Iceberg B09B grounding: a plausible trigger for more persistent marine cold-spells off Commonwealth Bay, East Antarctica

Abstract. Marine cold-spells (MCSs) remain much less studied than marine heatwaves, especially in Antarctic coastal seas where sea ice complicates sea-surface temperatures. Here we quantify MCSs in the Adélie Sill-Commonwealth Bay region of East Antarctica from 1982 to 2024 using daily OSTIA sea-surface temperature and an event-based detection framework. We then compare cold-spell variability with independent indicators of winter sea-ice state, freshwater forcing, and modified Circumpolar Deep Water (mCDW) diagnostics. The strongest long-term increase in MCS exposure is concentrated along a sill-centered corridor linking the Adélie Sill, Adélie Depression, and northern Commonwealth Bay. Before 2010, cold-spell activity was generally weak and spatially patchy. After the 2010 calving of the Mertz Glacier Tongue and the subsequent grounding of iceberg B09B, cold-spells became stronger, more spatially coherent, and more persistent, with domain-wide annual cumulative intensity shifting to substantially more negative values. Seasonal diagnostics show that this post-2011 strengthening is expressed most clearly in summer, while winter remains consistently cold and strongly ice-influenced throughout the record. Winter sea-ice diagnostics indicate a more open but less ice-retentive post-2011 surface state, with increased lead activity, higher open-water fraction, and reduced sea-ice volume proxy. At the same time, mCDW remains present beneath the shelf, but under a thicker cool upper layer. Together, these results identify an iceberg-driven reorganization of the local ice-ocean system in which subsurface heat persists but is less effectively connected to the surface. This framework provides a basis for testing whether similar step-like changes in cold extremes occur on other iceberg-affected Antarctic shelves.