The riddle of eastern tropical Pacific Ocean oxygen levels:  the role of the supply by intermediate-depth waters

Abstract. Observed oxygen minimum zones (OMZs) in the tropical Pacific Ocean are located above intermediate-depth waters (IDWs), defined here as the 500–1500 m water layer. Typical climate models do not represent IDW properties well and are characterized by OMZs that are too deep-reaching. We analyze the role of the IDW in the misrepresentation of oxygen levels in a heterogeneous subset of ocean models characterized by a horizontal resolution ranging from 0.1 to 2.8∘. First, we show that forcing the extratropical boundaries (30∘ S and N) to observed oxygen values results in a significant increase in oxygen levels in the intermediate eastern tropical region. Second, we highlight the fact that the Equatorial Intermediate Current System (EICS) is a key feature connecting the western and eastern part of the basin. Typical climate models lack in representing crucial aspects of this supply at intermediate depth, as the EICS is basically absent in models characterized by a resolution lower than 0.25∘. These two aspects add up to a “cascade of biases” that hampers the correct representation of oxygen levels at intermediate depth in the eastern tropical Pacific Ocean and potentially future OMZ projections.

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Intermediate water masses, a major supplier of oxygen for the eastern tropical Pacific ocean

10.5194/os-2020-17 ◽

2020 ◽

Author(s):

Olaf Duteil ◽

Ivy Frenger ◽

Julia Getzlaff

Keyword(s):

Pacific Ocean ◽

Climate Models ◽

Current System ◽

Tropical Pacific ◽

Water Masses ◽

Intermediate Water ◽

Tropical Pacific Ocean ◽

Eastern Tropical Pacific ◽

Oxygen Levels ◽

Equatorial Intermediate Current

Abstract. It is well known that Intermediate Water Masses (IWM) are sinking in high latitudes and ventilate the lower thermocline (500–1500 m depth). We here highlight how the IWM oxygen content and the IWM pathway along the Equatorial Intermediate Current System (EICS) towards the eastern tropical Pacific ocean are essential for the supply of oxygen to the lower thermocline and the Oxygen Minimum Zones (OMZs). To this end, we assess here a heterogeneous subset of ocean models characterized by a horizontal resolution ranging from 0.1° to 2.8°. Subtropical oxygen levels in the lower thermocline, i.e., IWM are statistically correlated with tropical oxygen levels and OMZs. Sensitivity simulations suggest that the oxygen biases of the subtropical IWM oxygen levels contribute to oxygen biases of the tropical thermocline as an increase of the IWM oxygen by 60 mmol m−3 results in a 10 mmol m−3 increase in the tropical ocean in a timescale of 50 years. In the equatorial regions, the IWM recirculates into the Equatorial Intermediate Current System (EICS). By comparing tracer and particle release simulations, we show that a developed EICS increases eastern tropical ventilation by 30 %. Typical climate models lack in representing crucial aspects of this supply: biases in IWM properties are prominent across climate models and the EICS is basically absent in models with typical resolutions of ~ 1°. We emphasize that these biases need to be reduced in global climate models to allow reliable projections of OMZs in a changing climate.

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