Reduced extremes of sub-daily temperature swings during the boreal summer in the Northern Hemisphere

Abrupt swings in temperature can exert negative impacts, ranging from human health to agricultural production. Here, we focus on a global assessment of the extremes in the temperature swings at sub-daily scales using Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) data. Overall, the regions with extremely large swings in hourly temperature (i.e., 99th percentile) are located in desert or arid regions, and the land masses exhibit larger temperature swings than the oceans. In contrast, the first percentile of the hourly temperature swings exhibits a different spatial pattern, with the lowest values (i.e., largest negative swings) located in the Rocky Mountain, South Australia, South and North Africa and some regions in Northwestern China. We identify a significant downward/upward trend in the 99th/1st percentile of sub-daily (i.e., hourly and 12 hr) temperature changes in the midlatitudes in the Northern Hemisphere, particularly during boreal summer. Overall, the regions with significant trends in the Northern Hemisphere are collocated with the paths of the jet streams and storm tracks. The significant downward/upward trends in the 99th/1st percentile of the sub-daily temperature swings over the Northern Hemisphere can be explained by a weakening in the Northern Hemisphere's summer circulation, as suggested by the downward trend in the eddy kinetic energy. These results indicate that a weak/strong persistence in the circulation may lead to less/more abrupt temperature swings (i.e., increase or decrease) caused by horizontal temperature advection.