Hydrographic CTD profiles capturing the onset of near-surface stratification in Southwest Greenland fjords during two consecutive spring 2025 field campaigns

Abstract. In situ hydrographic observations in Greenland fjords remain sparse, particularly during late winter and early spring, despite their importance for Arctic freshwater export, glacier–ocean interactions, and the seasonal development of stratification. A major limitation of many existing hydrographic datasets is the poor representation of the near-surface layer, where meltwater lenses, sharp salinity gradients, and strong density gradients are often concentrated within the upper few metres.

Here we present a quality-controlled Level 2 conductivity, temperature, and depth dataset comprising 73 vertical hydrographic profiles collected across Southwest Greenland fjords (60.2–69.5° N, 43.6–53.9° W) during two expeditions aboard the icebreaker Le Commandant Charcot: the Sea Ice Measurements for Satellite thickness retrieval Validation campaign (32 profiles, 24 March–4 April 2025) and the Freshwater fluxes and Atlantic-Nordic Seas hydrography campaign (41 profiles, 18–28 April 2025).

The dataset includes pressure, temperature, conductivity, practical salinity, and derived thermodynamic variables. Two RBR Concerto3 conductivity, temperature, and depth instruments sampling at 16 Hz and 2 Hz were processed through a reproducible eight-step workflow including atmospheric-pressure correction, automatic soak detection, signal conditioning, loop removal, thermodynamic calculations, and automated quality control following international ocean-data recommendations.

Unlike many standard conductivity, temperature, and depth products, the processing strategy preserves the shallowest valid observations and the native vertical sampling. This allows users to investigate thin freshwater lenses and sharp near-surface gradients rather than systematically removing the upper few metres of the water column. The dataset therefore provides a detailed view of the onset of meltwater-driven stratification and offers the flexibility to apply alternative vertical averaging or filtering strategies depending on the objectives of future analyses.

Thermohaline diagnostics identify four main hydrographic regimes: coastal and glacier meltwater-influenced waters, Polar Water, Winter Water, and modified Atlantic or Irminger Water. The comparison between the late-winter and early-spring campaigns documents the onset of surface restratification across Southwest Greenland fjords. The strongest freshening and shallow stratification are observed in fjords directly influenced by tidewater glaciers, while deeper water masses remain comparatively stable over the one-month period.

The dataset is distributed in NetCDF-4 format compliant with Climate and Forecast Metadata Conventions version 1.8 and Attribute Convention for Data Discovery version 1.3, and is publicly available through Zenodo. The processing code is archived on GitHub and Zenodo. By preserving the upper-ocean structure during a critically undersampled season, this dataset provides a valuable resource for studies of Arctic freshwater dynamics, glacier–ocean interactions, fjord stratification, ocean model evaluation, and satellite product validation.