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Ocean Science An interactive open-access journal of the European Geosciences Union
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Volume 14, issue 4 | Copyright
Ocean Sci., 14, 731-750, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 31 Jul 2018

Research article | 31 Jul 2018

Transport, properties, and life cycles of mesoscale eddies in the eastern tropical South Pacific

Rena Czeschel1, Florian Schütte1, Robert A. Weller2, and Lothar Stramma1 Rena Czeschel et al.
  • 1GEOMAR Helmholtz Centre for Ocean Research Kiel, Düsternbrooker Weg 20, 24105 Kiel, Germany
  • 2Woods Hole Oceanographic Institution (WHOI), 266 Woods Hole Rd, Woods Hole, MA 02543, USA

Abstract. The influence of mesoscale eddies on the flow field and the water masses, especially the oxygen distribution of the eastern tropical South Pacific, is investigated from a mooring, float, and satellite data set. Two anticyclonic (ACE1/2), one mode-water (MWE), and one cyclonic eddy (CE) are identified and followed in detail with satellite data on their westward transition with velocities of 3.2 to 6.0cms−1 from their generation region, the shelf of the Peruvian and Chilean upwelling regime, across the Stratus Ocean Reference Station (ORS;  ∼ 20°S, 85°W) to their decaying region far west in the oligotrophic open ocean. The ORS is located in the transition zone between the oxygen minimum zone and the well oxygenated South Pacific subtropical gyre. Velocity, hydrographic, and oxygen measurements at the mooring show the impact of eddies on the weak flow region of the eastern tropical South Pacific. Strong anomalies are related to the passage of eddies and are not associated with a seasonal signal in the open ocean. The mass transport of the four observed eddies across 85°W is between 1.1 and 1.8Sv. The eddy type-dependent available heat, salt, and oxygen anomalies are 8.1×1018J (ACE2), 1.0×1018J (MWE), and −8.9×1018J (CE) for heat; 25.2×1010kg (ACE2), −3.1×1010kg (MWE), and −41.5×1010kg (CE) for salt; and −3.6×1016µmol (ACE2), −3.5×1016µmol (MWE), and −6.5×1016µmol (CE) for oxygen showing a strong imbalance between anticyclones and cyclones for salt transports probably due to seasonal variability in water mass properties in the formation region of the eddies. Heat, salt, and oxygen fluxes out of the coastal region across the ORS region in the oligotrophic open South Pacific are estimated based on these eddy anomalies and on eddy statistics (gained out of 23 years of satellite data). Furthermore, four profiling floats were trapped in the ACE2 during its westward propagation between the formation region and the open ocean, which allows for conclusions on lateral mixing of water mass properties with time between the core of the eddy and the surrounding water. The strongest lateral mixing was found between the seasonal thermocline and the eddy core during the first half of the eddy lifetime.

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Short summary
The mean circulation on the poleward side of the oxygen minimum zone is overlain by eddy activity playing an important role in the distribution of water masses and oxygen within the OMZ. The activity of different types of eddies was investigated during their westward propagation from the formation area off Peru/Chile into the open ocean. The focus was on the development of eddies, seasonal conditions during their formation, and the change of water mass properties transported within the eddies.
The mean circulation on the poleward side of the oxygen minimum zone is overlain by eddy...