Seasonal weather regimes in the North Atlantic region: towards new seasonality?

Abstract. European climate variability is shaped by atmospheric dynamics and local physical processes over the North Atlantic region. As both have strong seasonal features, a better insight of their future seasonality is essential to anticipate changes in weather conditions for human and natural systems. We explore the weather seasonality of the North Atlantic over 1979–2017 and 1979–2100 by using seasonal weather regimes (SWRs) defined by clustering year-round daily fields of geopotential height at 500 hPa (Z500) from the ERA-Interim reanalysis and 12 climate models of the Coupled Model Intercomparison Project fifth phase (CMIP5). The spatial and temporal variability of SWR structures is investigated, as well as associated patterns of surface air temperature. Although the climate models have biases, they reproduce structures and evolutions of SWRs similar to the reanalysis over 1979–2017: decreasing frequency of winter conditions (starting later and ending earlier in the year) and increasing frequency of summer conditions (starting earlier and ending later). These changes are stronger over 1979–2100 than over 1979–2017, associated with a large increase of North Atlantic seasonal mean Z500 and temperature. When using more SWRs (i.e. more freedom in the definition of seasonality), the changes over 1979–2100 correspond to a long-term swap between SWRs, resulting in similar structures (seasonal cycle and weather patterns) with respect to the evolution of the seasonal cycle of Z500 and temperature. To understand whether the evolution of the SWRs is linked to uniform Z500 increase (i.e. uniform warming), or to changes in Z500 spatial patterns (i.e. changes in circulation patterns), we remove the calendar trend in the Z500 regional average to define SWRs based on detrended data (d-SWRs). The temporal properties of d-SWRs appear almost constant, whereas their spatial patterns change. This indicates that the calendar Z500 regional trend drives the evolution of the SWRs and that the changing spatial patterns in d-SWRs account for the heterogeneity of this trend. Our study suggests that historical winter conditions will continue to decrease in the future while historical summer conditions continue to increase. It also suggests that according to an increasing seasonal cycle, the seasonality of weather conditions would not change in a major way.