Abstract
While 20th century changes in many individual meteorological variables are well documented, the trends in multivariate synoptic-scale air masses—or weather types—largely remain unexplored. Utilizing a recently developed gridded weather typing classification system, this research investigates the changes in the frequency of weather types (WTs) across North America, 1979–2017. Averaged across the study domain as a whole, Humid Warm WTs are occurring 22 more days per year, while Dry Warm WTs have increased by 10 days/year. These increases are offset mostly by decreased frequency of Dry Cool (−17 days/year) and Cool WTs (−21 days/year). The largest absolute changes are in the Canadian Archipelago, where the Warm WT is occurring 42 more days/year and the Cool WT is occurring 48 fewer times per year. In western Canada all humid types are occurring more frequently, including a Humid Cool type that is occurring 16 more days/year. The Desert Southwest US and northern Mexico show significant increases in Dry Warm WTs (+33 to +40 days/year). Cold front and warm front passages show increases in most of the US and decreases in most of Canada. Describing these secular changes to the frequency of intuitive weather types may be an effective means of communicating these climate trends to policymakers and the general public, especially considering their large magnitude.
Highlights
During the past few decades the topic of climate change has received ample attention in the scientific, public and political arenas
Since temperature and precipitation trends over the last few decades are well-researched, but a changing climate can manifest itself in myriad ways, the simple aim of this research is to examine the changes in gridded weather type classification (GWTC) weather type (WT; Table 1) frequency over North America since 1979
Of transitional WTs [Cold Front Passage (CFP) and Warm Front Warm WT days increased by 3.4% (+13 days/year) on Passage (WFP)], with both increasing in frequency in much of the US and decreasing over all but the extreme western portions of Canada
Summary
During the past few decades the topic of climate change has received ample attention in the scientific, public and political arenas. The global average temperature has risen 0.85 °C,1 with the strongest impact felt disproportionately in the Arctic region.[2,3] Across the United States, the average temperature has increased about 1 °C since 1895, with the majority coming since 1970, and the rate of the rise has been increasing over that time.4Rising temperatures have ushered in greater evaporation rates, giving rise to an overall increase in landbased precipitation in the mid-latitudes,[1] though with much greater spatial variability than the changes generally noted with temperature. The absolute moisture content of the atmosphere—the link between rising temperatures and increased precipitation—has correspondingly risen[5,6] as relative humidity has stayed steady.[6] sea-level pressure has shown significant changes in low-latitude regions,[7] wind speeds have generally declined,[8] and many other atmospheric variables and phenomena have displayed significant changes in frequency or magnitude[4] in recent decades
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