Abstract
Abstract. Temporal changes in the water mass distribution and biogeochemical signals in the tropical eastern South Pacific are investigated with the help of an extended optimum multi-parameter (OMP) analysis, a technique for inverse modeling of mixing and biogeochemical processes through a multidimensional least-square fit. Two ship occupations of a meridional section along 85°50' W from 14° S to 1° N are analysed during relatively warm (El Niño/El Viejo, March 1993) and cold (La Niña/La Vieja, February 2009) upper-ocean phases. The largest El Niño–Southern Oscillation (ENSO) impact was found in the water properties and water mass distribution in the upper 200 m north of 10° S. ENSO promotes the vertical motion of the oxygen minimum zone (OMZ) associated with the hypoxic equatorial subsurface water (ESSW). During a cold phase the core of the ESSW is found at shallower layers, replacing shallow (top 200 m) subtropical surface water (STW). The heave of isopycnals due to ENSO partially explains the intrusion of oxygen-rich and nutrient-poor antarctic intermediate water (AAIW) into the depth range of 150–500 m. The other cause of the AAIW increase at shallower depths is that this water mass flowed along shallower isopycnals in 2009. The shift in the vertical location of AAIW reaching the OMZ induces changes in the amount of oxygen advected and respired inside the OMZ: the larger the oxygen supply, the greater the respiration and the lower the nitrate loss through denitrification. Variations in the intensity of the zonal currents in the equatorial current system, which ventilates the OMZ from the west, are used to explain the patchy latitudinal changes of seawater properties observed along the repeated section. Significant changes reach down to 800 m, suggesting that decadal variability (Pacific decadal oscillation) is also a potential driver in the observed variability.
Highlights
Oxygen minimum zones (OMZs) exist with different intensities in the upper thermocline of the eastern part of the subtropical gyres of the Pacific and Atlantic oceans as well as in the northern Indian Ocean (Karstensen et al, 2008; Paulmier and Ruiz-Pino, 2009)
The extended optimum multi-parameter (OMP) analysis has been applied to the data of each transect, but excluding those data points located within the mixed layer, as they are influenced by air/sea interaction processes which are not considered in the set of equations, and which may introduce sources and/or sinks to the thermodynamic parameters (Holte et al, 2012)
The distribution obtained for the P19 data (Fig. 2b) along the same latitude range shows similar patterns to those found in the M77 data, and complements an earlier distribution obtained by applying the classic OMP analysis (De PolHolz et al, 2007) to the southern part of the P19 section www.biogeosciences.net/10/6339/2013/
Summary
Oxygen minimum zones (OMZs) exist with different intensities in the upper thermocline of the eastern part of the subtropical gyres of the Pacific and Atlantic oceans as well as in the northern Indian Ocean (Karstensen et al, 2008; Paulmier and Ruiz-Pino, 2009). The PDO oscillates between warm (El Viejo) and cold (La Vieja) phases that go along with changes in sea surface height, sea surface temperature (SST), thermocline depth and ocean currents Given their different time scales, ENSO has more impact on upper water masses, while the PDO can induce more substantial changes over the whole permanent thermocline and intermediate layers, e.g. influencing the dissolved oxygen content down to 700 m in the water column (Stramma et al, 2010a; Czeschel et al, 2011). We will use the extended OMP analysis (Karstensen and Tomczak, 1998; Hupe and Karstensen, 2000), which decomposes the observed parameter distribution into contributions that originate from water mass mixing and those that stem from remineralization/respiration as well as denitrification processes In this way it is possible to investigate the relative roles of both ocean transport (linked to the advection and mixing of water masses) and biogeochemical processes on the changes arising between the two cruises
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