La Niña Effects on the MJO Modulation of North Pacific Atmospheric Rivers and Associated Western North American Precipitation

Abstract We investigate connections between the Madden‐Julian Oscillation (MJO) and North Pacific atmospheric rivers (ARs) and associated precipitation over North America during La Niña (LN) years. Overall, AR frequency is reduced over the northeastern Pacific and western North America in phases 2–5 of the MJO. However, under LN, increased AR activity appears over those regions in phases 4–6 and across the central North Pacific in Phase 3. Distinct effects of LN on the basic state and MJO contribute to the persistence of AR anomalous patterns in phases 3–6. For instance, MJO intensifies in Phase 3 under LN, favoring the coupling between northeastward integrated water vapor transport (IVT) and the MJO convection over the eastern Indian Ocean. Also, suppressed MJO convection in Phase 4 strengthens and shifts eastwards in LN, affecting the MJO teleconnection linked to North Pacific ARs. The northern Pacific subtropical jet extends northeastward in LN, supporting cyclonic flow over the Gulf of Alaska. The MJO propagates eastward more slowly over the Maritime Continent in LN, favoring a second coupling between eastward IVT and the MJO convection in Phase 5. Decreased AR precipitation and frequency of AR rainfall extremes in phases 2–5 over western North America fade in LN. Furthermore, increased AR precipitation and frequency of AR extremes over the Pacific Northwest and British Columbia intensify and shift from phases 3–4 to 4–6. Therefore, LN has nonlinear effects on the North Pacific MJO‐AR connections and their AR landfalling impacts on western North American precipitation.