A coral δ 18 O record of ENSO driven sea surface salinity variability in Fiji (south -western tropical Pacific)

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GEOPHYSICAL RESEARCH LETTERS, VOL. 27, NO. 23, PAGES 3897-3900, DECEMBER 1, 2000

A coral •180 recordof ENSO driven seasurfacesalinity variability in Fiji (south - western tropical Pacific) Nolwenn Le Bec, Anne Juillet-Leclerc Laboratoire des Sciences du Climat et de l'Environnement CEA-CNRS, Gif-sur-Yvette, France

Thierry Corr•ge Institutde Recherchepour le D6veloppement,Noum6a,New Caledonia

DominiqueBlamart Laboratoire des Sciences du Climat et de l'Environnement CEA-CNRS, Gif-sur-Yvette, France

Thierry Delcroix Institutde Recherchepour le Ddveloppement,Noumda,New Caledonia

Abstract. The role of salinity in the dynamics and thermodynamicsof E1 Nifio - SouthernOscillation (ENSO) events is increasingly being investigated. However, instrumentalrecordsof salinity are scarceand short in the tropical Pacific, and there is a clear need for a reliable salinity proxy to extendour knowledgeof ENSO through

increasinglyusedas naturalarchivesto tentativelyprovide multicenturyclimatereconstruction. The oxygen isotopic composition ($'80) of coral aragoniteskeletonsis a functionof both SST and$•80.....ter [Epsteinet al., 1953, McConnaughey,1989]. Coral •80 has been widely usedas a paleothermometer assuminga

time. Here, we present40 yearsof $'80 datafrom a Fiji fairly constant•80 ........ t•r value [Dunbar et al., 1994, coral(16ø48'S- 177ø27'E).The coral$'80 signalintegrates Wellington et al., 1996, Charles et al., 1997, Boiseau et bothseasurfacetemperature(SST) and seasurfacesalinity al., 1998, Cole et al., 2000]. Conversely,in regionswhere (SSS) variations.On a seasonaltimescale,$'80 is mainly temperature variationsare small,the $•80of coralhas been drivenby SST changeswhereason an interannualENSO usedto reconstruct changesin •5•80 ........ •r (andby extension, timescale, it is almost exclusively affected by SSS rainfall variability) [Cole and Fairbanks, 1990, Linsley et variability. Since interannualfluctuationsof SSS are rather al., 1994, Tudhopeet al., 1995]. In siteswhere the climatic

well correlated to the SouthernOscillationIndex in Fiji, andenvironmental settingis more complex,coral$•80 can coral$t80 can be usedto reconstruct paleo-salinitydata reflect a compositesignal [Quinn et al., 1996, Klein et al., with somelevel of confidence. This may help for tracking 1997]. ENSO influences back in time. In this paperwe presentthe calibrationof a Fiji (south westerntropicalPacific ocean, 16ø48'S - 177ø27'E) coral $•80 time seriesagainstthe instrumentalclimatic record, andvalidatetherobustness of $•80asa proxyfor salinity.

Introduction Much useful work to understand and model ENSO

has

been doneneglectingsalinity variations [see the special

Climatic and Oceanic Setting

The westernPacific warm pool is characterized by the TOGA DecadeJ. Geophys. Res. Oceans volume in June warmestSST (_>28øC)in the open ocean. Deep active 1998]. However, it is now well recognizedthat near-surface atmospheric convectionis locatedover the warm pool, and salinitychangesmay play a major role in the mixed layer over the IntertropicalandSouthPacific ConvergenceZones dynamicsandthermodynamics of the westernPacific warm (ITCZ and SPCZ; see Figure 1). In these zones, pool [Lukas and Lindstrom, 1991, Vialard and Delecluse, precipitation exceedsevaporation,leadingto the occurrence 1998], a region of enhanced ENSO-related air-sea of a "freshpool" (SSS_
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