The Changing Frequency of Spatiotemporally-Relative Weather Types across North America

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Most climate change research is focused on the trends in different metrics (means, maximums, minimums, extremes, etc.) of temperature and/or precipitation. Beyond this, however, trends in other variables have been noted, including absolute moisture metrics, relative humidity, sea-level pressure and many other atmospheric variables and phenomena. This multivariate nature of the weather at any given location naturally underlies the concept of an air mass or weather type – a category of weather that defines the holistic atmospheric situation at a particular time. Thus, 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 GWTC weather type (WT) frequency over North America since 1979. Generally, warm WTs (Humid Warm, Warm, Dry Warm) are increasing in frequency at the expense of the cool WTs (Humid Cool, Cool, Dry Cool), and the effects of Arctic Amplification are apparent. Humid Cool weather is significantly increasing in northwestern North America and transitional (frontal) weather types are increasing in the US and decreasing in Canada. The magnitude of changes is greater than we expected (+/- 30 to 40 days in some regions). We postulate that noting the extreme magnitude of the WT frequency changes found herein might be a more efficacious means (than explaining importance of a 2-3°C change) of communicating longer-term climate change trends to policymakers and the general public.