Influence of intraseasonal eastern boundary circulation variability on hydrography and biogeochemistry off Peru

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.

Energy Fluxes in Coastal Trapped Waves

The calculation of energy flux in coastal trapped wave modes is reviewed in the context of tidal energy pathways near the coast. The significant barotropic pressures and currents associated with coastal trapped wave modes mean that large errors in estimating the wave flux are incurred if only the baroclinic component is considered. A specific example is given showing that baroclinic flux constitutes only 10% of the flux in a mode-1 wave for a reasonable choice of stratification and bathymetry. The interpretation of baroclinic energy flux and barotropic-to-baroclinic conversion at the coast is discussed: in contrast to the open ocean, estimates of baroclinic energy flux do not represent a wave energy flux; neither does conversion represent the scattering of energy from the tidal Kelvin wave to higher modes.

Influence of intraseasonal eastern boundary circulation variability on hydrography and biogeochemistry off Peru

The Peruvian Upwelling System is characterized by high primary productivity fuelled by the supply of nutrients in a highly dynamic boundary circulation. The intraseasonal evolution of the physical and biogeochemical properties is analysed based on shipboard observations and remote sensing conducted between April and June 2017 off central Peru. The poleward transport in the subsurface Peru Chile Undercurrent was highly variable and strongly intensified between mid and end of May. This intensification was likely caused by a first baroclinic mode downwelling coastal trapped wave excited at the equator at about 95° W that propagated poleward along the South American coast. The intensified poleward flow shortens the time of water mass advection from the equatorial current system to the study site. The impact of the anomalous advection is mostly noticed in the nitrogen cycle because during the shorter time needed for poleward advection less fixed nitrogen loss occurs within the waters. This causes a strong increase of nitrate concentrations and a decrease in the nitrogen deficit. These changes suggest that the advection caused by the coastal trapped wave supersedes the simultaneous effect of anomalous downwelling in terms of nutrient response.