Impact of mesoscale eddies on water mass and oxygen distribution in the eastern tropical South Pacific

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 6.0 cm/s 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 to a seasonal signal in the open ocean. The mass transport of the four observed eddies across 85° W is between 1.1 and 1.8 Sv. The eddy type dependent available heat, salt and oxygen anomalies are 7.6 × 1018 J (ACE), 0.8 × 1018 J (MWE), −9.4 × 1018 J (CE) for heat, 23.9 × 1010 kg (ACE2), −3.6 × 1010 kg (MWE), −42.8 × 1010 kg (CE) for salt and −3.6 × 1016 μmol (ACE2), −3.5 × 1016 μmol (MWE), −6.5 × 1016 μmol (CE) for oxygen showing an imbalance between anticyclones and cyclones for heat and salt transports probably due to seasonal variability of 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 conclusions on the isolation of water mass properties and the lateral mixing with time between the core of the eddy and the surrounding water showing the strongest lateral mixing between the seasonal thermocline and the eddy core during the first half of the lifetime.