| Summary: | EUR-OCEANS Hot Topics Conference - A Changing Ocean, 6-8 November 2013, Gran Canaria, Spain Two ship occupations of a section along about 86°W in the tropical eastern South Pacific (ESP), one during El Niño (March 1993) and the other one during La Niña (February 2009), are used to investigate the changes in the distribution of water masses and the biogeochemical cycling in the upper ocean (top 1200 m) with the extended Optimum Multi-Parameter (OMP) analysis. Special attention is placed to changes in those water masses involved in the ventilation and maintenance of the oxygen minimum zone (OMZ). The El Niño-Southern Oscillation (ENSO) directly determines the water properties and water mass distribution in the upper 200 m of the water column. During La Niña, the reinforced trade winds drove an intense upwelling and the upper part of the OMZ is found at shallower depths north of 8ºS, replacing the more oxygenated surface waters and promoting denitrification. The intense nitrate loss signal found between 14-10ºS in 2009 is linked with an increased advection of waters from the Peruvian shelf. The heaving of isopycnals associated to ENSO partially explains the remarkable shoaling and upward expansion of the upper part of the Antarctic Intermediate Water (AAIW) observed in the southern part of the section, with AAIW replacing ESSW between 150 and 400 m. This situation brought a large increase in advected oxygen to this depth range of the OMZ which led to enhanced respiration and nitrate remineralization. Therefore, at an interannual time scale, the upward movement of isopycnals associated to La Niña favours the ventilation of the OMZ by AAIW. The shoaling of AAIW is also partially explained by the fact that AAIW flows along shallower isopycnals in 2009, which is in good agreement with the relatively long-term shoaling and density-reduction trend reported at the core of AAIW south of 15ºS in the ESP (Schmidtko and Johnson, 2012). Such changes in the advection of AAIW may mitigate the long-term trend of expanding OMZs ...
|
|---|