Multi-decadal warming of Atlantic water and associated decline of dissolved oxygen in a deep fjord

Dag L Aksnes,Jan Aure,Per-Otto Johansen,Geir Helge Johnsen,Anne Gro Vea Salvanes

doi:10.1016/j.ecss.2019.106392

Abstract

Previous studies have shown decline in dissolved oxygen of the ocean basins. A hypothesis for this development is that ocean warming through increased stratification has caused reduced ventilation of the interior ocean. Here we provide evidence that reduced ventilation, which has been associated with a 1 °C warming of the North Atlantic Water (NAW), has contributed to recent deoxygenation of the mesopelagic zone of a Norwegian fjord, Masfjorden. Our results suggest that after the North Atlantic “Great Salinity Anomalies” around 1980, this warming has led to a decreased frequency of high-density intrusions of oxygen rich NAW and thereby reduced the renewal of the basin water of Masfjorden. From this, we infer that the basin water of other deep fjords are prone to similar development and briefly discuss some potential implications of deoxygenation in the mesopelagic zone.

Highlights

Ocean warming and deoxygenation have become a large concern worldwide (Diaz and Rosenberg, 2008; Keeling et al, 2010, Breitburg et al, 2018; Levin, 2018)
The residence time of the basin water depends on three factors (Stigebrandt, 2012): The spectral distribution of the temporal density fluctuations of the water just above the sill depth outside the fjord, the rate of the density reduction of the basin water due to diapycnal mixing, and the time it takes to fill the basin with new water
We investigate the hypothesis that recent warming of the North Atlantic Water (NAW), through seawater density reduction, has contributed to reduced ventilation and oxygen decline in Norwegian fjord basins

Summary

Introduction

Ocean warming and deoxygenation have become a large concern worldwide (Diaz and Rosenberg, 2008; Keeling et al, 2010, Breitburg et al, 2018; Levin, 2018). The residence time of the basin water depends on three factors (Stigebrandt, 2012): The spectral distribution of the temporal density fluctuations of the water just above the sill depth outside the fjord, the rate of the density reduction of the basin water due to diapycnal mixing, and the time it takes to fill the basin with new water. For many fjords, this time scale is long relative to the time scale for density variations and numerous partial exchanges might take place. We investigate the hypothesis that recent warming of the North Atlantic Water (NAW), through seawater density reduction, has contributed to reduced ventilation and oxygen decline in Norwegian fjord basins

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Conclusion