Insights into the Causes and Predictability of the 2022/23 California Flooding

In late December of 2022 and the first half of January 2023 much of California experienced an unprecedented series of atmospheric rivers that produced heavy rains and near-record flooding. In this study, we examine the causes and predictability of the extreme rainfall employing the MERRA-2 reanalysis and the NASA GEOS AGCM. The results are based on simulations with the GEOS AGCM run in “replay” mode which allow us to identify the underlying remote forcing of the flooding event, and assess its predictability. The results show that key to the development of the heavy rains was a persistent positive Pacific North American (PNA) circulation pattern (with an associated low that extended onto the west coast) that developed in late December as a result of vorticity forcing in the western North Pacific. This was part of a chain of events initiated in mid-December with the development of a Rossby wave (as a result of forcing linked to the MJO) that propagated from the northern Indian Ocean into the North Pacific. The persistent cyclonic circulation off the west coast was unexpected in that it is more likely to occur in a response to El Niño (rather than La Niña as was the case) SST anomalies. As such, the lack of predictability on seasonal time scales reflects the substantial amount of internal atmospheric variability involved in the initiation and evolution of the event, including interactions of the PNA with a barotropically unstable North Pacific jet. Forecasts with the GEOS coupled model suggests that useful skill in predicting the extreme precipitation over California was in fact limited to lead times shorter than about 3 weeks.