Eddies and an extreme water mass anomaly observed in the eastern south Pacific at the Stratus mooring

Abstract

is located in the transition zone between the oxygen minimum zone (OMZ) and the well-oxygenated subtropical gyre. In February/March 2012, extremely anomalous water mass properties were observed in the thermocline at the Stratus ORS. The available eddy oxygen anomaly was 210.5 3 10 16 mmol. This anomalous water was contained in an anticyclonic mode-water eddy crossing the mooring site. This eddy was absorbed at that time by an anticyclonic feature located south of the Stratus mooring. This was the largest water property anomaly observed at the mooring during the 13.5 month deployment period. The sea surface height anomaly (SSHA) of the strong mode-water eddy in February/March 2012 was weak, and while the lowest and highest SSHA were related to weak eddies, SSHA is found not to be sufficient to specify the eddy strength for subsurface-intensified eddies. Still, the anticyclonic eddy, and its related water mass characteristics, could be tracked backward in time in SSHA satellite data to a formation region in April 2011 off the Chilean coast. The resulting mean westward propagation velocity was 5.5 cm s 21 . This extremely longlived eddy carried the water characteristics from the near-coastal Chilean water to the open ocean. The water mass stayed isolated during the 11 month travel time due to high rotational speed of about 20 cm s 21 leading to almost zero oxygen in the subsurface layer of the anticyclonic mode-water eddy with indications of high primary production just below the mixed layer.

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

“…These values are about five times smaller in comparison to a mode-water eddy that was also measured at the Stratus mooring in February/March 2012 (Stramma et al, 2014;Tab. 1).…”

“…An anticyclonic mode water eddy observed at the Stratus mooring in February/March 2012 indicated high primary production just below the mixed layer (Stramma et al, 2014). According to a global investigation the eastern South Pacific off Peru and Chile seems to have the highest amount of MWEs, which are also deep reaching compared to other regions (Zhang et al, 2017).…”

“…In addition velocity, pressure, temperature, conductivity sensors (for salinity computation) and 13 oxygen sensors were added to the mooring within the water column during the Table S1 (depth distribution of the velocity sensors for the 2011 to 2012 deployment period are given in Stramma et al (2014)). Annual mean velocity profiles were computed for the upper 600 m for the two Stratus mooring deployment periods 2011/2012 and 2014/2015 where 20 oxygen measurements were conducted.…”

“…Available heat, salt and oxygen anomalies (AHA, ASA and AOA) were calculated as described in Chaigneau et al (2011) and Stramma et al (2014). At the Stratus mooring eddy core anomalies were estimated by the difference between the mean of temperature, salinity and oxygen within the eddy boundaries and the background field estimated from the annual mean for the period 10 April 2014 to 9 25 April 2015.…”

“…ASA is even negative in 2015 due to the strong doming in the upper 200 m. As both mode-water eddies have about the same propagation speed and 25 volume, the differences of water mass properties point towards seasonal or interannual variations of the water mass characteristics during their formation. The MWE is generated in February, when upwellingfavorable alongshore winds weaken and SST increases (Gutiérrez et al, 2011), whereas the mode-water Stramma et al (2014) is generated in April, when the PCUC, which transports oxygen-deficient ESSW (Hormazabal et al, 2013), has its poleward maximum (Shaffer et al, 1999;Penven et al, 2005;Chaigneau et al, 2013). Additionally, the mode-water eddy observed by Stramma et al (2014) was generated in year 2011, which is considered as a La Niña-period, when generally lower oxygen and higher salinity values exist in the upper 100 m depth (Stramma et al, 2016) leading 5 to higher anomalies of salt and oxygen of the MWE observed by Stramma et al (2014) in comparison to the actual MWE.…”