Atmospheric rivers impacting Northern California and their modulation by a variable climate

Abstract

Understanding the role of climate variability in modulating the behavior of land-falling atmospheric rivers (ARs) is important for seasonal and subseasonal predictability for water resource management and flood control. We examine daily activity of ARs targeting the Northern California coast over six decades using observations of synoptic-scale circulation, high-resolution precipitation, and a long-term AR detection catalog to quantify distinct types of land-falling ARs categorized by their circulation features. We demonstrate how dramatically different atmospheric states evolve into landfalling ARs along distinct pathways that are modulated by interannual (El Niño/Southern Oscillation (ENSO) and the Pacific Decadal Oscillation) and subseasonal (Arctic Oscillation, Pacific North American Pattern, Western Pacific Oscillation, and the Eastern Pacific Oscillation) modes of large-scale climate variability. Different configurations of climate variability modes are shown to favor ARs having different characteristics in terms of synoptic evolution, integrated vapor transport and landfall orientation resulting in different patterns of precipitation over the landscape. In particular, our results show that while ENSO plays an important role in modulating the synoptic evolution of ARs and their orientation at landfall, subseasonal regional climate modes, which also influence landfall orientation as well as the position of the storm track, appear to be more influential than ENSO in modulating precipitation variability in California. This could have implications for seasonal to subseasonal (S2S) forecasting. Finally, we examine AR activity over the most recent and highly anomalous winter 2016-2017 and show how the unprecedented wet conditions in Northern California were at least partly due to the persistence of ARs characterized by a southward storm track and southerly orientation, which represent the type of ARs associated with heavy rainfall in California, and which are associated with the negative phase of subseasonal regional teleconnection patterns.

Simulating and Evaluating Atmospheric River‐Induced Precipitation Extremes Along the U.S. Pacific Coast: Case Studies From 1980–2017

“…We also note that statistical downscaling is an alternative approach for precipitation downscaling (Pierce et al, ). For example, Guirguis et al () recently assessed impacts of wetter/stronger ARs in a warming climate on fine‐resolution precipitation over Western North America using statistical (Localized Constructed Analogs) downscaling of precipitation.…”

North American extreme precipitation events and related large-scale meteorological patterns: a review of statistical methods, dynamics, modeling, and trends

“…ARs are also evident in a self-organizing map (SOM) analysis of moisture transport associated with extreme precipitation in the Intermountain West (Swales et al 2016). Analyses of the synoptic circulations associated with AR landfalls in Northern California further indicate that, although ENSO is not known to impact AR landfall frequencies in this region, it does impact AR orientations at landfall (Guirguis et al 2018b), promoting southerly (westerly) orientations during El Niño (La Niña) winters, which bolsters canonical ENSO precipitation anomalies in West Coast precipitation.…”

Experimental Subseasonal‐to‐Seasonal (S2S) Forecasting of Atmospheric Rivers Over the Western United States

“…3. Apply the Guan and Waliser (2015) AR detection algorithm, with the refinements in Guan et al (2018), to the IVT fields for each hindcast system, and for the contemporaneous reanalysis data set used to verify the hindcasts. AR objects are detected based on the seasonal 85th percentile threshold of IVT magnitude from the ERA-I data set at each grid cell and based on geometric constraints described in Guan and Waliser (2015).…”

“…Atmospheric rivers (ARs) are elongated regions of intense horizontal water vapor transport, which are often associated with a baroclinic midlatitude cyclone (Neiman et al, ; Ralph et al, ; Ralph et al, ; Zhang et al, ; Zhu & Newell, ). Many studies have revealed the important link between ARs and annual precipitation over different regions around the globe (Dettinger et al, ; Lavers et al, ; Ralph & Dettinger ; Neiman et al, ; Lavers & Villarini, ; Lamjiri et al, ; and others), their association with global floods and water availability (Ralph et al, ; Leung & Qian, ; Ralph & Dettinger ; Paltan et al, ; Corringham, ), their relationship to snowpack over the western U.S. (Goldenson et al, ; Guan et al, ; Huning et al, , ), their importance to extratropical climate and hydrology (Gorodetskaya et al, ; Nash et al, ), and their projected changes in a warming climate (e.g., Espinoza et al, ; Guirguis et al, ). In addition, a new AR scale has recently been introduced which groups ARs into categories based on their intensity, duration, and beneficial or hazardous impacts on society (Ralph et al, ).…”