Two ship occupations of a meridional section along about 86°W in the tropical eastern South Pacific, one during El Niño/El Viejo conditions (March 1993) and the other one during La Niña/La Vieja conditions (February 2009), are used to investigate the distribution and changes in water masses and biogeochemical cycling in the upper ocean (top 1200 m). Special attention is placed to the changes in the distribution of those water masses involved in the ventilation and maintenance of the oxygen minimum zone (OMZ). The extended Optimum Multi-Parameter (OMP) analysis is applied to the data in order to estimate the water mass contributions and the amounts of remineralized organic matter, respired oxygen and denitrified nitrate. The El Niño-Southern Oscillation (ENSO) chiefly determines the water properties and water mass distribution in the upper 300 m of the water column, affecting the location of the Shallow Salinity Minimum (SSM) originated by the subsurface intrusion of subantarctic waters and leading to substantial changes in biogeochemical processes. During the weak 1993 El Niño event, the intrusion of relatively oxygenated waters from the west displaced and deepened the OMZ, the northward progression of the SSM was reduced and the denitrification was dampened. At the time of the 2009 La Niña conditions, the reinforced trade winds drove an intense upwelling and the upper part of the OMZ was found at relatively shallower depths, replacing the more oxygenated surface waters, promoting denitrification and reinforcing and extending the SSM further north. The influence of the Pacific decadal oscillation (PDO) may be appreciated in the deep portion of the water column but it is not easily discernible in the upper layers as a result of the superimposed ENSO signature, also changing from a warm phase in 1993 to a cold phase in 2009

AGU Fall Meeting 3–7 December 2012, San Francisco, California

Peer Reviewed