Abstract
Abstract. The impact of the positive and negative phases of the Pacific Decadal Oscillation (PDO) on the extension of the poorly oxygenated regions of the eastern Pacific Ocean was assessed using a coupled ocean circulation–biogeochemical model. We show that during a “typical” PDO-positive phase the volume of the suboxic regions expands by 7 % over 50 years due to a slowdown of the large-scale circulation related to the decrease in the intensity of the trade winds. Changes in oxygen levels are mostly controlled by advective processes between 10∘ N and 10∘ S, whereas diffusive processes are dominant poleward of 10∘: in a “typical” PDO-positive phase the sluggish equatorial current system provides less oxygen to the eastern equatorial part of the basin while the oxygen transport by diffusive processes significantly decreases south of 10∘ S. The suboxic region located north of 10∘ N displays less sensitivity to the phase of the PDO as the local upwelling-related processes play a dominant role compared to the large-scale circulation in setting the oxygen concentration. Our study suggests that the prevailing PDO-positive conditions since 1975 may explain a significant part of the current deoxygenation occurring in the eastern Pacific Ocean.
Highlights
Oxygen is one of the most important chemical elements in the ocean, as marine organisms ranging from microorganisms to vertebrates use it for respiration
We show that during a “typical” Pacific Decadal Oscillation (PDO)-positive phase the volume of the suboxic regions expands by 7 % over 50 years due to a slowdown of the large-scale circulation related to the decrease in the intensity of the trade winds
In the eastern parts of the tropical oceans, the large export of organic material out of productive surface layers combined with sluggish circulation depletes oxygen levels at depth, resulting in the formation of large suboxic regions where the oxygen concentration falls below 20 mmol m−3 (Karstensen et al, 2008)
Summary
Oxygen is one of the most important chemical elements in the ocean, as marine organisms ranging from microorganisms to vertebrates use it for respiration. A subtropical increase in productivity related to an increase in the trade winds causes a negative oxygen anomaly in these mode waters, which is transported equatorward and leads to a delayed oxygen decrease in tropical regions as shown by Ridder and England (2014) in an Earth system model of intermediate complexity All of these studies highlight the potential effect of the PDO on oxygen concentrations and show that the processes at play are diverse and strongly region dependent. Rather than performing a “hindcast” experiment, in this study we isolate and assess the role of the phase of the PDO on the oxygen levels in the tropical eastern Pacific Ocean by forcing a coupled circulation–biogeochemical model using “typical” conditions characteristic of the negative and positive PDO phases.
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