Preprints
https://doi.org/10.5194/os-2020-123
https://doi.org/10.5194/os-2020-123

  28 Dec 2020

28 Dec 2020

Review status: this preprint is currently under review for the journal OS.

Oxygen and nutrient trends in the Tropical Oceans

Lothar Stramma and Sunke Schmidtko Lothar Stramma and Sunke Schmidtko
  • GEOMAR Helmholtz Centre for Ocean Research Kiel, Düsternbrooker Weg 20, 24105 Kiel, Germany

Abstract. A vertical expansion of the intermediate-depth low-oxygen zones (300 to 700 m) is seen in time series for selected tropical areas for the period 1960 to 2008, in the eastern tropical Atlantic, the equatorial Pacific and the eastern tropical Indian Ocean. These nearly five decade-long time series were extended to 68 years by including rare historic data starting in 1950 and more recent data. For the extended time series between 1950 and 2018 the deoxygenation trend for the layer 300 to 700 m is similar to the deoxygenation trend seen in the shorter time series. Additionally, temperature, salinity and nutrient time series in the upper ocean layer (50 to 300 m) of these areas were investigated since this layer provides critical pelagic habitat for biological communities. Generally, oxygen is decreasing in the 50 to 300 m layer except for an area in the eastern tropical South Atlantic. Nutrients also showed long-term trends in the 50 to 300 m layer in all ocean basins and indicates overlying variability related to climate modes. Nitrate increased in all areas. Phosphate also increased in the Atlantic and Indian Ocean areas, while it decreased in the two areas of the equatorial Pacific Ocean. Silicate decreased in the Atlantic and Pacific areas but increased in the eastern Indian Ocean. Hence oxygen and nutrients show trends in the tropical oceans, though nutrients trends are more variable between ocean areas than the oxygen trends, therefore we conclude that those trends are more dependent on local drivers in addition to a global trend. Different positive and negative trends in temperature, salinity, oxygen and nutrients indicate that oxygen and nutrient trends cannot be completely explained by local warming.

Lothar Stramma and Sunke Schmidtko

 
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Lothar Stramma and Sunke Schmidtko

Lothar Stramma and Sunke Schmidtko

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Short summary
Six tropical areas in the Pacific, Atlantic and Indian Ocean were investigated for trends for the years since 1950 in temperature, salinity, oxygen and nutrients for the layers 50 to 300 m and 300 to 700 m depths. Generally, temperature increases, oxygen decreases while nutrients often increase. Overlain variability on the trends seem to be related to climate modes. Different trends indicate that oxygen and nutrient trends cannot by completely explained by local warming.