Understanding the Intermodel Diversity Simulating the Year When the Warming Trend Is beyond the Internal Variability in CMIP5 Climate Models

Both external forcing including anthropogenic forcing and the internal variability (IV) in climate system are important factors to correctly project future climate. It has been expected that the time when the intensity of global warming exceeds the range of the climate trend derived by IV (hereafter, this will be referred to as the exceeding year) is around the early 21^st century. Since the exceeding of the IV trend range (IVTR) means that the unexperienced climate could be appeared, therefore, it is necessary to understand these processes. This study analyzes the exceeding point simulated in CMIP5 climate models in the historical run and the RCP8.5 run based on a long-term IVTR of Global Surface Temperature (GST). We found that the exceeding year simulated in CMIP5 climate model is quite diverse. In particular, the climate models which simulate a fast exceeding year tend to simulate strong Arctic Amplification. Those climate models simulate a higher sensitivity between the Arctic sea ice change and the surface heat flux changes along with a small sea ice extent in the Arctic. This result implies that the background state in the Arctic climate plays a key role to determine the exceeding year in CMIP5 climate models.