Interannual variability in contributions of the Equatorial Undercurrent (EUC) to Peruvian Upwelling.

Supervisors: Robert Marsh(Univ. Southampton), Luis Icochea (Univ. Nacional Agraria,Peru)
The impacts of the Pacific Equatorial Undercurrent in the Peruvian Upwelling system areanalysed here using a Lagrangian method (Blanke and Raynaud, 1997) for tracking particlesin a backward mode through the study area between 170ºW – 75ºW to 5ºN – 15ºS. Utilizingtemperature, salinity and velocity data from the high-resolution (1/12º or ~9.277 km)configuration of the ocean model NEMO in global eddy-resolving ORCA12 configuration(Blanke and Raynaud, 1997; Madec, 2008; Blakeret al., 2015), undertaking 20-year hindcastexperiments for the period 1988-2007. The simulation is validated with observational data fromNOAA at specific locations along the Pacific. Upwelled particles exceeding +1.9×10-5 m/s (50m/month), were back-tracked from 30meter depth off the Peruvian Coast, to obtain a “Grandensemble” for Particle concentration, Age, Depth, Temperature, Salinity and Density (Sigma-0). After 1-year tracking, most of the particles were originated in the Western equatorial Pacific(from 160ºW) between 3ºN-3ºS (the highest Particle concentration area), characterized bytemperatures colder than 17ºC and salinities between 34.9 to 35.2 in its core (between ~1ºN to~1ºS), however, saltier salinities were also observed eastern and southern ~130ºW and 2ºSrespectively. Along the isopycnal of sq= ~26.5 from deeper region, two important branchesbifurcating from the EUC, mainly distributed along ~3ºS and ~8ºS, were also identified.Percentage of particles recruited by the EUC (17.5% up to 47% in average from 160ºW to92ºW) during specific El Niño and La Niña events were also calculated, to reveal that duringextreme El Niño, moderate El Niño and during weak La Niña events, more strongly upwellingparticles off Peru come from the western Pacific, but with different level of intensification. Incontrast, during extreme La Niña, upwelling particles are recruited from the nearby region.Finally, we established relationships between changes in EUC transport in the central (160ºW)and eastern (92ºW) Pacific. Negative correlation was found for 95ºW (-0.13, p-value < 0.01)and 92ºW (-0.17, p-value < 0.01) at 0-lag. From the coefficient of Determination (R2) for ±12months-lag correlation we found that in general, volume transported at 160ºW, are observedaround 30-35 days later at 95ºW (+0.31, p-value < 0.01) and 92ºW (+0.29, p-value < 0.01),explaining in 9.7% and 8.4% of transport variability respectively. It may therefore be possibleto predict EUC transport variability around the Galapagos Islands, and associated changes inPeruvian upwelling. These results are of particular interest for Peru and the Peruvian fishingsector due to the great consequences (positive or negative) that warm and cold interannualmodulations can have in the biogeochemical-based processes, and the ecosystem approach tofishery management, ergo in the socio-economic sector of the country.