Life Cycle of the North Pacific Atmospheric Rivers Modulated by the Madden-Julian Oscillation

Atmospheric rivers (ARs) are intensive poleward moisture transport which can have significant socioeconomic impacts on coastal regions, especially the western North America. However, the life cycle of an AR event which represents the spatiotemporal evolution of moisture transport and its intraseasonal variability associated with the large-scale circulation remain poorly understood. A tracking algorithm is developed to identify the life cycle of an AR event including origin, termination, lifetime, intensity, and propagation track. We utilized the tracking algorithm to understand the modulation of AR life cycles by the Madden-Julian Oscillation (MJO) which is the dominant mode of tropical intraseasonal variability. As the MJO propagates from the Indian Ocean to the western Pacific, the origin frequency and evolution of AR events are modulated by the MJO-associated atmospheric circulation and moisture anomalies, in addition to mid-latitude circulation patterns. In particular, the life cycles of AR events are strongly modulated by the MJO when enhanced convection is over the Indian Ocean (i.e. MJO phase 2-3) and western Pacific (i.e. MJO phase 6-7). By comparing the relative contributions from the MJO-related wind and moisture, We found that the dynamical component associated with the MJO-related wind and mean moisture has the major contribution to the total moisture flux convergence. This MJO-AR relationship is further confirmed by GCM large ensemble simulations. Results of this study suggest that the MJO have persistent influence on ARs through the life cycles from origins to terminations.