Spatial Heterogeneity and Multi-Decadal Dynamics of Antarctic Deep-Water Polynyas

 

Antarctic open-ocean polynyas (OOPs) are critical windows for air-sea interaction, significantly enhancing heat exchange and driving Antarctic Bottom Water (AABW) formation. While their importance is recognized, their long-term spatiotemporal evolution and the underlying subsurface drivers remain incompletely understood. This study identifies recurrent deep-water OOPs and reveals a pronounced longitudinal asymmetry and complex subsurface dynamics through a refined 20-year analysis.

Using the DEEP-AA daily polynya edge dataset (2003–2022), we tracked open-water features during austral winters (April–October). By applying rigorous spatiotemporal filters—excluding short-lived (<20 days), small-scale (<100 km²), or coastal-influenced features—we identified 16 primary OOPs. Among these, eight were classified as "deep-water OOPs" based on their location over abyssal depths (>3,500 m) or major topographic features, such as seamounts and mid-ocean ridges.

Our results highlight a strong clustering of deep-water OOPs within the 90°W–90°E sector, governed by coupled dynamic-thermodynamic processes. In the 55°S–65°S latitudinal band, Antarctic Circumpolar Current (ACC) shear amplifies Ekman transport and mesoscale eddy activity. Topographic features, such as Maud Rise, induce flow divergence that sustains surface openings. In contrast, the northward deflection of the ACC west of 90°W promotes sea-ice expansion in the Ross and Amundsen Seas. These OOPs act as vital conduits for AABW formation; topographic uplift facilitates the upward transport of warm Circumpolar Deep Water (CDW; T > 0°C, S ≈ 34.6 psu) into the mixed layer, effectively inhibiting ice growth.

A key novelty of this study is the identification of "Silent Periods"—intervals where the surface appears frozen (days to months) due to extreme winter cooling, yet subsurface heat transport remains active. We found that residual thermal anomalies create an "Oceanic Heat Memory" effect, which preconditions the polynya for rapid reactivation. Conventional sea-ice concentration (SIC) threshold methods fail to capture these subsurface signals, consistently underestimating both polynya persistence and total ocean-to-atmosphere heat fluxes.

These findings demonstrate that seabed topography, environmental conditions, and ocean circulation are the primary determinants of deep-water polynyas distribution. By elucidating the mechanisms of topographic preconditioning and the limitations of surface-only observations, this work provides essential insights for improving ice-ocean coupling in Earth System Models and refining projections of Southern Ocean climate change.