The Role of Atmospheric Drivers in Sudden Transitions of California Winter Precipitation

Abstract:

The seasonal repartition of rainfall over California (CA) is critical for the management of the state’s water resources. CA Water year (October-April) total rainfall amounts typically depend on a handful of atmospheric river events during the season, making seasonal forecasting particularly difficult as the frequency and location of these events is difficult to predict months ahead of time. Certain years exhibit a transition from dry conditions early on, to wet conditions for the rest of the water year (and conversely for wet to dry conditions). Predicting such transitions is particularly challenging for seasonal forecasts, and whether such transitions may be predictable months ahead is still unknown.

Here we investigate the transition from wet to dry conditions during the 2012/13 water year. The 2012/13 winter was characterized by a La Niña in the tropical Pacific, strong Madden-Julian Oscillation activity, a sudden stratospheric warming and the easterly phase of the Quasi-Biennial Oscillation. The potential influence of these different drivers is examined in AGCM experiments with the Whole Atmosphere Community Climate Model, by nudging atmospheric variability towards reanalyses in the tropics and/or high-latitudes (outside the area of interest). It is found that the sudden transition in CA rainfall was influenced by both tropical and high-latitude processes. Our model results are compared to forecasts/reforecasts from the S2S and NMME projects, to determine which important processes may have been missed by the seasonal forecast systems.