AbstractsEarth & Environmental Science

Water mass variability in the eastern South Pacific and the ventilation of the oxygen minimum zone

by Pedro José Llanillo del Río




Institution: Universitat Politècnica de Catalunya
Department:
Year: 2014
Record ID: 1125742
Full text PDF: http://hdl.handle.net/10803/284199


Abstract

This dissertation aims at extending our knowledge of the ESP OMZ through two main approaches. The first approach is based on tracer analysis and inverse modeling techniques. Such techniques are applied to hydrographic datasets in order to examine the water mass structure, its variability and its role on the ventilation of the ESP OMZ. The second approach brings a more dynamical perspective to the study of the ventilation of the ESP OMZ, with the assessment of the annual-mean advective and turbulent oxygen transports into the ESP OMZ from Argo and dissolved oxygen datasets. We describe the distribution of water masses offshore Chile, Peru and Ecuador and discuss their spreading pathways. A thorough characterization of the water masses has been accomplished, resulting in the first set of ESP water types accounting for inorganic nutrients and dissolved oxygen. The low oxygen waters that compose the ESP OMZ are mainly Equatorial Subsurface Water (ESSW). The ESP OMZ is ventilated from the south by the Antarctic Intermediate Water (AAIW) and by the shallower Subantarctic Water (SAAW). These water masses are transported into this region by the Peru Chile Current (PCC) or by the adjacent flow that forms part of the subtropical gyre. We also examine the changes induced by two opposite phases of ENSO in water mass distribution and biogeochemical activity. During La Niña, vigorous upwelling promotes the rise in depth of the upper part of the ESP OMZ and denitrification strengthens in the subsurface layer. Furthermore, the upward displacement of isopycnals induced by La Niña favors the ventilation of a different depth range of the OMZ by the upper portion of the AAIW. The opposite occurs during El Niño conditions. We find that, with a larger oxygen supply, respiration increases balancing most of the extra oxygen gain. This suggests that there is an excess of organic matter waiting to be remineralized whenever oxygen supply increases slightly and this situation favors the maintenance of the ESP OMZ. Furthermore, our results show that AAIW flowed along shallower isopycnals in 2009 than in 1993. Such shoaling is not caused by the ENSO phenomenon and changes the way AAIW ventilates the ESP OMZ. This finding might explain why an increase in oxygen content has been reported between 200 and 700m off Chile. A global picture of the main processes and predominant paths of oxygen supply into ESP OMZ is provided here for the first time. Two main advective routes are found, the traditional equatorial pathway and a previously unreported subtropical pathway. Remarkably, the subtropical pathway provides more net oxygen gain than the equatorial pathway at the core of the ESP OMZ. This finding challenges the common assumption that the ESP OMZ is only ventilated by the eastward flowing zonal currents of the Equatorial Current System. This result is endorsed by the independent water mass analyses accomplished in the first part of this dissertation where the fingerprint of AAIW and SAAW is clearly found in the ESP OMZ. In addition, an unreported…