PNNL

Abstract

Marked uncertainty in California (CA) precipitation projections challenges their use in adaptation planning in the region already experiencing severe water stress. Under global warming, a westerly jet extension in the North Pacific analogous to the El Niño-like teleconnection has been suggested as a key mechanism for CA winter precipitation changes. However, this teleconnection has not been reconciled with the well-known El Niño-like warming response or the controversial role of internal variability. Analysis of 319 climate simulations from several multi-model and large ensembles shows that internal variability contributes >70% and >50% of uncertainty in the CA precipitation changes and the El Niño-like warming, respectively. The Interdecadal Pacific Oscillation (IPO) plays a key role in each contribution and in connecting the two via the westerly jet extension. This novel, unifying understanding of the role of internal variability in CA precipitation provides critical guidance for reducing and communicating uncertainty to inform adaptation planning.