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

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

M. I. Graco, S. Purca, B. Dewitte, B. Dewitte, B. Dewitte, C. G. Castro, O. Morón, J. Ledesma, G. Flores, D. Gutiérrez

Over the last decades, the Humboldt Current upwellingecosystem, particularly the northern component off the coast of Peru, has drawn theinterest of the scientific community because of its unique characteristics:it is the upwelling system with the biggest catch productivity despite thefact it is embedded in a shallow and intense oxygen minimum zone (OMZ).It is also an area of intense nitrogen loss and anammox activity andexperiences large interannual variability associated with the equatorialremote forcing. In this context, we examined the oceanographic andbiogeochemical variability associated with the OMZ off central Peru from amonthly time series (1996–2011) recorded off the coast of Callao (12° 02′ S, 77° 29′ W). The data reveal a rich spectrumof variability in the OMZ that includes frequencies ranging from seasonal tointerannual scales. Due to the efficient oceanic teleconnection off Peru,the observed variability is interpreted in the light of an estimate of theequatorial Kelvin wave contribution to sea level anomalies consideringthe 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 strongregime is associated with the strong 1997–1998 equatorial Pacific El Niño,during which the OMZ adjusted to Kelvin-wave-induced downwelling conditionsthat switched off the upwelling and drastically reduced nutrientavailability. The weak regime corresponds to the post-2000 periodassociated with the occurrence of moderate central Pacific El Niño eventsand enhanced equatorial Kelvin wave activity, in which mean upwellingconditions are maintained. It is shown that the characteristics of thecoupling between physics and biogeochemistry is distinct between the tworegimes with the weak regime being associated with a larger explainedvariance in biogeochemical properties not linearly related to the ENSO oceanicteleconnection. The data also reveal a long-term trend from 1999corresponding to a deepening of the oxygen-deficient waters and warming.The implications of our results for understanding the OMZ dynamics off Peru arediscussed.

2017

10.5194/bg-14-4601-2017

Biogeosciences

Copernicus Publications

Geology, Ecology, Life