Atmospheric rivers (ARs) are now widely known for their association with high‐impact weather events and long‐term water supply in many regions. Researchers within the scientific community have developed numerous methods to identify and track of ARs—a necessary step for analyses on gridded data sets, and objective attribution of impacts to ARs. These different methods have been developed to answer specific research questions and hence use different criteria (e.g., geometry, threshold values of key variables, and time dependence). Furthermore, these methods are often employed using different reanalysis data sets, time periods, and regions of interest. The goal of the Atmospheric River Tracking Method Intercomparison Project (ARTMIP) is to understand and quantify uncertainties in AR science that arise due to differences in these methods. This paper presents results for key AR‐related metrics based on 20+ different AR identification and tracking methods applied to Modern‐Era Retrospective Analysis for Research and Applications Version 2 reanalysis data from January 1980 through June 2017. We show that AR frequency, duration, and seasonality exhibit a wide range of results, while the meridional distribution of these metrics along selected coastal (but not interior) transects are quite similar across methods. Furthermore, methods are grouped into criteria‐based clusters, within which the range of results is reduced. AR case studies and an evaluation of individual method deviation from an all‐method mean highlight advantages/disadvantages of certain approaches. For example, methods with less (more) restrictive criteria identify more (less) ARs and AR‐related impacts. Finally, this paper concludes with a discussion and recommendations for those conducting AR‐related research to consider.

The Andes is the longest cordillera in the world and extends from northern South America to the southern extreme of the continent (from 11 • N to 53 • S). The Andes runs through seven countries and is characterized by a wide variety of ecosystems strongly related to the contrasting climate over its eastern and western sides, as well as along its latitudinal extension. This region faces very high potential impacts of climate change, which could affect food and water security for about 90 million people. In addition, climate change represents an important threat on biodiversity, particularly in the tropical Andes, which is the most biodiverse region on Earth. From a scientific and societal view, the Andes exhibits specific challenges because of its unique landscape and the fragile equilibrium between the growing population and its environment. In this manuscript, we provide an updated review of the most relevant scientific literature regarding the hydroclimate of the Andes with an integrated view of the entire Andes range. This review paper is presented in two parts. Part I is dedicated to summarize the scientific knowledge about the main climatic features of the Andes, with emphasis on mean large-scale atmospheric circulation, the Andes-Amazon hydroclimate interconnections and the most distinctive diurnal and annual cycles of precipitation. Part II, which is also included in the research topic "Connecting Mountain Hydroclimate Through the American Cordilleras," focuses on the hydroclimate variability of the Andes at the sub-continental scale, including the effects of El Niño-Southern Oscillation.

Winter orographic precipitation over the Andes between 308 and 378S is examined using precipitation gauges in the mountains and adjacent lowlands. Because of the limited number of precipitation gauges, this paper focuses on the large-scale variation in cross-barrier precipitation and does not take into account the fine ridge-valley scale. The maximum amount of precipitation was observed on the windward slope of the mountain range below the crest, which was twice that observed on the low-windward side between 32.58 and 348S. Toward the east of the crest, precipitation amounts drop sharply, generating a strong cross-barrier gradient. The rain shadow effect is greater in the north (328-34.58S) than in the south (358-36.58S) of the low-lee side, which is probably due to more baroclinic activity in southernmost latitudes and a southward decrease in the height of the Andes enabling more spillover precipitation. The effect of the Andes on winter precipitation is so marked that it modifies the precipitation regimes in the adjacent windward and leeward lowlands north of 358S. Based on the fact that ;75% of the wintertime precipitation accumulated in the fourth quartile, through four or five heavy events on average, the synoptic-scale patterns of the heavy (into fourth quartile) orographic precipitation events were identified. Heavy events are strongly related to strong water vapor transport from the Pacific Ocean in the pre-cold-front environment of extratropical cyclones, which would have the form of atmospheric rivers as depicted in the reanalysis and rawinsonde data. The composite fields revealed a marked difference between two subgroups of heavy precipitation events. The extreme (100th-95th percentiles) events are associated with deeper cyclones than those for intense (95th-75th percentiles) events. These deeper cyclones lead to much stronger plumes of water vapor content and cross-barrier moisture flux against the high Andes, resulting in heavier orographic precipitation for extreme events. In addition, regional airflow characteristics suggest that the low-level flow is typically blocked and diverted poleward in the form of an along-barrier jet. On the lee side, downslope flow dominates during heavy events, producing prominent rain shadow effects as denoted by the domain of downslope winds extending to low-leeward side (i.e., zonda wind).

This study quantifies the impact of atmospheric rivers (ARs) on precipitation in southern South America. An AR detection algorithm was developed based on integrated water vapor transport (IVT) from 6-hourly CFSR reanalysis data over a 16-yr period (2001–16). AR landfalls were linked to precipitation using a comprehensive observing network that spanned large variations in terrain along and across the Andes from 27° to 55°S, including some sites with hourly data. Along the Pacific (west) coast, AR landfalls are most frequent between 38° and 50°S, averaging 35–40 days yr−1. This decreases rapidly to the south and north of this maximum, as well as to the east of the Andes. Landfalling ARs are more frequent in winter/spring (summer/fall) to the north (south) of ~43°S. ARs contribute 45%–60% of the annual precipitation in subtropical Chile (37°–32°S) and 40%–55% along the midlatitude west coast (37°–47°S). These values significantly exceed those in western North America, likely due to the Andes being taller. In subtropical and midlatitude regions, roughly half of all events with top-quartile precipitation rates occur under AR conditions. Median daily and hourly precipitation in ARs is 2–3 times that of other storms. The results of this study extend knowledge of the key roles of ARs on precipitation, weather, and climate in the South American region. They enable comparisons with other areas globally, provide context for specific events, and support local nowcasting and forecasting.