Meridional and zonal changes in water properties along the continental slope off central and northern Chile

The Humboldt-09 cruise covered a narrow meridional band along the Chilean continental slope (44–23o S). Here we use physical and biochemical data from a long meridional section (4000 km) and three short zonal sections (100 km) to describe the distribution of the different water masses found in this region. Six water masses were identified: Subantarctic Water (SAAW), Summer Subantarctic Water (SSAW), Subtropical Water (STW), Equatorial Subsurface Water (ESSW), Antarctic Intermediate Water (AAIW), and Pacific Deep Water (PDW). For the first time, a novel set of source water mass properties (or water types) is introduced for SSAW, and nutrient and dissolved oxygen water types are proposed for all the water masses. Optimum multiparameter (OMP) analysis was used through an iterative process to obtain a sound definition of the water types that minimizes the residuals of the method. Both the classic OMP and the quasi-extended OMP models reproduced the data rather well. Finally, the spatial distribution of the different water masses was calculated with the quasi-extended OMP, which is not influenced by the respiration of organic matter. The distribution of the different water masses is presented over the meridional and zonal transects and in property-property diagrams. A smooth meridional transition from subantarctic to tropical and equatorial water masses is observed in this area. This transition takes place in surface, central, and intermediate waters over distances of the order of 1000 km. The meridional transition contrasts with the abrupt zonal changes found in the cross-slope direction, which are of comparable magnitude but over distances of the order of 100 km. Both AAIW and SAAW (fresh and well oxygenated) partially mix with the hypoxic ESSW and, therefore, play an important role in the ventilation of the southern part of the oxygen minimum zone. La campana Humboldt-09 abarco una estrecha franja latitudinal a lo largo del talud continental chileno, de 44 a 23oS. En este trabajo se utilizan los datos fisicos y bioquimicos obtenidos a lo largo de una seccion latitudinal (4000 km) y de tres secciones zonales cortas (100 km) para describir la distribucion de las diferentes masas de agua presentes en la region. Se identificaron seis masas de agua: Agua Subantartica (SAAW, por sus siglas en ingles), Agua Subantartica de Verano (SSAW), Agua Subtropical (STW), Agua Ecuatorial Subsuperficial (ESSW), Agua Antartica Intermedia (AAIW) y Agua Profunda del Pacifico (PDW). Se propone, por primera vez, un conjunto completo de propiedades de la masa de agua en origen (o valores tipo) para caracterizar SSAW, asi como valores tipo de oxigeno disuelto y nutrientes para todas las masas de agua. Para su correcta definicion se empleo el analisis optimo multiparametrico (OMP) de manera iterativa, seleccionando el conjunto de valores tipo que genero menores residuos. Tanto el modelo OMP clasico como el cuasi-extendido reproducen de manera fiel las observaciones. Finalmente, se determino la distribucion espacial de las diferentes masas de agua con el OMP cuasi-extendido, que no se encuentra influenciado por la respiracion de la materia organica. La distribucion de las distintas masas de agua se representa en las diferentes secciones y en los diagramas propiedad-propiedad. En esta region se aprecia una progresiva transicion latitudinal de masas de agua de origen subantartico a masas de agua de origen tropical y ecuatorial. Esta transicion tiene lugar en los 1000 m superiores, en distancias del orden de 1000 km. Esto contrasta con los rapidos cambios observados en las secciones zonales, de magnitud comparable pero en distancias del orden de 100 km. Tanto el AAIW como el SAAW (relativamente menos salinas y bien oxigenadas) se mezclan parcialmente con la hipoxica ESSW y, en consecuencia, juegan un papel importante en la ventilacion de la parte austral de la zona de minimo oxigeno.

[1]  K. Denman,et al.  Least squares multiple tracer analysis of water mass composition , 1987 .

[2]  F. F. Pérèz,et al.  Mixing analysis of nutrients, oxygen and inorganic carbon in the Canary Islands region , 2001 .

[3]  M. Tomczak,et al.  Optimum multiparameter analysis of mixing in the thermocline of the eastern Indian Ocean , 1989 .

[4]  Wallace S. Broecker,et al.  “NO”, a conservative water-mass tracer , 1974 .

[5]  J. Reid.  Transpacific hydrographic sections at Lats. 43°S and 28°S: the SCORPIO expedition—III. Upper water and a note on southward flow at mid-depth , 1973 .

[6]  L. Talley,et al.  Formation rates of Subantarctic mode water and Antarctic intermediate water within the South Pacific , 2011 .

[7]  O. Pizarro,et al.  Circulation and variability in the Chile Basin , 2004 .

[8]  Nelson Silva,et al.  Water masses in the Humboldt Current System: Properties, distribution, and the nitrate deficit as a chemical water mass tracer for Equatorial Subsurface Water off Chile , 2009 .

[9]  Johannes Karstensen,et al.  Age determination of mixed water masses using CFC and oxygen data , 1998 .

[10]  W. Peterson,et al.  An overview of interactions among oceanography, marine ecosystems, climatic and human disruptions along the eastern margins of the Pacific Ocean , 2004 .

[11]  M. Tomczak,et al.  Optimum multiparameter analysis of the water mass structure in the Atlantic Ocean thermocline , 1999 .

[12]  K. Koltermann,et al.  WOCE Global Hydrographic Climatology , 2004 .

[13]  J. Sprintall,et al.  Expanding Oxygen-Minimum Zones in the Tropical Oceans , 2008, Science.

[14]  L. Codispoti,et al.  Nitrification, denitrification and nitrous oxide cycling in the eastern tropical South Pacific ocean , 1985 .

[15]  F. G. Figueiras,et al.  Theoretical limits of oxygen: carbon and oxygen: nitrogen ratios during photosynthesis and mineralisation of organic matter in the sea , 1998 .

[16]  O. Ulloa,et al.  Hydrography of the eastern South Pacific Ocean: results from the Sonne 102 cruise, May–June 1995 , 2004 .

[17]  S. Zuta,et al.  Oceanografía de las aguas costeras del Perú , 1970 .

[18]  Jorge Tam,et al.  Average circulation, seasonal cycle, and mesoscale dynamics of the Peru Current System: A modeling approach , 2005 .

[19]  M. Tomczak.  A multi-parameter extension of temperature/salinity diagram techniques for the analysis of non-isopycnal mixing , 1981 .

[20]  W. Thomas.  On denitrification in the northeastern tropical Pacific Ocean , 1966 .

[21]  J. S. Godfrey,et al.  Regional Oceanography: An Introduction , 1994 .

[22]  F. F. Pérèz,et al.  Displacement of water masses and remineralization rates off the Iberian Peninsula by nutrient anomalies , 1993 .