Modulation of atmospheric rivers near Alaska and the U.S. West Coast by northeast Pacific height anomalies

Abstract

Atmospheric rivers (ARs) can cause wide‐ranging impacts upon landfall at high northern latitudes, but comparatively little is known about the dynamics supporting these ARs in contrast to their midlatitude counterparts. Here ARs near the U.S. West Coast and the Gulf of Alaska during 1979–2015 are compared. ARs are found to occur in both regions with similar frequency, but with different seasonality. Composited atmospheric conditions from the NASA Modern‐Era Retrospective Analysis for Research and Applications data set reveal that a broad height anomaly over the northeast Pacific is influential to AR activity in both regions. When a positive height anomaly exists over the northeast Pacific, AR activity is often deflected poleward toward Alaska, while the U.S. West Coast experiences a decrease in AR activity. The opposing relationship also applies; that is, AR activity is decreased near Alaska and increased along the U.S. West Coast in the presence of a negative height anomaly. Quantitatively, nearly 79% of Gulf of Alaska ARs are associated with a positive northeast Pacific height anomaly and 86% of U.S. West Coast ARs are associated with a negative anomaly. Results suggest that this relationship applies across a range of time scales, to include subseasonal and interannual, not just with respect to individual transient waves. Both ARs and height anomalies are found to be associated with Rossby wave breaking, thereby dynamically linking AR activity with broader North Pacific dynamics.

Skillful empirical subseasonal prediction of landfalling atmospheric river activity using the Madden–Julian oscillation and quasi-biennial oscillation

“…1b, ARs occur at frequencies of~12.4% and 11.5% of all days during DJFM for the British Columbia and California boundaries, respectively. Not shown is the substantial year-to-year variability in AR occurrences 32,34 indicating that landfalling ARs are also influenced by longer time scale modes of variability such as the ENSO cycle.…”

“…In both panels, the composite height and IVT anomaly patterns are conducive for an AR activity tradeoff between the two landfall boundaries. 34 Overall, the example composites shown in Fig. 4 highlight that the anomalous AR response patterns (e.g., Fig.…”

“…32 However, regions along the coast experience a pronounced seasonality in AR frequency of occurrence that generally varies with latitude. 33,34 The landfall boundaries used in this study, as identified in Fig. 1a, are no exception.…”

“…For example, the increase in AR activity near British Columbia~2 weeks following MJO phase 2 contrasts with a simultaneous decrease in activity near California (see also ref. 34 ). In addition to the modulation of AR activity following active MJO periods, Fig.…”

Evaluation of the moisture sources in two extreme landfalling atmospheric river events using an Eulerian WRF tracers tool

“…The relationship between ARs and flood events has been extensively analyzed for the US West Coast region (e.g., Higgins et al, 2000;Ralph et al, 2005Ralph et al, , 2006Ralph et al, , 2004Ralph et al, , 2013Bao et al, 2006;Neiman et al, 2008a, b;Leung and Qian, 2009;Dettinger, 2011;Dettinger et al, 2011;Warner et al, 2012Warner et al, , 2014Rutz et al, 2013;Kim et al, 2013), Europe (Lavers et al, 2011(Lavers et al, , 2012Villarini, 2013, 2014;Ramos et al, 2015Ramos et al, , 2016Eiras-Barca et al, 2016;Brands et al, 2016) and other regions world-wide (e.g., Mahoney et al, 2016;Mundhenk et al, 2016). It is important to better understand the physical mechanisms leading to extreme flooding associated with ARs, considering their impacts on human and natural systems and the mounting evidence that ARs are projected to become more frequent and intense in the future (e.g., Dettinger, 2011;Payne and Magnusdottir, 2015).…”