We investigate changes in the Madden-Julian Oscillation (MJO) modulation of cool-season North Pacific atmospheric rivers (ARs) and associated AR-landfall North American precipitation 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 4-5 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 3-5. For instance, MJO intensifies in phase 3 under LN, favoring the coupling between anomalous 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 associated with North Pacific ARs. The northern Pacific subtropical jet extends northeastwards in LN, supporting an extratropical 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 years. Furthermore, increased AR precipitation and frequency of AR extremes over the Pacific Northwest and British Columbia intensify and shift from phases 3-4 to 4-5. Therefore, LN has nonlinear effects on the North Pacific MJO-AR connections and their AR landfalling impacts on western North American precipitation.

The impacts of the Madden-Julian Oscillation (MJO) on the South American monsoon season (December-February) and possible changes during El Niño (EN) and La Niña (LN) events are analyzed in the UK Met Office Unified Model Global Ocean Mixed Layer configuration (MetUM-GOML3). Experiments sixty years long, with and without El Niño-Southern Oscillation (ENSO), considering different spatial resolutions, are performed to assess if ENSO influences several MJO characteristics, including the teleconnections to South America (SA). Simulations without ENSO show: (i) an extratropical teleconnection triggered by enhanced convection in the central-east subtropical South Pacific (CSSP) and its strongest impact on central-east South American precipitation in phase 8, earlier than in observations (phase 1). (ii) An extratropical teleconnection, triggered by suppressed convection over the same region, with strongest impact on South American precipitation in phase 4, with opposite sign. (iii) Increased resolution enhances the MJO convection and the South American circulation-precipitation dipole. ENSO affects the basic state and the MJO convection, which modulate teleconnections to SA in simulations with ENSO cycles. EN (LN) strengthens (deteriorates) MJO propagation and its convection. However, both EN and LN produce enhanced convection over the CSSP in phase 8. The extratropical teleconnections and their impacts are stronger under ENSO with respect to those in simulations without ENSO. Hence, both simulated ENSO states generate forcing that more efficiently triggers teleconnections than simulations without ENSO, indicating nonlinear ENSO effects on MJO anomalies over SA.