Circulation in the OMZs
Mid-depth circulation of the eastern tropical South Pacific
and its link to the oxygen minimum zone *
by Rena Czeschel(a), Lothar Stramma (a), Franziska U.Schwarzkopf(a), Benjamin S.Giese(b), Andreas Funk (a) and Johannes Karstensen (a)
* Corresponding author : Rena Czeschel
(a) : Helmholtz Centre for Ocean Research Kiel, GEOMAR, Kiel Germany
(b) : Department of Oceanography Texas A&M University, College Station, TX,USA
Observations indicate increasing oxygen minimum zones (OMZs) in the tropical Pacific over decades, with a westward extension and a vertical expansion of the intermediate depth (300-700m) low oxygen zones during the last 50 years. Reduced oxygen levels may have dramatic consequences for ecosystems and coastal economics. In the Pacific, the supply of oxygen-rich water to the OMZs is related to zonal eastward near-equatorial flowing currents (Schott et al, 2004) and expanding OMZs must be thought to be connected to variability in zonal currents (Stramma et al, 2008) . It is thus important to understand the present-day mid-depth circulation and to investigate oxygen changes in the low-oxygen areas. Subsurface circulation in the eastern tropical South Pacific OMZ is investigated using ADCP measurements, Argo floats tracks and models fields, to better understand the weak mean flow field in the OMZ and the spreading pathways.
Data and Method
Subsurface currents of the OMZ in the eastern tropical South Pacific are investigated with a focus at 400m depth, close to the core of the OMZ. Mid-depth circulation is investigated using shipboard CTD and ADCP sections carried out on two cruise legs of the RV Meteor in early 2009. During the second cruise leg, hydrographic measurements were made along 14°S, 6°S and 3°35’S as well as 85°50’W between 14°S and 2°N. The 85°50W section was measured in March 1993 (Tsuchiya and Talley, 1998) and provides the basic water mass information for the 2009 survey.
Figure 1 : Horizontal distribution of a) ADCP velocity vectors at 400 m depth recorded between 5 January and 14 February 2009 with current bands indicated by arrows and the mean climatological oxygen distribution at 400 m (in μmol/kg, with 5μmol/kg contour spacing, thin lines) from WOA05 [Boyer et al., 2006] and b) altimeter derived sea level anomaly in cm on 4 February 2009 [http://www.aviso.oceanobs.com]. ADCP and CTD sections are shown as black lines. Arrows indicate the related flow direction.
Ten profiling Argo floats with Aanderaa oxygen sensors were deployed between 2 and 11 february 2009 along the 85°50W sections at 10°S, 8°S, 6°S, and 2°S in pairs of 2 floats, one of each pair with a parking depth at 400m and one with a parking depth at 1000m. The floats, all programmed with a 10-day cycle, recorded data until mid-august 2010, providing a 1.5 years data recordings. The velocity distribution and the float paths are related to the oxygen distribution as recorded in a number of CTD-oxygen sections. The results are compared with the 1/10° Tropical Pacific Model (TROPAC01) fields and the output from the Simple Ocean Data Assimilation (SODA 2.1.6) model.
Figure 2 : Zonal velocity distribution (in cm s-1) at 400 m depth averaged from February 2002 to January 2003 for a)- TROPAC01 and b)- SODA. Float trajectories in 400 m depth for February 2009 to mid-August 2010 are also shown. Eastward velocity is plotted white and red, westward velocity is plotted grey and blue.
The first observation is a large agreement for the models field and the observed data, as shown on figure 2. Both models generally represent the float tracks well, excepted for eddies and meandering tracks which can’t be observed by the annual mean for the model velocity fields but can be observed in the float velocity.
Near the core of the OMZ at 400m depth, the currents in the center of the OMZ are relatively stagnant and the Argo floats deployed at 8°S and 10°S stay much longer in the region than the floats at the northern side of the OMZ, which travel fast westward within the equatorial current system. Considering only the 400m flow component of the float deployed at 8°S, it returned to a location it crossed in May 2009 in August 2010, 15 months later. Wyrtki  concluded that a sluggish circulation led to long residence time in the OMZ of this region.
Oxygen concentration from the float deployed at 4°S (figure 3) shows a predominant vertical increase in oxygen >40μmol/kg as it propagates, while the minimum in the core of the OMZ stays at a relatively low value of less than 4μmol/kg. The strongest vertical increase in oxygen took place in January and February 2010 at the time when the float shifted north by about 200 km. After the end of 2009, the depth of the lowest oxygen values shifted upwards from about 400m depth to 300m in august 2010. At 400m depth, oxygen increased to more than 20 μmol/kg in May 2010, similar to what is expected from the mean climatological oxygen distribution from WOA05 (see figure 1a).
Figure 3 : Time series from February 2009 to mid-August 2010 of the float (WMO #3901081) deployed at 4°S with 400 m parking depth for a) westward (gray) and eastward (white) displacements (in °) and b) oxygen profiles (in μmol/kg) of the upper 1500 m measured in 10-day intervals. The corresponding longitude is marked on top of b). The float is drifting westward with the SEIC until April 2010 when it entered the SICC flowing eastward (see figure 2). .
The horizontal velocity distribution observed from ADCP measurements at 400m depth for the early 2009 (figure 1) reproduce several of the previously described zonal equatorial intermediate current bands (e.g. Rowe et al., 2000 ; Stramma et a al., 2010). The ADCP distribution for the upper 700m between 14°S and 2°N at 85°50‘W section in February 2009 show a complicated zonal equatorial current system. At 3°35’S, the meridional velocity distribution show current bands with low vertical expansion, while at 6°S and 14°S, most current bands extend vertically over the upper 700m.
The schematic flow field for the mid-depth OMZ layer at about 400 m depth (Figure 4) combines the results described here with results that have been previously reported for the equatorial currents near the equator [Stramma et al., 2010; Rowe et al., 2000] and the mesoscale eddy-field [Chaigneau et al., 2008]. The region south of the OMZ is covered by the northern part of the subtropical gyre, while to the north the region is governed by the complicated zonal equatorial flow field. The eastern Pacific between 8°S and 10°S is also a region with high eddy occurrence. The eddies move westward from the formation region near the shelf and carry water with low DO from the shelf region westward with velocities of 3 to 6 cm/s [Chaigneau et al., 2008]. A cyclonic eddy with low oxygen in its core clearly demonstrates the contribution of low oxygen water to the OMZ. Hence the westward extending core of the OMZ at 6 to 10°S seems to be maintained by stagnant flow supplied with additional low DO water from the near shelf region by westward propagating eddies. The southern area with high frequency of eddy occurrence at 16°S to 18°S feeds low DO water into the HC/PCC to the SEC transition area leading to low DO values at the southeastern part of the OMZ.
Figure 4 : Schematic mid-depth flow field at about 400 m depth. The mean climatological dissolved oxygen distribution at 400 m from WOA05 [Boyer et al., 2006] is included (in µmol/kg, with 5 µmol/kg contour spacing, thin lines). Areas of high frequency of eddy occurrence [Chaigneau et al., 2008] are marked by dashed lines
Most current bands identified in the upper ocean (Kessler, 2006) were also observed in the OMZ layer especially south of about 4°S, although they are weaker and more variable than in the upper ocean. The mean flow field of the mid-depth southeast Pacific at 400m depth is obtained and expected to be well-resolved and accurate. This study shows that argo floats with oxygen sensors might play an important role to construct oxygen times series in regions with sparse or infrequent ship-based hydrographic surveys.
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