Contrasting factors on the trends in hot days and warm nights over Northern Hemisphere land during summer

Abstract

The authors examined the contrasting factors behind the distinct increases in hot days (HDs, daytime hot extremes) and warm nights (WNs, nighttime hot extremes) over land in the Northern Hemisphere during the boreal summers (June to August) of 1980-2018. While the occurrence of HDs has increased gradually since 1980, that of WNs increased abruptly during the late 1990s and has changed little since then. This sudden increase observed in the occurrence of WNs was found to be related to the low frequency variability in sea surface temperature, including the Pacific Decadal Oscillation/Interdecadal Pacific Oscillation. Comparisons of observational data with historical simulations by CMIP5 climate models under different forcings suggest that the increasing trend in observed HDs was largely due to anthropogenic forcing, but that the observed regime shift-like increase in WNs could not be reproduced by external forcing alone. Unlike the observations, the CMIP5 models exhibited accelerated increasing trends in both HDs and WNs, indicating distinct factors controlling the observed trends in HDs and WNs.