CMIP6 model simulation of concurrent continental warming holes in Eurasia and North America since 1990 and their relation to the Indo-Pacific SST warming

Abstract

Observations during 1990–2014 depict concurrent cooling trends in land surface air temperature (LSAT) over central-eastern Eurasia (CEE) and northwestern North America (NWNA), while significant warming trends are observed in other areas across the Northern Hemisphere (NH). This lack of surface warming of the continental CEE and NWNA is termed continental warming holes (CWH) here. The ability of the 6th phase of the Coupled Model Intercomparison Project (CMIP6) to reproduce the LSAT trends during 1990–2014 over the continental NH is evaluated, and the evaluation includes all CMIP6 historical simulations from 32 unique models and their multi-model ensemble mean (MMEM). The results show that the performances of the different models vary greatly. GISS-E2–1-G-CC model performs best among the 32 models, considering reproducing the correct sign of the LSAT trends as well as reproducing the trend magnitude. However, the MMEM simulates uniformly significant warming trends in NH and fails to capture the continental warming holes, indicating a role of internal variability in the CWH. The mechanisms of the continental warming holes are further investigated using reanalysis datasets, CMIP6 models and AGCM simulations. GISS-E2–1-G-CC also shows a better simulation ability, including the trends in NH sea level pressure and tropical sea surface temperature than other models. Both reanalysis data and GISS-E2–1-G-CC model show high-pressure trends over mid-high latitudes, which corresponded to enhanced Siberia High and weakened Aleutian Low. These atmospheric circulation changes result in cold advection over the CEE and NWNA regions, offsetting anthropogenic warming and leading to the emergence of the warming holes. Further analysis and AGCM simulations link these atmospheric circulation trends to Indo-Pacific SST warming. In response to the Indo-Pacific SST warming, tropical tropospheric warming leads to the equatorward shift of the mid-latitude jet stream, and subsequently, anomalous easterly and high pressure over mid-high latitudes tend to influence the Siberia High and Aleutian low, resulting in the LSAT cooling over the CEE and NWNA.