Reconstructing the Sea-Surface Temperature at the Equatorial Pacific using tree-ring proxies from the Peruvian central Andes.

Some El Niño events are characterized by very warm conditions in the far-eastern Equatorial Pacific (FEP), but cool conditions elsewhere in the central Equatorial Pacific (CEP). The impact of these so called Coastal El Niño (CEN) events is particularly strong, associated with extreme rainfall events over the coastal northern Peru, Ecuador and adjacent Andean slopes. Despite the fact CEN events represent a high cost for social and economic development of these countries, little is known in terms of frequency, mechanisms and predictability and only very recently science and society are paying attention to these episodes. Here we use the tree-ring width (TRW), as well as stable oxygen (δ¹⁸O) and carbon (δ¹³C) isotopes measured in tree rings to reconstruct Sea-Surface Temperature (SST) in the Equatorial Pacific Ocean described by the two first Empirical Orthogonal Function (EOF). While EOF1 corresponds to the CEP region, EOF2 represents the FEP region.

The newly developed TRW and isotopic records span from 1890 to 2007 and were built from Polylepis rodolfovasquezii trees located at 4,360 m a.s.l in an Andean forest in Peru (11.72°S, 75.14°W). Our results show significant (-) correlation between tree-ring δ¹³C and temperature during the previous growing season peak, while non climatic signal was found in TRW and δ¹⁸O records. During the current growing season tree-ring δ¹⁸O is the proxy that exhibits the highest sensitivity to both, precipitation (-) and temperature (+) compared with the two other tree-ring parameters. In addition, the δ¹⁸O record displays more consistent correlation patterns with both EOFs, suggesting that δ¹⁸O may contain stronger climate signals than TRW and tree-ring δ¹³C. Lastly, the use of a sequential leave-20-out calibration-validation technique for reconstructing the variability of EOFs indicated that the δ18O record was effective to reconstruct EOF1. However, incorporating a multi-proxy strategy, which includes TRW, δ18O, and δ13C, enhanced the overall quality of the reconstruction. In contrast, the multi-proxy approach was not enough to reconstruct EOF2. We conclude that expanding the geographical distribution of proxy records into new in-land areas around the FEP, where SST variability has a local impact on hydroclimate, it is a priority in order to reconstruct CEN. Tree-ring stable isotopic records are valuable to complement existing TRW chronology to overcome the inherent difficulties on using tropical Andean species for paleoclimate research.