Articles | Volume 14, issue 20
Biogeosciences, 14, 4601–4617, 2017
Biogeosciences, 14, 4601–4617, 2017

Research article 19 Oct 2017

Research article | 19 Oct 2017

The OMZ and nutrient features as a signature of interannual and low-frequency variability in the Peruvian upwelling system

Michelle I. Graco1, Sara Purca1, Boris Dewitte2,3,4, Carmen G. Castro5, Octavio Morón1, Jesús Ledesma1, Georgina Flores1, and Dimitri Gutiérrez1 Michelle I. Graco et al.
  • 1Dirección General de Investigaciones Oceanográficas y cambio Climático, Instituto del Mar del Perú (IMARPE), P.O. Box 22, Callao, Perú
  • 2Laboratoire d`Etudes en Géophysique et Océanographie Spatiale (LEGOS)/IRD, Toulouse, France
  • 3Universidad Católica del Norte, Facultad de Ciencias del Mar, Coquimbo, Chile
  • 4Centro de Estudios Avanzado en Zonas Áridas (CEAZA), Coquimbo, Chile
  • 5CSIC, Instituto de Investigaciones Marinas, Eduardo Cabello 6, 36208 Vigo, Spain

Abstract. Over the last decades, the Humboldt Current upwelling ecosystem, particularly the northern component off the coast of Peru, has drawn the interest of the scientific community because of its unique characteristics: it is the upwelling system with the biggest catch productivity despite the fact it is embedded in a shallow and intense oxygen minimum zone (OMZ). It is also an area of intense nitrogen loss and anammox activity and experiences large interannual variability associated with the equatorial remote forcing. In this context, we examined the oceanographic and biogeochemical variability associated with the OMZ off central Peru from a monthly time series (1996–2011) recorded off the coast of Callao (12° 02′ S, 77° 29′ W). The data reveal a rich spectrum of variability in the OMZ that includes frequencies ranging from seasonal to interannual scales. Due to the efficient oceanic teleconnection off Peru, the observed variability is interpreted in the light of an estimate of the equatorial Kelvin wave contribution to sea level anomalies considering the peculiarities of its vertical structure (i.e., the first two baroclinic modes). The span of the data set allows us to contrast two OMZ regimes. The strong regime is associated with the strong 1997–1998 equatorial Pacific El Niño, during which the OMZ adjusted to Kelvin-wave-induced downwelling conditions that switched off the upwelling and drastically reduced nutrient availability. The weak regime corresponds to the post-2000 period associated with the occurrence of moderate central Pacific El Niño events and enhanced equatorial Kelvin wave activity, in which mean upwelling conditions are maintained. It is shown that the characteristics of the coupling between physics and biogeochemistry is distinct between the two regimes with the weak regime being associated with a larger explained variance in biogeochemical properties not linearly related to the ENSO oceanic teleconnection. The data also reveal a long-term trend from 1999 corresponding to a deepening of the oxygen-deficient waters and warming. The implications of our results for understanding the OMZ dynamics off Peru are discussed.

Short summary
The Peruvian coastal upwelling ecosystem is a natural laboratory to study climatic variability and climate change. We examined the variability in the OMZ in the last decades in connection with the equatorial Pacific strong 1997–1998 El Niño event and the influence of central Pacific El Niño events and enhanced equatorial Kelvin wave activity since 2000. The data reveal two contrasting regimes and a long-term trend corresponding to a deepening of the oxygen-deficient waters and warming.
Final-revised paper