Tropical Atlantic’s strongest current offsets climate change-induced low ocean oxygen levels

The ocean’s circulating waters play an essential role in regulating climate: they store and transport oxygen, carbon, nutrients, heat and freshwater all around the world. In addition to the changes they undergo in response to natural climate variability, ocean currents have also been changing as a result of global warming. An EU-backed study has now used long-term data on the most energetic current in the tropical Atlantic, the Equatorial Undercurrent (EUC), to examine the relationship between equatorial current fluctuations and variations in the ocean’s oxygen content.

Since the 1950s, oceans around the world have lost about 2 % of their oxygen content, with large declines in the tropical oceans. However, the current study reveals a different result for its target region. Supported by the EU-funded TRIATLAS project, the French-German research team found that the EUC grew more than 20 % stronger from 2008 to 2018. Furthermore, this intensification resulted in increasing oxygen concentrations in the upper layer of the equatorial Atlantic, counteracting global warming-induced deoxygenation in the region.

“At first, this statement sounds encouraging, but it does not describe the entire complexity of the system,” comments Prof. Dr Peter Brandt of TRIATLAS project partner GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany, in a news item posted on the ‘Science Daily’ website. “We found that the strengthening of the Equatorial Undercurrent is mainly caused by a strengthening of the trade winds in the western tropical North Atlantic,” adds Prof. Dr Brandt, who is also the first author of the study published in the journal ‘Nature Geoscience’.The research team analysed a 60-year data set to gain insight into the recent rise in oxygen content in the upper equatorial Atlantic. They concluded that the oxygen increase is connected to variability occurring over several decades, with low oxygen concentrations in the 1990s and early 2000s and high oxygen concentrations in the 1960s and 1970s. “In this respect, our results do not contradict the global trend, but indicate that the observed current intensification likely will switch back into a phase of weaker currents associated with enhanced oxygen reduction. It shows the need for long-term observations in order to be able to separate natural fluctuations of the climate system from trends such as oxygen depletion caused by climate warming,” states Prof. Dr Brandt.

The study’s results show a connection between changes in oxygen content and the size of tropical pelagic fish habitats that could affect marine ecosystems and fisheries. “Habitat compression or expansion for tropical pelagic fish can lead to altered predator-prey relationships, but also make it particularly difficult to assess overfishing of economically relevant fish species, such as tuna,” notes co-author Dr Rainer Kiko of Sorbonne University’s oceanographic laboratory in Villefranche-sur-Mer.

The 4-year TRIATLAS (Tropical and South Atlantic climate-based marine ecosystem predictions for sustainable management) project aims to assess the current situation of the southern and tropical Atlantic Ocean’s marine ecosystem and predict future changes. Coordinated by the University of Bergen in Norway, the project brings together 33 European, African and South American institutions specialising in climate change, oceanography and social sciences. TRIATLAS ends in May 2023.

For more information, please see:

TRIATLAS project website