Abstract
1. The North Atlantic Oscillation (NAO) exerts considerable control on U.K. weather. This study investigates the impact of the NAO on butterfly abundance and phenology using 34 years of data from the U.K. Butterfly Monitoring Scheme (UKBMS).2. The study uses a multi-species indicator to show that the NAO does not affect overall U.K. butterfly population size. However, the abundance of bivoltine butterfly species, which have longer flight seasons, were found to be more likely to respond positively to the NAO compared with univoltine species, which show little or a negative response.3. A positive winter NAO index is associated with warmer weather and earlier flight dates for Anthocharis cardamines (Lepidoptera: Pieridae), Melanargia galathea (Lepidoptera: Nymphalidae), Aphantopus hyperantus (Lepidoptera: Nymphalidae), Pyronia tithonus (Lepidoptera: Nymphalidae), Lasiommata megera (Lepidoptera: Nymphalidae) and Polyommatus icarus (Lepidoptera: Lycaenidae). In bivoltine species, the NAO affects the phenology of the first generation, the timing of which indirectly controls the timing of the second generation.4. The NAO influences the timing of U.K. butterfly flight seasons more strongly than it influences population size.