North Atlantic Oscillation controls on oxygen and hydrogen isotope gradients in winter precipitation across Europe; implications for palaeoclimate studies

Winter (October to March) precipitation delta O-18(P) and delta D-P values in central Europe correlate with the winter North Atlantic Oscillation index (wNAOi), but the causal mechanisms remain poorly understood. Here we analyse the relationships between precipitation-weighted delta O-18(P) and delta D-P datasets (delta O-18(pw) and delta D-pw) from European GNIP and ANIP stations and the wNAOi, with a focus on isotope gradients. We demonstrate that longitudinal delta O-18(pw) and delta D-pw gradients across Europe (continental effect) depend on the wNAOi state, with steeper gradients associated with more negative wNAOi states. Changing gradients reflect a combination of air temperature and variable amounts of precipitable water as a function of the wNAOi. The relationships between the wNAOi, delta O-18(pw) and delta D-pw can provide additional information from palaeoclimate archives such as European speleothems that primarily record winter delta O-18(pw). Comparisons between present-day and past European longitudinal delta O-18 gradients inferred from Holocene speleothems suggest that atmospheric pressure configurations akin to negative wNAO modes dominated the early Holocene, whereas patterns resembling positive wNAO modes were more common in the late Holocene, possibly caused by persistent shifts in the relative locations of the Azores High and the Icelandic Low.