A Distinct Atmospheric Mode for California Precipitation
Scientists at the University of California - Los Angeles have demonstrated the existence of an influential mode of North Pacific atmospheric pressure variability that regulates wet season precipitation variability over both northern and southern California (the “California precipitation mode” (CPM)). The team finds that in addition to controlling wet-season mean precipitation, the CPM coincides with most of the wet extreme days and dry days over California. CMIP6 models capture the CPM remarkably well, and they also reproduce the CPM’s strong association with California wet season precipitation, boosting confidence in the models' dynamics relating to regional hydrologic extremes.
The study identifies an influential mode (i.e., the CPM) that regulates wet season precipitation variability over California. The mode captures large-scale dynamics over North Pacific that are important for hydrologic extremes. It is also closely linked to atmospheric rivers (ARs) that make landfall over California. Besides, because CMIP6 models reproduce the CPM’s association with California precipitation, the CPM could be a useful way to understand the origins of model biases and select the more credible models for further analysis. The team recommends that such metric could be integrated into the process of climate model evaluation and selection for downscaling.
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 the “California precipitation mode” (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.