Historic seasonality in Iberian trees: 400 yrs of winter North Atlantic Oscillation and summer hydroclimate

Abstract:

The North Atlantic Oscillation (NAO) is Europe’s dominant mode of climate variability. Its negative phase represents a change in the westerlies that brings wet conditions to the Iberian Peninsula (IP). The NAO exhibits decadal and centennial non-stationary behaviour with high temporal variability challenging the reliability of its reconstruction and predictability. Here, we use ring-width and stable isotopes in cellulose of trees from the northwest Iberian Peninsula to link NAO and regional hydroclimate over the last 400 years. Non-linear methods based on extremes allow the capture of distinct seasonal climatic variability recorded by tree-ring parameters and asymmetric signals of the associated atmospheric features. Tree growth is largely pre-conditioned by winter-spring precipitation, rather than water availability during the growing season. Thus, wider rings allow for a skillful wintertime NAO reconstruction. In contrast, extreme values in stable isotopic records are linked to early-summer IP hydroclimatic conditions. Extreme wet/dry summers detected by these trees compare favourably to pluvial/drought episodes in historical records based on rogation ceremonies from Catholic liturgy in northeastern Spain. Independent sources of past climate variability validate our findings that attribute the non-linear moisture signals recorded by extreme tree-ring values to distinct large-scale atmospheric patterns and allow for targeted seasonal 400-yr reconstructions of wintertime NAO and summer hydroclimate variability.