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

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.