The Role of Atmospheric Rivers in Extratropical and Polar Hydroclimate

Atmospheric rivers (ARs) are narrow, long, transient, water vapor-rich corridors of the atmosphere that are responsible for over 90% of the poleward water vapor transport in and across midlatitudes. However, the role of ARs in modulating extratropical and polar hydroclimate features (e.g., water vapor content and precipitation) has not been fully studied, even though moistening of the polar atmosphere is both a key result and amplifier of Arctic warming and sea ice melt, and precipitation is key to the surface mass balance of polar sea ice and ice sheets. This study uses the Modern-Era Retrospective analysis for Research and Applications, Version 2 reanalysis to characterize the roles of AR water vapor transport on the column-integrated atmospheric water vapor budget in the extratropical and polar regions of both hemispheres. Meridional water vapor transport by ARs across a given latitude (examined for 40 degrees, 50 degrees, 60 degrees, and 70 degrees) is strongly related to variations in area-averaged (i.e., over the cap poleward of the given latitude) total water vapor storage and precipitation poleward of that latitude. For the climatological annual cycle, both AR transport (i.e., nonlocal sources) and total evaporation (i.e., local sources) are most correlated with total precipitation, although with slightly different phases. However, for monthly anomalies, the water budget at higher latitudes is largely dominated by the relationship between AR transport and precipitation. For pentad and daily anomalies, AR transport is related to both precipitation and water vapor storage variations. These results demonstrate the important role of episodic, extreme water vapor transports by ARs in modulating extratropical and polar hydroclimate. Plain Language Summary The term atmospheric river (AR) was coined by scientists Zhu and Newell in the early 1990s with the main result highlighting the importance of relatively infrequent, long conduits of strong moisture transport being responsible for most of the poleward transport of moisture across the midlatitudes and into the polar regions. While it is generally understood that this moisture is critical to the water and energy budgets of high latitudes, there have been no studies that have ever quantified the relationship between AR poleward moisture transports and the hydroclimate features of high latitudes. After a long hiatus in the consideration of the role of ARs on global climate since those of Zhu and Newell, this study quantifies the connections between water vapor transport by ARs across specific latitudes (e.g., 40 degrees) and the hydroclimate poleward of this latitude. The findings show there are strong, time scale-dependent (e.g., daily and monthly) connections between ARs and high-latitude hydroclimate features. For example, the findings show a strong relationship between AR water vapor transport at a given latitude and the area-averaged total precipitation of the region poleward. This and other results in this study indicate the importance of ARs in shaping our global weather and climate.