Disentangling the roles of internal atmospheric variability and ENSO on
California precipitation to explain recent seasonal forecast errors
Abstract
Water years (WY) 2017 and 2023 were anomalously wet for California, each
helping to terminate multiyear droughts. In both cases, this was
unexpected given La Niña conditions, with most seasonal forecasts
favoring drier-than-normal winters. We analyze over seven decades of
precipitation and snow records along with mid-tropospheric circulation
to identify recurring weather patterns driving California precipitation
and Sierra Nevada snowpack. Tropical forcing by ENSO causes subtle but
important differences in these wet weather patterns, which largely
drives the canonical ENSO-precipitation relationship. However, the
seasonal frequency of these weather patterns is not strongly modulated
by ENSO and remains a primary source of uncertainty for seasonal
forecasting. Seasonal frequency of ENSO-independent weather patterns was
a major cause of anomalous precipitation in WY2017, record-setting snow
in WY2023, and differences in precipitation outcome during recent El
Niño winters 1983,1998 and 2016. Improved understanding of recurrent
atmospheric weather patterns could help to improve seasonal forecasts.