomplete physical components. Therefore, the development of high-resolution circumpolar coupled ocean–sea ice–ice shelf models could improve our understanding of the evolution of the Southern Ocean. In this study, we use the c66m version of the Massachusetts Institute of Technology General Circulation Model, including a sea ice component and an ice shelf component, to configure the coupled Southern Ocean–Sea ice–Ice shelf Model (SOSIM v1.0). Adopting the Refined Topography dataset version 2 for the geometry of seafloor and ice draft, SOSIM features a horizontal resolution of ~5 km and 70 vertical layers. Forced by the European Centre for Medium-Range Weather Forecasts Reanalysis v5, a long-term integration of SOSIM is run forward from 1979 to 2022, with daily outputs for estimating the oceanic state, sea ice evolution, and basal mass balance of ice shelves. A comprehensive evaluation of the performance of SOSIM has been conducted against multiple observational and reanalysis datasets. Identified biases include an underestimated Antarctic Circumpolar Current transport, an overestimated Antarctic Slope Current, a warm drift in abyssal waters, an exaggerated seasonality of sea ice extent, and an underestimated total ice shelf mass loss. Despite these limitations, SOSIM still captures large-scale hydrographic structures, the annual variability of sea ice, and cross-slope exchanges over shelf seas. Furthermore, SOSIM is set to serve as the dynamical core for the next-generation Southern Ocean Ice Prediction System being developed in China.

In the central Ross Sea continental shelf, the modified Circumpolar Deep Water could intrude the continental shelf between March and July, and reach about 76 °S. According to seal-CTD data, in March and April, the potential temperature of the modified Circumpolar Deep Water was up to - 0.55 °C, and was widely distributed within the depths of 100 - 300 m. In May and June, the potential temperature of the modified Circumpolar Deep Water was up to - 0.65 °C, and was found to the north of 76 °S above the depth of 350 m, but discontinuous above the depth of 200 m. In July, the modified Circumpolar Deep Water sharply cooled, with the maximum potential temperature being -1.45 °C.By analysing the seal-CTD observations and the numerical model results, this study found strong mixing in the central part of the shelf, owing to topography-induced upwelling. The resulted mixed layer warming is also attributed to the intrusion of the modified Circumpolar Deep Water. This oceanic process, along with the katabatic wind forcing, contributed to forming unique sea ice distribution characteristics in the Ross Sea Polynya, featured by ‘’less ice, more ice, less ice” from the front of the Ross Ice Shelf to the upwelling zone. This kind of sea ice distribution characteristics can also promote the northward expansion of the Ross Sea Polynya.

Satellite-based observations and a pan-Arctic coupled sea ice-ocean model are utilized to study the effect of ocean geostrophic currents on large-scale sea ice circulation in the Canada Basin, Arctic Ocean. We find that surface winds primarily drive sea ice drifts in the west–east direction, while the geostrophic currents in the Beaufort Gyre promote north–south ice drifts. Wind fluctuations can create variable ice drifts, yet geostrophic currents respond more slowly due to their larger vertical scale, serving as a slowly-evolving conveyor belt for maintaining the anticyclonic ice circulation. It is further demonstrated that the bathymetry can regulate the movement of sea ice via constraining the expansion of ocean circulation. This mechanism is indirect in the sense that the ice is far from the seafloor. Our research underscores the necessity of considering the bathymetry-constrained geostrophic currents in understanding Arctic sea ice dynamics. With the rapid retreat of Arctic sea ice, the multi-scale interactions between ice drifts and ocean currents may have significant implications for the Arctic ecosystem, climate, and shipping corridors.

Flow of dense shelf water provide an efficient mechanism for pumping CO₂ to the deep ocean along the continental shelf slope, particularly around the Antarctic bottom water (AABW) formation areas where much of the global bottom water is formed. However, the contribution of the formation of AABW to sequestering anthropogenic carbon (C_ant) and its consequences remain unclear. Here, we show prominent transport of C_ant (25.0 ± 4.7 Tg C yr⁻¹) into the deep ocean (>2,000 m) in four AABW formation regions around Antarctica based on an integrated observational data set (1974–2018). This maintains a lower C_ant in the upper waters than that of other open oceans to sustain a stronger CO₂ uptake capacity (16.9 ± 3.8 Tg C yr⁻¹). Nevertheless, the accumulation of C_ant can further trigger acidification of AABW at a rate of −0.0006 ± 0.0001 pH unit yr⁻¹. Our findings elucidate the prominent role of AABW in controlling the Southern Ocean carbon uptake and storage to mitigate climate change, whereas its side effects (e.g., acidification) could also spread to other ocean basins via the global ocean conveyor belt.