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Evolution of Interannual and Decadal/Interdecadal variability of the SPCZ since the late 18th century using a network of Fiji coral d18O time-series

Date
le 20-12-2012 à 09:45

Lieu
Salle Stendhal, DGO, Bâtiment B18

Intervenant(s)
Emilie DASSIE, Post doctoral fellow au Lamont-Doherty Earth Observatory, Columbia University, USA

Résumé
The South Pacific Convergence Zone (SPCZ) region is of high interest due to its impact on tropical rainfall and the export of moisture from the tropics. Documenting the amplitude and periodicity of SPCZ displacements on interannual (mainly influenced by El Niño Southern Oscillation (ENSO)) and decadal/interdecadal (D/I) time-scales is of high importance not only for risk management but also for understanding the dynamics of the SPCZ and for improving General Circulation Model climate predictions.
We have generated a unique regional coral d18O network from the Fiji Islands, which are located just south of the main SPCZ rainfall axis. This network consists of five monthly-resolved coral d18O time-series from different regions of Fiji. Evaluation of interseries correlation coefficients indicates that ENSO-band (2 to 9 years) and D/I-band (10 to 50 years) d18O variability is highly reproducible. Correlation to instrumental surface salinity (SSS) data indicates that interannual coral d18O variability in Fiji is the result of interannual changes in SSS that are coupled to ENSO.
The composite reconstruction (arithmetic average of the five Fiji d18O records), extending from 1790 to 2004 AD, was compared to known climatic indices such as the Southern Oscillation and Interdecadal Pacific Oscillation indices over the 20th century. Pearson Product-Moment correlations between the coral d18O composite and these target indices of the Pacific basin are r= 0.71 and 0.41 for ENSO and D/I timescales respectively, with p-values better than 0.001. These high values indicate that our Fiji composite d18O record is suitable for reconstructing past ENSO and D/I variability prior to the instrumental record. Our Fiji ENSO-band and D/I-band reconstructions from the Southwest Pacific also share important similarities with other Pacific coral d18O records such as the d18O record from Malo-Channel (Vanuatu). The Fiji composite d18O reconstruction also correlates negatively with interannual and D/I coral d18O variability from equatorial sites in the Pacific (e.g. Maiana). Collectively, our results from the SPCZ region reflect the validity of the coral d18O composite to track regional climatic variability at both interannual and decadal/interdecadal timescales.
From the late-18th century to the late-19th century the D/I band dominates the Fiji composite while the ENSO-band amplitude is relatively small. Starting around 1885, this tendency reverses and we observed a drastic decrease in the D/I signal amplitude, with its variance reduced by more than 50%, while the ENSO-band signal increases progressively in amplitude toward the present, reaching unprecedented values during the mid-20th century. This switch around 1885 A.D. is unique over the last 250 years and may correspond to a reorganization of Pacific-wide climate.
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