Observations and modeling of the 2021 deep-water renewal event in Masfjorden, a sill fjord in Western Norway

Deep-water renewal (DWR) is an important process in sill fjords, as it brings oxygen-rich water to the greater depths of the isolated fjord basin. In Masfjorden, a sill fjord in Western Norway, the first full DWR event in 11 years was observed in April 2021. Prior to the DWR event, near-hypoxic conditions were observed close to the bottom of the basin. Two high-density episodes at the sill and water intrusions in the fjord basin were identified from mooring records, showing that the DWR event took place following northerly winds sufficiently strong for coastal upwelling to lift dense water up to sill level. The observations were used to evaluate a high-resolution regional model’s performance regarding deep-water renewal in a fjord basin. The regional model showed the development of coastal upwelling leading up to a high-density episode at the sill, which coincided with the observed high-density episode. • A deep-water renewal event was observed in April 2021 in a Western Norwegian fjord • The renewal event was triggered by upwelling favorable winds • Upwelling favorable wind events were quantified using the wind impulse • Regional models captured the timing of the observed renewal event