Diagnosis of the role of the tropics in driving serial clustering of atmospheric rivers over the U.S. West Coast during January 2023

Between late December 2022 and mid January 2023, a series of extratropical cyclones and associated atmospheric rivers impacted California in rapid succession, producing extreme precipitation accumulations that led to high-impact flooding across the state. This serial clustering of cyclones and ARs occurred in connection with repeated propagation of baroclinic Rossby wave packets toward the U.S. West Coast along a zonally extended North Pacific jet stream. Observational evidence suggests that the wave packets were forced in part by upper-level divergent outflow linked to enhanced tropical convection over the Maritime Continent and western Pacific associated with the Madden-Julian Oscillation (MJO). In this study, a reforecast experiment is conducted with a prototype version of the NOAA Unified Forecast System model to test the hypothesis that the remote MJO-related tropical convection played a principal role in the serial clustering of cyclones/ARs over the U.S. West Coast during this event. Specifically, a nudged forecast, in which a “perfect” representation of the tropical conditions is imposed by continuously nudging forecast state variables in the tropics (10°S–10°N) to reanalysis, is compared with a control forecast, in which the model is run freely without nudging. The forecasts, each composed of 30 ensemble members, were initialized on 15 Dec 2022 and run out to 35 days in order to capture the event of interest during weeks 3–5 of the forecast. Results demonstrate that the nudged forecast, featuring an accurate representation of MJO-related convection and upper-level divergent outflow, captures the serial clustering of cyclones and ARs and the resulting extreme precipitation over the U.S. West Coast. Conversely, the control forecast, exhibiting large errors in the structure, intensity, and location of the MJO-related convection and outflow, fails to capture the serial clustering process and instead produces ridging and relatively dry conditions over the West Coast. A detailed analysis of the forecasts is conducted to diagnose the chain of synoptic-dynamic processes linking improved representation of tropical processes, especially those related to the MJO, to improved representation of the extratropical flow and precipitation over the U.S. West Coast.