NAO signatures in southwestern Iberian tree rings

ROIG, F.A., BARRIOPEDRO, D., GARCÍA, R., PATÓN, D., MONGE, S.

Carmona, Sevilla

Workshop; First MedCLIVAR Workshop on Reconstruction of Past Mediterranean Climate; 2006

European Science Fundation and others

Exactly-dated ring-width chronologies derived from Pyrenean Oak and SweetChestnut trees growing on northern Extremadura, Spain have been developed and usedto evaluate their potential as proxies of the regional precipitation and the North AtlanticOscillation (NAO) index variability. The relationships between tree-rings, instrumentalclimatic records and three versions of the NAO index were computed for different timesub-periods of the last century. Tree-ring records reflect with variable intensity bothshort-term (high-frequency) and long-term (lower-frequency) variations of climate.Summer precipitation appears to be the major factor controlling tree-growth atinterannual timescale. However, strong signals of winter precipitation and NAOvariability are found in the low-frequency variations of tree-growth. The positiveresponse of tree-growth to summer rainfall is mostly interpreted as a proxy for moisturestress during a growing season characterized by scarce precipitation. But sincefluctuations in accumulated rainfall over SW Iberia are controlled by winterprecipitation, the accumulated rainfall and winter NAO indices are strongly correlatedwith tree-ring records at interdecadal timescales. The relationship between tree ringsand the winter NAO indices appears to be especially prominent during the second halfof the 20th century. The improvement of the NAO-tree ring linkage at the end of therecord is in agreement with a concurrent increase of the winter NAO signal in theannual precipitation and a greater sensibility of the tree-growth to local precipitation.These results have significant implications for future paleoclimatic studies, particularlythose related to the past natural variability of the NAO and the extent of its influence onprecipitation variability over NAO-sensitive regions such as SW Iberia.