Abstract
Abstract. The intraseasonal evolution of physical and biogeochemical properties during a coastal trapped wave event off central Peru is analysed using data from an extensive shipboard observational programme conducted between April and June 2017, and remote sensing data. The poleward velocities in the Peru–Chile Undercurrent were highly variable and strongly intensified to above 0.5 m s−1 between the middle and end of May. This intensification was likely caused by a first-baroclinic-mode downwelling coastal trapped wave, excited by a westerly wind anomaly at the Equator and originating at about 95∘ W. Local winds along the South American coast did not impact the wave. Although there is general agreement between the observed cross-shore-depth velocity structure of the coastal trapped wave and the velocity structure of first vertical mode solution of a linear wave model, there are differences in the details of the two flow distributions. The enhanced poleward flow increased water mass advection from the equatorial current system to the study site. The resulting shorter alongshore transit times between the Equator and the coast off central Peru led to a strong increase in nitrate concentrations, less anoxic water, likely less fixed nitrogen loss to N2 and a decrease of the nitrogen deficit compared to the situation before the poleward flow intensification. This study highlights the role of changes in the alongshore advection due to coastal trapped waves for the nutrient budget and the cumulative strength of N cycling in the Peruvian oxygen minimum zone. Enhanced availability of nitrate may impact a range of pelagic and benthic elemental cycles, as it represents a major electron acceptor for organic carbon degradation during denitrification and is involved in sulfide oxidation in sediments.
Highlights
The Peruvian upwelling system (PUS) is one of the most productive regions in the world’s ocean with an economically important fishing industry (e.g. Carr, 2002; Chavez et al, 2008)
The Sea level anomaly (SLA) increased towards the coast, implying that a poleward geostrophic velocity anomaly was present at the sea surface as well
Mean alongshore flow at 12◦ S determined from vessel-mounted ocean surveyor acoustic Doppler current profiler (vmADCP) data sampled during 22 cruises showed maximum Peru–Chile Undercurrent (PCUC) core velocities of 0.1–0.15 m s−1 (Chaigneau et al, 2013), similar to the situation observed during 18–25 April and 24 June (Fig. 2b and g)
Summary
The Peruvian upwelling system (PUS) is one of the most productive regions in the world’s ocean with an economically important fishing industry (e.g. Carr, 2002; Chavez et al, 2008). Located in the eastern tropical South Pacific (ETSP), the high surface productivity of the PUS is most pronounced within a 100 km wide stretch along the Peruvian coast between 4 and 16◦ S (Pennington et al, 2006). Equatorward winds favour upwelling throughout the year (Bakun and Nelson, 1991; Strub et al, 1998) and enable a supply of nutrients from subsurface waters and benthic boundary layer to the euphotic zone, stimulating high primary productivity (Pennington et al, 2006). Low-oxygen waters supplied to the PUS and enhanced local oxygen consumption due to remineralization of exported organic matter, leading to the development of a pronounced oxygen minimum zone (OMZ). Gunther, 1936; Chaigneau et al, 2013) It carries low-oxygen and high-nutrient equatorial subsur-
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