Schweizer, Ellen (2023) Regional oxygen variability and trends within the North Atlantic Oxygen Minimum Zone (NA-OMZ). (Master thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 56 pp.

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Abstract

The Eastern Tropical North Atlantic hosts the North Atlantic Oxygen Minimum Zone (NA-OMZ) in the open ocean, and the neighboring Canary Current upwelling system along West African coast. The NA-OMZ has expanded and shoaled in the last seven decades presumably due to a warming-driven reduction of oxygen solubility, and enhanced stratification, which reduces the ventilation of the interior ocean. In the NA-OMZ the accelerated oxygen loss is superimposed to the natural, seasonal and longer-term oxygen variability. These oxygen changes and general enhancement of hypoxic conditions threaten the sustainability of marine ecosystems and small pelagic fisheries, which are the most important marine resources in west African coastal countries. In this study, by using a new, extensive, and quality-controlled oxygen database, we have addressed the regional oxygen variability over the past 70 years, with focus on the two oxygen minima, i.e. the shallow and the deep minima. In order to better understand the regional differences and the drivers of oxygen variability, the NA-OMZ has been divided into 15 different sub-regions, covering, for the first time, from the coast to the offshore area between 5-25°N and 10-28°W. The results presented here show that the amplitude of the decadal variability is more than twice that of the seasonal variability. Moreover, our results show that the long-term trend is consistent with a merging of the two oxygen minima, i.e. the shallow oxygen minimum has generally deepened and the deep oxygen minimum has become shallower while both have lost oxygen. It was possible to show that the northern boundary of the NA-OMZ, restricted by the Cape Verde zonal front, plays an important role in the northward extension and thus, in the dynamics of the NA-OMZ. The ventilation from the North, which showed strong implications to supply the NA-OMZ with oxygen-rich North Atlantic Central Water is likely modulated by the Atlantic Multidecadal Oscillation (AMO). This study supports the previous findings of the weakening of the ventilation from the West, which is reflected in a change of the regional water-mass properties over a wide depth range. This could be related to both, the weakening of zonal downward propagating Yanai waves, and the weakening of the North Atlantic Counter Current. This comprehensive study provides for the first time an hypothesis that three periods corresponding to different AMO phases can be distinguished in the period 1950-2020 in terms of regional dynamics in the NA-OMZ. The dynamics are driven first by a AMO-dominated northern ventilation by the Canary Current (CC) and the North Equatorial Current (NEC), second on a weakening western ventilation by the North Equatorial Counter Current (NECC) and deep jets, and third by north-western ventilation involving the Cape-Verde Current (CVC).

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