In January and February 2013, a subsurface anticyclonic eddy formed in the Peru-Chile Undercurrent, allowing researchers to document the creation of such an anomaly for the first time.
Using seven autonomous robotic gliders, researchers took recordings of the newly-formed eddy, which measured nearly 100 kilometres across. Such eddies are essential for the transport of oxygen, nutrients and heat throughout the oceans.
The Gulf Stream and the Agulhas Current are not the only currents found in the oceans; there are also countless eddies. These eddies are constantly on the move, can measure up to 300 kilometres in diameter, and last up to five years. There are different ways an eddy may be formed, with long-lasting examples first forming on the eastern boundaries of the oceans before moving westward. Because they form over a span of just a few weeks, they are difficult to predict, making a direct observation difficult to arrange.
In 2013, researchers from the GEOMAR Helmholtz Centre for Ocean Research Kiel, of the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research in Bremerhaven (AWI) and the Max Planck Institute for Marine Microbiology in Bremen had an opportunity to document the formation of an eddy off the Peruvian coast for the first time. Their findings have been published in the international Journal of Geophysical Research: Oceans.
The eddy had formed in precisely the same region where researchers had deployed seven robotic gliders from the German research vessel Meteor. The 1.5 metre long gliders functioned like underwater sailplanes. They manoeuvred back and forth at about 1,000 metres depth, by means of small wings and a sophisticated trim in a forward motion.
The gliders recorded more than 10,000 profiles of temperature, salinity, oxygen and chlorophyll as the eddy formed. Such data allowed the researchers to study the eddy's impact on the salt, oxygen and nutrient distribution in greater detail than would have been possible from ship-based measurements.
Oceanographer and co-author Prof Dr Torsten Kanzow from AWI explained that using satellite communication, they received the data from the gliders in real time and were thus able to adjust the cruise schedule to optimise the sampling strategy.
Describing the main findings of the study, lead author Sören Thomsen from GEOMAR explained that the data showed that the water inside the eddy's core originated from the bottom layers of the continental slope. The waters from this location possess properties that are different from the waters in the open ocean.
“The coastal areas are biologically very productive. As a result, many plants and animals die there too. They sink to the bottom and are decomposed by bacteria. Of course, these biogeochemical processes affect the characteristics of the bottom waters,” said co-author Dr Marcus Dengler from GEOMAR.
After the eddy draws in water from the continental slope, it sends it westward to the open Pacific Ocean. Throughout this process, there is almost no exchange between the eddy and the surrounding waters, so the differences in the properties are maintained. According to Dr Dengler, their research showed that a large part of the eddy's anomalous properties originated from the region where the eddy was formed.
As the eddy transports water masses away from the Peruvian continental margin, it creates space for nutrient-richer water to ascend from the depths. Hence, the eddy plays a crucial role in the maintenance of the high biological productivity of the Peruvian coast.
“This is highly relevant for the people. After all, fishing is an important economic factor in the region,” said Thomsen.
Link to the study: onlinelibrary.wiley.com/doi
Further Information: www.geomar.de