Abstract
Abstract. An oxygen decrease 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 5-decade 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. Due to the low amount of data available, the results are often not statistically significant within the 95 % confidence interval but nevertheless indicate trends worth discussing. 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 indicate overlying variability related to climate modes. Nitrate increased in all areas. Phosphate also increased in the Atlantic Ocean 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.
Highlights
Temperature, oxygen and nutrient changes in the ocean have various impacts on the ecosystem
Phosphate increased in the Atlantic Ocean and Indian Ocean areas, while it decreased in the two areas of the equatorial Pacific Ocean
Despite the overlying variability, the long-term deoxygenation in the tropical oceans is continuous for the 68-year period (Fig. S1)
Summary
Temperature, oxygen and nutrient changes in the ocean have various impacts on the ecosystem. Schmidtko et al (2017) reported that the global oceanic oxygen content of 227.4 ± 1.1 Pmol (1015 mol) has decreased by more than two percent (4.8 ± 2.1 Pmol) These oxygen changes vary by region, with some areas showing increasing oxygen values on timescales related to climate modes. By means of a coupled ecosystem circulation model Oschlies (2001) described for the North Atlantic that the long-term change in the North Atlantic Oscillation (NAO; e.g., Hurell and Deser, 2010) between the 1960s and 1990s may have induced significant regional changes in the upper ocean’s nutrient supply These include a decrease of nitrate supply to the surface waters of by about 30 % near Bermuda and in midlatitudes and a simultaneous 60 % increase to nitrate flux in the upwelling region off West Africa. The thermocline shift could be due to ocean warming and various climate modes, the averaging across the depths could lead to an influence on the trend of the 50 to 300 m layer
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