Simulation and projection of blocking highs in key regions of Eurasia by CMIP5 models

Abstract

It has been argued that the Coupled Model Intercomparison Project phase 5 (CMIP5) models underestimate the frequency of atmospheric blocking, while projecting a decreasing trend of blocking in the 21st century in the Northern Hemisphere. This average trend may not be true for regional blockings. Focusing on three key regions in Eurasia (the Urals, Baikal, and Okhotsk regions) where blocking significantly influences the weather and climate of East Asia, this study first evaluates the performance of the CMIP5 models by comparing historical simulations with National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis (NNR). Possible changes in the first half of the 21st century are then analyzed using the RCP4.5 and RCP8.5 experiments. It is found that instantaneous blocking frequencies are underestimated in the Urals and Baikal regions for the whole year and in the Okhotsk region in summertime but are overestimated in Okhotsk in wintertime. Blocking episode frequency in the Urals and Baikal regions is underestimated by most of the 13 CMIP5 models, especially the short-duration blocking episodes (4-5 days), and the simulations are better in wintertime than in summertime. However, in the Okhotsk region, the modeled frequency of blocking episodes projecis close to the value from NNR in summertime but is overestimated in wintertime. Model projections of instantaneous blocking frequency for the first half of the 21st century (2016-2065) show that both RCP4.5 and RCP8.5 runs yield an increasing frequency except during June-August in Eurasia. The multimodel ensemble-mean frequency of blocking episodes clearly decreases in the whole year in the Urals and Baikal regions (especially blocking episodes with short duration) and increases a little in summertime in the Okhotsk region in the first half of the 21st century. The model ensemble-mean frequency of blocking episodes with long duration (more than 9 days) decreases by ~40 % in the Urals region but increases by no more than 5 % in Okhotsk region.