1 Introduction

Atmospheric rivers (ARs) are elongated strands of horizontal water vapor transport, accounting for over 90% of the poleward water vapor transport across midlatitudes (Zhu & Newell, 1998). These “rivers in the sky” have important implications for extreme precipitation when they make landfall, particularly along the west coasts of many midlatitude continents (e.g., North America, South America, and western Europe) and especially when encountering orographic lifting (e.g., Neiman et al., 2009; Ralph et al., 2004). ARs are important contributors to extreme weather and precipitation events, and while their presence can contribute to beneficial rainfall and snowfall (Dettinger et al., 2011; Guan et al., 2010), which can mitigate droughts (Dettinger, 2013), they can also lead to flooding (e.g., Lavers et al., 2011; Leung & Qian, 2009; Neiman et al., 2011; Ralph et al., 2006, 2013; Ralph & Dettinger, 2011) and extreme winds (Waliser & Guan, 2017). These important impacts have motivated a number of climate change studies on ARs, with studies to date focusing mainly only on the west coasts of North America (Dettinger, 2011; Gao et al., 2015; Hagos et al., 2016; Payne & Magnusdottir, 2015; Pierce et al., 2013; Radić et al., 2015; Shields & Kiehl, 2016a, 2016b; Warner et al., 2015) and Europe (Gao et al., 2016; Lavers et al., 2013; Ramos et al., 2016; Shields & Kiehl, 2016a).

The first climate change study on ARs was conducted by Dettinger (2011) and focused on landfalling ARs in California using seven Coupled Model Intercomparison Project (CMIP) Phase 3 models with the A2 greenhouse‐gas emissions scenario. This study found AR frequency increases of about 30% depending on the model by the end of the 21st century and noted increases in storm temperature, length of AR season, and peak AR intensity values. Note that climate change studies on ARs have generally defined AR frequency as the fraction of days a particular grid point has an AR detected over it. Warner et al. (2015) extended the consideration to a larger area along the west coast of North America using 10 CMIP5 (Taylor et al., 2012) models for a historical period (1970–1999) and projection period (2070–2099) for the Representative Concentration Pathway (RCP) 8.5 warming scenario. This study conducted a multimodel mean (MMM) analysis with results indicating ~230–290% increase in AR days. The analysis also showed that there would be an increase in extreme values of integrated water vapor transport (IVT) magnitude and IWV of ~30%. Precipitation values for the MMM showed ~15–39% increase. Using more CMIP5 models (a total of 24), Gao et al. (2015) indicated an ~50–600% increase in AR days under RCP8.5, depending on the season and landfall location along western North America. Another study by Payne and Magnusdottir (2015) using 28 CMIP5 models found 23–35% increases in projected AR landfall dates in this region under RCP8.5. Hagos et al. (2016) showed increases in projected AR landfall days by 35% under RCP8.5 based on a 29‐member ensemble of the National Center for Atmospheric Research Community Earth System Model. Other studies have also examined projected AR changes in western North America, as summarized in Table 1.

Table 1. Comparison of Mean Changes in AR Frequency (Percent of Time Steps) and IVT (kg · m−1 · s−1) Between the Current Study and Previous Studies for the Western U.S. and Western Europe Publication Historical period Projection period Geographic region AR Freq (± %) AR IVT (± %) Dettinger ( 2011 ) 1961–2000 2046–2065; 2081–2100 CA Coast +30 +10 Pierce et al. ( 2013 ) 1985–1994 2060s CA Coast +25–100 ‐‐ Warner et al. ( 2015 ) 1970–1999 2070–2099 U.S. West Coast +230–290 +30 Payne and Magnusdottir ( 2015 ) 1980–2005 2070–2100 U.S. West Coast +23–35 ‐‐ Gao et al. ( 2015 ) 1975–2004 2070–2099 U.S. West Coast +50–600 ‐‐ Hagos et al. ( 2016 ) 1920–2005 2006–2099 U.S. West Coast +35 ‐‐ Shields and Kiehl ( 2016a ) 1960–2005 2055–2100 U.S. West Coast +8 ‐‐ Espinoza et al. (2018, current study) 1979–2002 2073–2096 U.S. West Coast +45 +30 Lavers et al. ( 2013 ) 1980–2005 2074–2099 W. Europe +50–100 ‐‐ Gao et al. ( 2016 ) 1975–2004 2070–2099 W. Europe +127–275 +20–50 Ramos et al. ( 2016 ) 1980–2005 2074–2099 Europe +100–300 +30 Shields and Kiehl ( 2016a ) 1960–2005 2055–2100 North Atlantic +4 ‐‐ Espinoza et al. (2018, current study) 1979–2002 2073–2096 W. Europe +60 +30

For the European region, Lavers et al. (2013) used five CMIP5 models, comparing historical (1980–2005) and projection (2074–2099) periods for the RCP4.5 and RCP8.5 scenarios. This study also used an IVT‐based threshold approach for detecting ARs. This study found that AR frequency approximately doubled in Britain under RCP8.5 and determined that the change was dominated by the thermodynamic (moistening) response to warming rather than from the influence of wind changes. Gao et al. (2016) conducted a study with a focus on comparing the influences of thermodynamic and dynamic effects on ARs and the quantification of the number of AR days across the European sector. By using 24 CMIP5 models, this study found that AR frequency increased by ~127–275% by the end of the century under RCP8.5. Not only did the study find that the projected increases in AR frequency were influenced by thermodynamic processes but found that variability in wind speed and direction related to shifts in the midlatitude jet stream played a dominant role in the changes of ARs in the European sector. Other studies have also examined projected AR changes in Europe, as summarized in Table 1.

While all the studies discussed and cited above have tended toward the same general conclusions (Table 1), that is, finding an increase in AR frequency and IVT, they have been limited to two regions in the Northern Hemisphere. A uniform global assessment of climate change impacts on ARs has not been performed despite the global presence and impacts of ARs (Guan & Waliser, 2015; Waliser et al., 2012; Waliser & Guan, 2017; Zhu & Newell, 1998). For example, despite the number of studies performed on western North America and western Europe, the differences in data sets and methodologies used make it challenging to use these studies to compare impacts of climate change effects on ARs in these two regions. This study addresses this research gap by analyzing climate change impacts on AR frequencies and IVT using a globally consistent approach on historical climate simulations and future projections of climate change from CMIP5.