Vertical mixing alleviates autumnal oxygen deficiency in the central North Sea

Williams, Charlotte A. J.; Hull, Tom; Kaiser, Jan; Mahaffey, Claire; Greenwood, Naomi; Toberman, Matthew; Palmer, Matthew R.

There is an immediate need to better understand and monitor shelf sea dissolved oxygen (Oinline-formula2) concentrations. Here we use high-resolution glider observations of turbulence and Oinline-formula2 concentrations to directly estimate the vertical Oinline-formula2 flux into the bottom mixed layer (BML) immediately before the autumn breakdown of stratification in a seasonally stratified shelf sea. We present a novel method to resolve the oxycline across sharp gradients due to slow optode response time and optode positioning in a flow “shadow zone” on Slocum gliders. The vertical Oinline-formula2 flux to the low-Oinline-formula2 BML was found to be between 2.5 to 6.4 mmol minline-formula−2 dinline-formula−1. Episodic intense mixing events were responsible for the majority (up to 90 %) of this oxygen supply despite making up 40 % of the observations. Without these intense mixing events, BML Oinline-formula2 concentrations would approach ecologically concerning levels by the end of the stratified period. Understanding the driving forces behind episodic mixing and how these may change under future climate scenarios and renewable energy infrastructure is key for monitoring shelf sea health.



Williams, Charlotte A. J. / Hull, Tom / Kaiser, Jan / et al: Vertical mixing alleviates autumnal oxygen deficiency in the central North Sea. 2024. Copernicus Publications.


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