Interannual variability in contributions of the Equatorial Undercurrent (EUC) to Peruvian upwelling source water

Abstract

Time-varying sources of upwelling waters off the coast of northern Peru are analyzed in a Lagrangian framework, tracking virtual particles backwards in time for 12 months. Particle trajectories are calculated with temperature, salinity and velocity fields from a hindcast spanning 1988–2007, obtained with an eddy-resolving (1/12∘) global configuration of the Nucleus for European Modelling of the Ocean (NEMO) ocean model. At 30 and 100 m, where coastal upwelling rates exceed 50 m month−1, particles are seeded at monthly intervals in proportion to the upwelling rate. Ensemble maps of particle concentration, age, depth, temperature, salinity and density reveal that a substantial but variable fraction of the particles upwelling off Peru arrives via the Equatorial Undercurrent (EUC). Particles follow the EUC core within the depth range 125–175 m, characterized by temperatures <17 ∘C, salinities in the range 34.9–35.2 and densities of σθ=25.5–26.5. Additional inflows are via two slightly deeper branches further south from the main system, at around ≈3 and ≈8∘ S. Averaged across the hindcast, annual-mean percentages of particles upwelling at 30 m (100 m) associated with the EUC vary from 57.4 % (52 %) at 92∘ W to 19.2 % (17.9 %) at 165∘ W. Considerable interannual variability in these percentages reveals that more of the Peruvian upwelling can be traced back to the EUC during warm events, such as El Niño. In contrast, upwelling waters are of more local origin during cold events such as La Niña. Despite weaker EUC transport during El Niño, relative flattening of the equatorial thermocline brings the EUC upwelling waters much closer to the Peruvian coast than under neutral or La Niña conditions. Annually averaging EUC transport at specific longitudes, a notable negative-to-positive transition is evident during the major El Niño/La Niña events of 1997/99. On short timescales, a degree of longitudinal coherence is evident in EUC transport, with transport anomalies at 160∘ W evident at the Galápagos Islands (92∘ W) around 30–35 d later. It is concluded that the Peruvian upwelling system is subject to a variable EUC influence, on a wide range of timescales, most notably the interannual timescale of El Niño–Southern Oscillation (ENSO). Identifying this variability as a driver of shifts in population and catch data for several key species, during the study period, these new findings might inform sustainable management of commercially important fisheries off northern Peru.