Estimation of seasonal weather persistence during the Holocene from varved records

Global warming is leading to more persistent boreal summers in the Northern Hemisphere, characterised by a longer season with more extreme heat waves. These seasonal changes have already led to many ecological and socio-economic implications. Statistical significance between summer weather persistence and its potential drivers is weak due to the short instrumental period, in turn preventing skilful predictions. In this study, we develop a new proxy for seasonal weather persistence based on varve thickness measurements to extend observations further back in time. We used two lakes, Nautajärvi in Finland and Diss Mere in England, which provide i) a continuous, seasonally resolved varved records that cover most of the Holocene Epoch; ii) a detailed understanding of the annual depositional varve model supported by lake monitoring, demonstrating that the seasonal sedimentation responds to summer and winter-type weather conditions; and iii) a consistent varve structure over the entire study period, so data are comparable through time. In both lakes, the varve model is made of two seasonal layers, representing the limnological summer and winter. The thickness of the seasonal layers indicate either the length of the season (i.e. longer period of sedimentation) and/or occurrence of extreme events (e.g. intensification of the sedimentological process). This agrees with the definition of seasonal weather persistence, which refers to either stationarity or recurrence of characteristic surface weather or atmospheric circulation of a season. In order to estimate the relative persistence of the summer and winter season over a year, we calculate the percentage of the winter and summer layers contributing to the varve (annual) thickness in the two lakes. Our results show that the early and late Holocene annual mean climate was controlled by winter weather variability, while the mid-Holocene has a predominant summer component. The Holocene winter-summer-winter weather pattern in Europe follows the evolution of the regional latitudinal temperature gradient (LTG) supporting current hypothesis that a weakening of the LTG in response to Global Warming is the origin of more persistent summers today.