High Values of the Arctic Amplification in the Early Decades of the 21st Century: Causes of Discrepancy by CMIP6 Models Between Observation and Simulation

Arctic Amplification (AA) in the first decade of the 21st century has reached values between 4 and 5, with a subsequent decrease to current values of about 3.6, while the value was from 2 to 3 during the twentieth century. The ensemble mean of the CMIP6 models has difficulty in reproducing the recently observed high values of the AA. In this report, we identify the main reason for this difficulty to be the CMIP6 models overestimate of the mean global temperature trend since about 1990. The largest values of the AA are observed in winter and spring. A sharp AA peak in 1987 spring was caused by a peak in the Arctic temperature trend occurring at the same time as a dip in the trend of mean global temperature. The winter AA has increased almost monotonically since 1990. Dividing the AA between the Arctic land and ocean areas shows that the ocean area makes a larger contribution to the AA. Our future projection of the AA suggests an increasing AA for about the next decade, followed by a slow decrease to about 3.5 in the 2050s. The Arctic is warming faster than the average warming of the whole earth. The Arctic Amplification (AA) is defined as the ratio of the Arctic to global mean warming rates. Thus, the AA increases when the rate of Arctic warming increases, when the rate of global warming decreases, or when both happen at the same time. For most of the twentieth century, the AA was between 2 and 3. However, during the first few years of the 21st century, the AA has reached over four. The current climate models are not able to reproduce the observed early 21st-century high values of AA. We find that the main reason for this difficulty is the models' overestimate of the global warming rates after 1990. The early 21st century high values of the AA are caused by a higher temperature trend over the Arctic and a lower global temperature trendThe CMIP6 models' difficulty in reproducing the observed AA is due to the models' overestimate of the rate of mean global warming after 1990The future projection of the AA suggests an increasing AA for about the next decade with a slowly decreasing trend after that