Warming Pattern over the Northern Hemisphere Midlatitudes in Boreal Summer 1979–2020

Journal Article · 31 May 2022 · Journal of Climate

DOI: https://doi.org/10.1175/JCLI-D-21-0437.1 · OSTI ID:1867324

Teng, Haiyan ^[1]; Leung, Ruby ^[1]; Branstator, Grant ^[2]; Lu, Jian ^[1]; Ding, Qinghua ^[3]

---

1. a Pacific Northwest National Laboratory, Richland, Washington
2. b National Center for Atmospheric Research, Boulder, Colorado
3. c Department of Geography, and Earth Research Institute, University of California, Santa Barbara, Santa Barbara, California

Abstract Significant surface air temperature warming during summer 1979–2020 is not uniformly distributed in the northern midlatitudes over land but rather is confined to several longitudinal sectors including Europe, central Siberia and Mongolia, and both coasts of North America. These hot spots are accompanied by a chain of high pressure ridges from an anomalous, circumglobal Rossby wave train in the upper troposphere. From reanalysis data and several baseline experiments from phase 6 of the Coupled Model Intercomparison Project (CMIP6), we find that the circulation trend pattern is associated with fluctuations of the Atlantic multidecadal variability (AMV) and the interdecadal Pacific oscillation. The phase shift of AMV in the 1990s is particularly noteworthy for accelerating warming averaged over the northern midlatitude land. The amplitude of the observed trend in both surface air temperature and the upper-level geopotential height generally falls beyond the range of multidecadal trends simulated by the CMIP6 preindustrial control runs, supporting the likelihood that anthropogenic forcing played a critical role in the observed trend. On the other hand, the fidelity of the simulated low-frequency modes of variability and their teleconnections, especially on multidecadal time scales, is difficult to assess because of the relatively short observational records. Our mechanistic modeling results indicate that synoptic eddy–mean flow interaction is a key to the formation of the anomalous wave train but how the multidecadal modes can modulate the synoptic eddies through atmosphere–ocean and atmosphere–land interactions remains poorly understood. This gap in our knowledge makes it challenging to quantify the roles of the low-frequency modes and external forcings in causing the observed multidecadal trends.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Biological and Environmental Research (BER)
• Grant/Contract Number: AC05-76RL01830
• OSTI ID: 1867324
• Report Number(s): PNNL-SA-163065
• Journal Information: Journal of Climate, Journal Name: Journal of Climate Journal Issue: 11 Vol. 35; ISSN 0894-8755
• Publisher: American Meteorological SocietyCopyright Statement
• Country of Publication: United States
• Language: English