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

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Title: La Niña Effects on the MJO Modulation of North Pacific Atmospheric Rivers and Associated Western North American Precipitation.
Authors: Fernandes, L. G.1 (AUTHOR) lais.fernandes@pdx.edu, Loikith, P. C.1 (AUTHOR), Mora, M.2 (AUTHOR)
Source: Journal of Geophysical Research. Atmospheres. 8/28/2025, Vol. 130 Issue 16, p1-22. 22p.
Subject Terms: *Meteorological precipitation, *Rainfall, *Climate change, La Niña, Madden-Julian oscillation, Western countries, Atmospheric rivers
Geographic Terms: Pacific Northwest, Pacific Coast (B.C.), North Pacific Ocean, North America, Pacific Ocean
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. Plain Language Summary: Extreme rainfall is most commonly associated with atmospheric rivers (ARs) between November and March (known as the "cool‐season") over western North America. In this study, we investigate whether the La Niña (LN) phase of the El Niño‐Southern Oscillation phenomenon affects the relationship between the lifecycle of ARs originating in the North Pacific, their respective landfalling precipitation impacts in western North America, and the Madden‐Julian Oscillation (MJO), the leading mode of subseasonal variability in the tropics. ARs are usually less active over the northeastern Pacific and western North America during certain MJO phases (2–5), favoring reduced AR precipitation over those regions. However, in LN years, changes in the atmosphere and MJO characteristics support AR activity increasing from MJO phases 4 through 6 over the northeastern Pacific and western North America. Hence, increased AR precipitation and frequency of AR rainfall extremes occur over the Pacific Northwest and British Columbia during those same MJO phases (4–6) when LN is active. These findings may help refine the forecast of AR activity when both climate modes (LN and MJO) are active. Key Points: Changes in the atmospheric background and the Madden‐Julian Oscillation (MJO) by La Niña (LN) affect MJO‐AR connections across the North PacificIncreased atmospheric river (AR) activity occurs in MJO phases 4‐5‐6 across the northeastern Pacific during LNMaximum AR precipitation and extremes across the Pacific Northwest shift from MJO phases 3–4 to 4–6 in LN years [ABSTRACT FROM AUTHOR]
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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. Plain Language Summary: Extreme rainfall is most commonly associated with atmospheric rivers (ARs) between November and March (known as the "cool‐season") over western North America. In this study, we investigate whether the La Niña (LN) phase of the El Niño‐Southern Oscillation phenomenon affects the relationship between the lifecycle of ARs originating in the North Pacific, their respective landfalling precipitation impacts in western North America, and the Madden‐Julian Oscillation (MJO), the leading mode of subseasonal variability in the tropics. ARs are usually less active over the northeastern Pacific and western North America during certain MJO phases (2–5), favoring reduced AR precipitation over those regions. However, in LN years, changes in the atmosphere and MJO characteristics support AR activity increasing from MJO phases 4 through 6 over the northeastern Pacific and western North America. Hence, increased AR precipitation and frequency of AR rainfall extremes occur over the Pacific Northwest and British Columbia during those same MJO phases (4–6) when LN is active. These findings may help refine the forecast of AR activity when both climate modes (LN and MJO) are active. Key Points: Changes in the atmospheric background and the Madden‐Julian Oscillation (MJO) by La Niña (LN) affect MJO‐AR connections across the North PacificIncreased atmospheric river (AR) activity occurs in MJO phases 4‐5‐6 across the northeastern Pacific during LNMaximum AR precipitation and extremes across the Pacific Northwest shift from MJO phases 3–4 to 4–6 in LN years [ABSTRACT FROM AUTHOR]
ISSN:2169897X
DOI:10.1029/2024JD042355