Characteristics and Variability of Winter Northern Pacific Atmospheric River Flavors

Atmospheric rivers (ARs) are intensive poleward moisture transport events that are essential to the global hydrological cycle and are often linked to extreme weather events. We categorize the winter North Pacific ARs into two flavors: wind-dominated (windy ARs) and moisture-dominated (wet ARs) using 40 years of hourly data from fifth generation of the European Centre for Medium-Range Weather Forecasts Interim Reanalysis. We compare the differences between windy ARs and wet ARs including the lifecycle characteristics (such as genesis locations and changes of meteorological elements through the lifecycle), overall AR frequency, landfall impacts, and variability. The windy ARs are more likely to occur in the midlatitudes, while wet ARs are more active in the subtropics. Windy ARs are associated with intensive surface pressure lows, where the strong pressure gradient can support the strong wind within ARs. Due to larger size and longer lifetime, wet ARs are more likely to produce more precipitation over a lifecycle. By scaling the landfalling ARs, we show that wet ARs dominate the high-category ARs (Category 4 and 5) with higher spatial frequency and more precipitation, and windy ARs have higher contributions in the lower AR categories especially over British Columbia. Windy ARs are modulated by El Nino Southern Oscillation (ENSO) teleconnections via the anomalous geopotential height and extended subtropical jet. Wet ARs are affected by the anomalous sea surface temperature over the midlatitudes related to ENSO. Sensitivity analysis with an alternate AR detection algorithm shows consistent results on AR flavors but with disagreement on the amplitude. Plain Language Summary Atmospheric rivers (ARs) are long and narrow plumes in the atmosphere that transport water vapor from the tropics to high latitudes. ARs generally have strong wind and rich moisture, but the relative strength of wind and moisture can be different for individual ARs. This study divides the ARs into two flavors: windy ARs (stronger wind and less moisture) and wet ARs (weaker wind and more moisture) and compares the differences within. Results show that windy ARs are more likely to occur in the midlatitudes and make landfall in British Columbia. Wet ARs are more active over the subtropics and more prevalent over the U.S. West Coast, especially in California. Windy ARs and wet ARs respond differently to climate variability like El Nino Southern Oscillation. This study could help us to understand the processes related to AR lifecycles and their variability, and help to address how ARs will change in the future warming climate.

Automated summary

Atmospheric rivers (ARs) are important for the global hydrological cycle and often associated with extreme weather events. This study categorizes winter North Pacific ARs into windy and wet ARs and compares their differences in lifecycle characteristics, overall frequency, landfall impacts, and variability.