Global Trends in Air Mass Frequencies: Multivariate Indicators of Climate Change

While the trends in numerous individual climate variables have been well-documented, research examining the multivariate manifestations of climate change is comparatively sparse. In this research, interannual variability and long-term trends in multivariate air masses for the entire globe were analyzed, from 1979-2018. These air masses were then also used to develop two global-scale indicators of multivariate climate change: a Warm/Cool Index (WCI) and a Global Extremes Index (GEI). Results show that spatiotemporally Warm, Humid and Humid-Warm air masses are all occurring significantly more frequently, covering anywhere from 5-10 million km² more area of the Earth's surface per decade, with the greatest changes happening in the tropical oceans. Both the WCI and GEI are significantly increasing over the 40-year period of study, indicating that warm-based air masses and extreme air masses are becoming more frequent. These two indicators were also compared against 26 `traditional' indicators of climate change along with the trends in 7 different near-surface climate variables. The WCI and GEI exhibit a moderate (GEI) to strong (WCI) relationship with the global mean temperature record and most temperature-based indicators. In addition, significant changes in cloud cover, dew points, and meridional winds in the tropical Pacific Ocean were well-captured by these two new indices. The GEI also shows a moderate nonlinear relationship with the El Nino Southern Oscillation, and strong linear relationships with traditional precipitation-based climate indicators. Most existing climate indicators are based upon a single variable (often temperature or precipitation) and/or are regionally defined; however, significant changes are occurring in a variety of different atmospheric variables across the globe. Since they are derived from multivariate air masses, the WCI and GEI are unique in that they are able to synergize all of these different manifestations of climate change, while also maintaining a global scope.