Regional assessments of the wave energy resource tend to focus on averaged quantities, and so provide potential developers with no sense of temporal variability beyond seasonal means. In particular, such assessments give no indication of interannual variability - something that is important for determining the potential of a region for wave energy convertor (WEC) technology. Here, we apply a third-generation wave model at high resolution to assess the wave resource of the northwest European shelf seas, an area where many wave energy test sites exist, and where many wave energy projects are under development. Our simulations demonstrate that there is much greater uncertainty in the NW European shelf wave resource during October-March, in contrast to the period April-September. In the more energetic regions of the NW European shelf seas, e.g. to the northwest of Scotland, the uncertainty was considerably greater. The winter NW European shelf wave power resource correlated well with the North Atlantic Oscillation (NAO). Therefore, provided trends in the NAO can be identified over the coming decades, it may be possible to estimate how the European wave resource will similarly vary over this time period.