Role of paleogeography on large-scale circulation during the early Eocene

Abstract. The configuration of continents and oceans has a major influence on Earth's climate by shaping large-scale atmospheric circulation patterns. In this study, we investigate the effect of early Eocene paleogeography, specifically from the Ypresian stage, on extratropical eddies. In our analysis we highlight the influence of the epicontinental West Siberian Sea as well as the impact of the lack of the Antarctic Circumpolar Current on mid-latitude cyclones and blocking events. Previous work from the Deep-Time Model Intercomparison Project (DeepMIP) has shown changes in atmospheric eddy heat transport under early Eocene boundary conditions. This motivates our analyse of mid-latitude cyclones and blocking events under early Eocene boundary conditions. For the analysis we use six-hourly output from an atmosphere-only CESM1.2 simulation, which is an extension of the corresponding DeepMIP 1 × CO2 experiment. In our simulation, cyclonic activity strengthens at the northern mid-latitudes and weakens at the southern mid-latitudes under early Eocene boundary conditions compared to pre-industrial. Moreover, the blocking signal, which is dominated by the northern hemisphere under pre-industrial conditions, becomes less pronounced. Our results show that, through air–sea interactions the paleogeographic features of the early Eocene produce a more balanced heat transport between the hemispheres and atmospheric processes compared to pre-industrial conditions.