An Examination of North Pacific Jet Regimes Conducive to Landfalling Atmospheric Rivers along the West Coast of North America

Every year, the west coast of North America experiences significant economic damage and societal disruption due to the extreme precipitation associated with landfalling atmospheric rivers (ARs). ARs not only may produce significant economic and societal impacts, but also may contribute disproportionately to precipitation anomaly statistics along the west coast of the North America. Recent research suggests that the frequency of landfalling ARs along the west coast of North America may increase, due to increased horizontal moisture transport, in a warmer climate. In view of these considerations, it is important to diagnose the physical processes and the antecedent environments that govern landfalling ARs in order to improve medium-range forecasts of extreme precipitation events along the west coast of the North America.

This study employs an archive of landfalling ARs along the west coast of North America constructed by the Center for Western Weather and Water Extremes in order to examine the large-scale upper-tropospheric flow patterns associated with landfalling ARs during September−May 1979−2017. In this study, the large-scale upper-tropospheric flow patterns prior to landfalling ARs are examined in the context of the two-leading empirical orthogonal functions (EOFs) of 250-hPa zonal wind over the North Pacific Ocean. The first EOF corresponds to a zonal extension or retraction of the climatological exit region of the North Pacific Jet (NPJ), while the second EOF corresponds to a poleward or equatorward shift of the climatological exit region of the NPJ. The projection of 250-hPa zonal wind anomalies at one or multiple times prior to AR landfall onto these two leading EOFs provides an objective characterization of the instantaneous state or evolution of the upper-tropospheric flow pattern over the North Pacific prior to AR landfall, which may be represented on a two-dimensional NPJ Phase Diagram. The analysis leverages the NPJ Phase Diagram to examine the degree to which AR landfall latitude and AR intensity vary as a function of the structure and evolution of the NPJ prior to AR landfall along the west coast of North America.