North Atlantic Oscillation, Jet and Blocking in CESM1 Large Ensemble Simulations

Atmospheric blocking is a weather phenomenon that occurs when an upper-level anticyclone blocks the mid-latitude westerly flow causing it to divert to the north or south of the anticyclone. The associated anomalous circulation can often lead to severe weather events such as heat waves, cold spells and droughts. It has been shown that the winter North Atlantic Oscillation (NAO) is anticorrelated with the number of blocking days near Greenland and a southerly displacement of the jet. Many climate models have been shown to underestimate the number of blockings near Greenland. In this study, the daily jet latitude index is constructed to quantify the origin of blocking bias in the 30 member ensemble simulations of the Community Earth System Model version 1 (CESM1). A previous study showed that the predecessor model of CESM1 exhibited a Gaussian distribution of the jet latitude at 850 hPa, while the reanalysis showed a trimodal distribution. On the other hand, the 20th century historical simulations of the CESM1 exhibit trimodal distributions of the jet latitude at 850 hPa, consistent with the reanalysis. However, the range of the jet latitude simulated in the CESM1 is still narrower compared to the reanalysis. The relationship between the jet latitude and blocking days, as well as the jet latitude change in the 21st century are also examined.