Year-round vertically resolved observations of upper ocean hydrography and biogeochemistry in the Atlantic Water inflow north of Svalbard

A novel Subsea Winched Profiling System (SWIPS) was deployed in the Atlantic Water (AW) inflow to the Arctic Ocean north of Svalbard, providing high-resolution, year-round observations of upper ocean hydrography and biogeochemistry. Between July 2022 and July 2023, SWIPS collected 85 vertical profiles from ∼125 m to 10 m depth at 4-day intervals, capturing seasonal transitions and fine-scale variability across open water and ice-covered conditions. The autonomous system provides a sustained Eulerian perspective of upper ocean dynamics, resolving the evolving water mass distribution, seasonal stratification, the deepening of the mixed layer in autumn and winter, and the persistent influence of AW beneath a strongly stratified surface layer. SWIPS captured an under-ice phytoplankton bloom in May 2023, occurring under > 80% sea ice concentrations and preceding the onset of Polar Day by more than one week. During peak bloom periods, satellite-derived chlorophyll concentrations underestimated in-situ values by up to an order of magnitude due to persistent subsurface chlorophyll maxima and ice cover. The profiler also detected two episodes of anomalous winter hydrography during which AW reached the surface and disrupted the expected cold, stratified regime. The hydrographic data and satellite sea surface temperature suggest these events were driven by upstream AW advection from Fram Strait and facilitated localized convection to depths exceeding 100 m, reinforcing the role of remote forcing in shaping local upper ocean and ice conditions. By capturing both gradual seasonal evolution and short-lived anomalies, SWIPS provides critical in-situ observations that complement traditional observational methods and improve understanding of ocean–ice–ecosystem interactions under Arctic amplification.