Madden‐Julian Oscillation and Atmospheric Rivers: New Insights on Water Source and Transport for Extreme Rainfall Over the Western U.S.

Abstract Atmospheric rivers (ARs) were first documented by Zhu and Newell for transporting global water vapor. ARs contribute to extreme rainfall, especially over the Western United States. The primary water vapor source of ARs is from the tropical ocean, where convective systems bring the moist flux upward from the surface to the troposphere. Previous studies have investigated ARs in connection to the Madden‐Julian Oscillation (MJO) using the Real‐time Multivariate MJO (RMM) Index, which is based on Empirical Orthogonal Function (EOF) analysis of outgoing long‐wave radiation and upper‐level wind. The question of what is the physical mechanism connecting the MJO and ARs remains unclear. This study aims to provide new insights into the effect of MJO convection as a water vapor source for ARs by investigating the direct connection between the MJO convection and ARs using the Large‐scale Precipitation Tracking (LPT) developed by Kerns and Chen. We track the MJO large‐scale precipitation and ARs in time and space using satellite data and reanalysis data from 2000 to 2024. We find that large‐scale convection of the MJO serves as a major water vapor source for ARs during the boreal winter (December–March) when the MJO LPT systems extended further in the west‐central Pacific. During these months, ARs are twice as likely to occur when the MJO convection is active. ARs are stronger when they are physically connected to MJO convection. These stronger ARs are more likely to lead to increased extreme rainfall and flood risk along the U.S. West Coast.