Modulation of Landfalling Atmospheric River Activity by Northeast Pacific Height Anomalies

Atmospheric rivers (ARs) can cause wide-ranging socio-economic and environmental impacts upon landfall.  This presentation explores the dynamical links between the occurrence of these extreme events impacting the west coast of North America and the large-scale atmospheric circulation over the North Pacific, with a special emphasis on the potential predictability of anomalous AR activity provided by the identified relationships.  Composited atmospheric conditions from the NASA Modern-Era Retrospective Analysis for Research and Applications (MERRA) dataset are shown that reveal broad, equivalent barotropic anomalies over the northeast Pacific that influence AR activity in regions such as near the U.S. West Coast and in the Gulf of Alaska.  These anomalies are presented in terms of geopotential height anomalies, but may also be characterized as pressure and/or circulation anomalies.  It is shown that 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 is also shown, that is, AR activity decreases near Alaska and increases along the U.S. West Coast in the presence of a negative height anomaly.  Quantitatively, we reveal that nearly 79% of so-called Gulf of Alaska ARs are associated with positive northeast Pacific height anomalies and 86% of U.S. West Coast ARs are associated with negative anomalies.  The results presented further indicate that this mechanistic relationship between AR activity and large-scale anomalies over the northeast Pacific is robust and applies across a range of time scales, to include subseasonal and interannual, not just with respect to individual transient waves.  Both landfalling ARs and height anomalies are shown to be associated with Rossby wave breaking, thereby dynamically linking landfalling AR activity with broader North Pacific dynamics.  The link between intraseasonal variability of convection in the tropical Pacific—such as that associated with the Madden-Julian oscillation (MJO)—and Rossby wave breaking over the North Pacific is also summarized.  Finally, we describe how knowledge of the actual and/or predicted state of the MJO may prove skillful in predicting periods of anomalous AR activity—communicated as altered regional strike probabilities—at the subseasonal time scale.