Middepth circulation of the eastern tropical South Pacific and its link to the oxygen minimum zone

Abstract

[1] There is an incomplete description of the middepth circulation and its link to the oxygen minimum zone (OMZ) in the eastern tropical South Pacific. Subsurface currents of the OMZ in the eastern tropical South Pacific are investigated with a focus at 400 m depth, close to the core of the OMZ, using several acoustic Doppler current profiler sections recorded in January and February 2009. Five profiling floats with oxygen sensors were deployed along 85°50′W in February 2009 with a drift depth at 400 m. Their spreading paths are compared with the model flow field from a 1/10°tropical Pacific model (TROPAC01) and the Simple Ocean Data Assimilation (SODA) model. Overall the mean currents in the eastern tropical South Pacific are weak so that eddy variability influences the flow and ultimately feeds oxygen-poor water to the OMZ. The center of the OMZ is a stagnant area so that floats stay much longer in this region and can even reverse direction. In one case, one float deployed at 8°S, returned to the same location after 15 months. On the northern side of the OMZ in the equatorial current system, floats move rapidly to the west. Most current bands reported for the near-surface layer exist also in the depth range of the OMZ. A schematic circulation flow field for the OMZ core depth is derived which shows the northern part of the South Pacific subtropical gyre south of the OMZ and the complicated zonal equatorial flow field north of the OMZ.

Decadal Variability of Eddy Kinetic Energy in the South Pacific Subtropical Countercurrent in an Ocean General Circulation Model

“…Between 498S and 318N in the Pacific, a grid with 1/108 resolution is nested into the ORCA05 grid using adaptive grid refinement in Fortran (AGRIF; Debreu et al 2008). TROPAC01 has been compared to observations in Czeschel et al (2011) andvan Sebille et al (2014). For more information on TROPAC01, the reader is referred to Schwarzkopf (2016).…”

Nitrous oxide dynamics in low oxygen regions of the Pacific: insights from the MEMENTO database

“…4c). This region, which is located along the Peru-Chile undercurrent (PCUC), was selected because altimeter (Strub et al, 1995;Pizarro et al, 2002) and current-meter data (Shaffer et al, 1995;Czeschel et al, 2011) indicate a predominantly unidirectional flow along the coast, allowing a comparison in water mass chemical changes along the direction of water mass flow. Additionally, denitrification-driven N 2 O consumption is likely occurring in this region based on decreases in N 2 O and N * concentrations (N * is the deviation in inorganic nutrients from Redfield ratios, or (NO…”

“…(5), an assumed residence time of 0.32 yr (Table 1), and a N 2 O loss of 0.0407 mmol m −3 (Table 1), we calculate a net N 2 O consumption rate of 0.01-1 mmol N 2 O m −3 yr −1 within the 26.3 and 26.5 σ θ density layer of focus. There is a relatively large uncertainty of one order of magnitude in the above rate due to uncertainties in water mass residence time (Table 1) as well as to uncertainties in flow of the PCUC caused by eddies (e.g., Czeschel et al, 2011;Altabet et al, 2012).…”

“…4 for a map of the station locations). The NITROP-85 cruise, described in detail in Friederich et al (1992), took place in February, 1985 and proceeded southward along the PCUC from 7.7-16 • S. The second dataset (the Meteor 77, or M77, cruise) was sampled in February, 2009 from the region surrounding 6-14 • S. Only two stations from the M77 cruise were within the PCUC as based on the co-sampled current data presented in Czeschel et al (2011), and so only these stations were compared to the NITROP-85 dataset. N 2 O loss was determined from the change in N 2 O concentrations between transit based on a linear model fit to the data (shown in Fig.…”

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

“…In contrast, the mid-depth circulation in the eastern South Pacific Ocean is sluggish in the region of the OMZ. As the mean currents are weak, eddy variability strongly influences the flow and ultimately supplies oxygen-poor water to the OMZ (Czeschel et al, 2011). A rough estimate of the oxygen budget 10 of the eastern tropical Pacific ocean (Stramma et al, 2010) was used to estimate 22% by vertical mixing, 33% by advection and the largest component of 45% by eddy mixing (Brandt et al, 2015).…”