This study presents the first dynamically downscaled projections of ocean acidification (OA) for the main Hawaiian Islands using coupled Regional Ocean Modeling System (ROMS) and Carbon, Ocean Biogeochemistry and Lower Trophics (COBALT) models integrated with Coupled Model Intercomparison Project Phase 6 (CMIP6) scenario outputs from the Community Earth System Model 2 (CESM2). We analyze three Shared Socioeconomic Pathways (SSP1-2.6, SSP2-4.5, and SSP3-7.0) and introduce a novelty metric to quantify departure from historical variability. Our results indicate unprecedented levels of OA within the next three decades across all scenarios, with aragonite saturation state (ΩA), pH, and substrate-to-inhibitor ratio projected to decline significantly. By 2100, under SSP3-7.0, ΩA novelty could exceed reference variability by a factor of 12. Spatial analysis reveals heterogeneous OA impacts, with windward coastlines consistently exhibiting higher novelty levels. Importantly, we find contrasting spatial patterns of OA indices due to varying sensitivities to temperature and dissolved inorganic carbon, resulting in higher ΩA novelty in northern areas and higher pH and substrate-to-inhibitor ratio novelty in southern regions.

Capes and cape-associated shoals represent sites of convergent sediment transport, and can provide points of relative coastal stability, navigation hazards, and offshore sand resources. Shoal evolution is commonly impacted by the regional wave climate. In the Arctic, changing sea-ice conditions are leading to (1) longer open-water seasons when waves can contribute to sediment transport, and (2) an intensified wave climate (related to duration of open water and expanding fetch). At Blossom Shoals offshore of Icy Cape in the Chukchi Sea, these changes have led to a five-fold increase in the amount of time that sand is mobile at a 31-m water depth site between the period 1953-1989 and the period 1990-2022. Wave conditions conducive to sand transport are still limited to less than 2% of the year, however - and thus it is not surprising that the overall morphology of the shoals has changed little in 70 years, despite evidence of active sand transport in the form of 1-m-scale sand waves on the flanks of the shoals which heal ice keel scours formed during the winter. Suspended-sediment transport is relatively weak due to limited sources of mud nearby, but can be observed in a net northeastward direction during the winter (driven by the Alaska Coastal Current under the ice) and in a southwestward direction during open-water wind events. Longer open-water seasons mean that annual net northeastward transport of fine sediment may weaken, with implications for the residence time of fine-grained sediments and particle-associated nutrients in the Chukchi Sea.