Session: Glacial Climates (LGM, Last deglaciation, Ice sheet uncertainties, Glacial-interglacial cycles)
Author: Frederik Schenk / email@example.com / Stockholm University
Co-author: Arne Johansson, Royal Institute of Technology, KTH Stockholm;
Barbara Wohlfarth, Stockholm University;
The late deglaciation over the Euro-Atlantic region is characterized by rapid climate shifts between warm and cold states. Proxy- and modelling-evidence suggest a consistent link between cold annual/winter climates with cold ocean states over the North Atlantic. However, it remains unclear whether and, if so, how these cold ocean states should lead to cold European summers in the presence of very high and further increasing summer insolation.
Here we present results from ongoing high resolution (~100 km) time slice simulations for the late deglaciation with the Community Earth System Model (CESM1). We study the link between cold/warm ocean states and European summer temperatures under different solar and greenhouse gas forcing. In these simulations (Bølling, Older Dryas, Allerød, Younger Dryas and Early Holocene), we use a realistic paleo-topography with ice sheets and low sea-level stands. Global SSTs and sea-ice concentrations for the different warm/cold states are prescribed from a previous coarse resolution (~375 km) but fully coupled transient simulation with CCSM3 (TraCE).
We show that atmospheric blocking over Europe is a dominant mechanism leading to warm summers during the late deglaciation in addition to orbital forcing. The cold ocean state of the Younger Dryas (YD, GS-1) leads to enhanced blocking. A cold-ocean-only experiment confirms that the ocean state alone leads already to warmer summers. Increased solar forcing weakens blocking over central Europe but instead leads to stronger warming over continental Eurasia. The warm but very short summers during the YD are confirmed by plant indicator species from European lakes. The strong cooling in other seasons with extreme winters may explain the dominance of cold proxy evidence.
We briefly show that the summer temperature response and atmospheric blocking are model resolution dependent. In addition, we show how the positive feedback of low soil moisture, late summer heating and atmospheric blocking in response to cold SSTs in our simulation are confirmed by recent observations linking unusually cold North Atlantic SSTs with European heat waves.