Characteristics and Variability of Winter Northern Pacific Atmospheric River Flavors

Abstract

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 Niño 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.

Evaluating the Representations of Atmospheric Rivers and Their Associated Precipitation in Reanalyses With Satellite Observations

“…It has been well documented that ARs are associated with enhanced precipitation (e.g., Arabzadeh et al., 2020; Gao et al., 2016; Lavers & Villarini, 2013). It is also quite common that reanalysis‐based precipitation is directly used in AR studies (Collow et al., 2020; Gao et al., 2016; S. Kim et al., 2022; Maclennan et al., 2022; Pasquier et al., 2019; Y. Zhou et al., 2022). Yet, reanalysis‐based precipitation is not directly constrained by observations.…”

Multiscale Nature of Atmospheric Rivers

“…This contrasts with Zhou et al. (2022) who used principal component analysis to distinguish between the windy and wet flavors of the wintertime North Pacific ARs where the thresholds between the two flavors are implicitly specified.…”

“…Note that MI is uniquely determined regardless of the analysis period and domain, and thus the thresholds between different flavors can be explicitly specified, facilitating the comparison of AR characteristics across different seasons and regions. This contrasts with Zhou et al (2022) who used principal component analysis to distinguish between the windy and wet flavors of the wintertime North Pacific ARs where the thresholds between the two flavors are implicitly specified.…”

Future Changes in Global Atmospheric Rivers Projected by CMIP6 Models

“…The PCCs between the zonal and meridional components of the 850 hPa wind vector (UV850) difference and IVT difference reach 0.66 and 0.53 separately. We use UV850 as a representative measure of AR winds due to its proximity to the core level where the low‐level jet is typically located (Cordeira et al., 2013; Zhou et al., 2022). Besides, the PCCs between the zonal and meridional components of the UV500 (UV200) differences and the IVT difference are 0.34 (0.10) and 0.29 (0.10), which are lower compared to the results with UV850, underscoring the significance of low‐level winds in influencing the behavior of ARs in the models.…”