Shifting trade winds behind lack of oxygen in tropical seas

Chuck Bednar for – Your Universe Online
For decades, scientists have witnessed the oxygen minimum zones (OMZ) in the tropical oceans expanding, reducing the habitat of some types of fish in the process, and now they finally know why this phenomenon is occurring.
OMZs are ideal living conditions for certain types of specially adapted microorganisms, but are essentially uninhabitable for fish, marine mammals and other larger forms of life. They exist in different intensities at the eastern edges of all tropical oceans, but the reasons for their expansion have long remained a mystery.
Now, however, marine scientists from the Helmholtz Centre for Ocean Research Kiel (GEOMAR) and the Kiel Collaborative Research Centre (SFB 754) have developed a model simulation of climate and biological processes that provides one possible explanation – fluctuations in the trade winds north and south of the equator.
These trade winds help supply oxygen to tropical sea water, so changes in these winds could also be responsible for the recent growth of the oxygen minimum zones, Dr. Olaf Duteil, lead author of a new study appearing in the journal Geophysical Research Letters, explained.
Since nutrient-rich water from the depths reaches the surface in OMZs, plankton living in these regions typically thrive there, which means that they also ultimately die in these areas. Once they pass on, they begin to sink to the ocean floor as bacteria begin decomposing them.
This process consumes the oxygen, while at the same time, currents begin transporting oxygen-rich water from the subtropics towards the tropics, where the oxygen minimum zones lie. Dr. Duteil compares the process to a bathtub.
“When I open the tap, I fill the bathtub with water or ‘oxygen’, respectively. When the siphon is open, too, we lose oxygen at the same time. We then have an instable equilibrium between input and output. If I turn off the tap a little, the tub empties slowly,” he explained.
He and his colleagues used computer simulations to determine the current oxygen balance and the strength of the water currents. As a result, they found that the oxygen flow to the tropics is directly related to the strength of the trade winds, which vary on a decadal time scale, and have been in a weak phase since the mid 1970s.
“These variations haven never been investigated in relation to the oxygen budget of tropical oceans,” said co-author Prof. Dr. Claus Böning from GEOMAR. The current status of the trade winds could help explain the detected enlargement of the oxygen minimum zones, Dr. Duteil said. Once the trade winds re-enter a stronger phase, the authors expect the process to reverse.
Climate change could also play a role in the OMZ expansion, said co-author and SFB 754 speaker Prof. Andreas Oschlies: “There is evidence that global change affects the major wind systems of the Earth. That would have a direct impact on the oxygen transport in the subtropical and tropical ocean, but it is important that according to this study the trade winds in any case as must be considered as a factor for long-term development of tropical oxygen minimum zones.”
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