Atmospheric rivers over the North Pacific basin

Atmospheric rivers (ARs) develop over synoptic scale time-scales and form spatially narrow plumes of moisture that can stretch over thousands of kilometers in the lower troposphere, extending from the tropics into the extratropics. Meteorological case studies have tied ARs most notably to severe flooding and precipitation events along the western coast of North America. Despite significant advances in the characterization of their meteorological and hydrological impacts on land, the full dynamical variability of ARs over the Pacific basin is poorly known. The intensity of land-falling ARs is variable and the processes leading to their intensification are still being investigated. Here we address the control of extratropical dynamics on AR behavior and intensity over the Pacific basin and the secondary role of the tropics.

We show that ARs at lower levels correspond closely to Rossby wave propagation/breaking in the upper troposphere. While it is possible for ARs to form in association with linear Rossby wave propagation, the majority of the events studied were associated with either anticyclonic or cyclonic wave breaking. Furthermore, ARs associated with a well mixed PV field, resulting from several breaking events, tended to be weaker than those associated with new breaking. Connection to the tropics was not a necessary precondition for AR formation, with several events having low amounts of poleward moisture flux. However, location of poleward moisture transport was important in determining the intensity of ARs, with pineapple express events, a more severe subset of ARs, having a moisture source located further eastward in the tropics. The effect of the Madden-Julian Oscillation on extratropical processes impacting ARs will also be addressed.