The hydrologic cycle in California is strongly influenced by wet season
(November to April) precipitation. Here, we demonstrate the existence of
an influential mode of North Pacific atmospheric pressure variability
that regulates wet season precipitation variability over both northern
and southern California. This mode, named as the “California
precipitation” (CPM), is statistically distinct from other well-known
modes of pressure variability such as the Pacific-North American
pattern. In addition to controlling wet-season mean precipitation,
positive days of the CPM coincide with up to 90% of the extreme
(>99th percentile) precipitation days and 76% of detected
atmospheric rivers (ARs) days, while the negative days correspond with
60% of the dry days. CMIP6 models capture the CPM remarkably well,
including its statistical separation from the other well-known modes of
pressure variability. The models also reproduce the CPM’s strong
association with California wet season precipitation, giving confidence
in the models’ dynamics relating to regional hydrologic extremes.
However, the models also exhibit biases in regional hydrologic extremes.
The CPM is a useful way to understand the origins of those biases and
select the more credible models for further analysis: Models with
unrealistically strong gradients in the CPM pressure pattern generally
over-simulate larger wet extremes and produce excessively long dry
intervals in the historical period. Thus the hydrologic biases can be
traced to the particular aspects of North Pacific atmospheric dynamics.