Extreme Rainfall and Flooding in Southern California: A Perfect Multiscale Interaction between the MJO, the North Pacific Jet and Atmospheric Rivers During the 2023-2024 El Niño

High-impact extreme events often result from multiscale interactions between processes at the interface of the global weather-climate system. In early February 2024, a powerful Atmospheric River (AR) hit Southern California, resulting in widespread extreme rainfall and severe hydro-geologic and socio-economic impacts. The AR was associated with explosive, low-latitude extra-tropical cyclogenesis and was preceded by an unusually large and long-lived Madden-Julian Oscillation (MJO). While a connection between MJOs and west- coast ARs has long been established, not all MJOs similarly affect AR activity. Such pronounced variability in intensity, duration and regional impacts is tied to the interaction between atmospheric and oceanic processes across multiple scales, thus posing a critical challenge for forecasters and risk managers. This study aims to identify the multiscale pathway that supported the extreme rain event and examine its impacts. By combining remote-sensing precipitation estimates with 2020 census data, we first quantify the population exposure to extreme rainfall and flooding in the coastal region. We then leverage a novel MJO tracking algorithm to examine the characteristics of the MJO and how it interacted with the large-scale environment across the Pacific basin. El Niño favored an unusually persistent eastward propagation of the MJO, which reached the central Pacific while maintaining an exceedingly large convective envelope. The almost unprecedented MJO characteristics allowed for a prolonged interaction with the North Pacific Jet (NPJ). The MJO outflow modulated the NPJ by rearranging the upper-level Potential Vorticity (PV) distribution through negative PV advection for approximately three weeks. The consequent jet extension and acceleration supported explosive downstream cyclogenesis, whose AR-driven moisture transport fueled the extreme rainfall over Southern California.