Flipping the Western United States El Niño Precipitation Pattern during the 2015/16 Winter

El Niño is a key source of seasonal precipitation prediction.  A “typical” El Niño leads to wet (dry) wintertime anomalies over the southern (northern) half of the Western United States (WUS). However, during the 2015/16 strong El Niño, the WUS winter precipitation pattern was roughly opposite to this canonical (average of the record) anomaly pattern, and most seasonal prediction systems failed to foresee this peculiar anomaly associated with anomalous atmospheric blocking circulation over the California coastal region. Here we explore the extent to which this past winter’s WUS precipitation anomalies may have been predictable. A suite of high-resolution seasonal prediction experiments shows that although the predictability of the 2015/16 WUS precipitation was substantially lower than in 1997/98, the unusual 2015/16 wintertime flipped El Niño pattern was predictable when the entire climate system (ocean, atmosphere and land) is initialized by observations.  When the ocean alone is initialized, the coupled model fails to predict the 2015/16 pattern, but can reproduce the 1997/98 pattern. Further sensitivity experiments suggest that the atmospheric/land initial conditions play complementary non-linear roles in predicting the 2015/16 WUS winter precipitation pattern. This study highlights the importance of atmospheric/land initial conditions and the substantial impact of intrinsic noise in controlling the WUS regional precipitation anomalies even under the broad influence of global-scale El Niño teleconnection, previously thought to be the principal source of the WUS seasonal precipitation prediction.