Composite physical–biological El Niño and La Niña conditions in the California Current System in CESM1-POP2-BEC

Abstract

El Niño-Southern Oscillation (ENSO) is recognized as one of the potentially predictable drivers of California Current System (CCS) variability. In this study, we analyze a 67-year coarse-resolution (∼1°) simulation using the ocean model CESM-POP2-BEC forced by NCEP/NCAR reanalysis winds to develop a model composite of the physical–biological response of the CCS during ENSO events. The model results are also compared with available observations. The composite anomalies for sea surface temperature (SST), pycnocline depth, 0m-100m vertically averaged chlorophyll, 0m-100m vertically averaged zooplankton, 25m-100m vertically averaged nitrate, and oxygen at 200m depth exhibit large-scale coherent relationships between physics and the ecosystem, including reduced nutrient and plankton concentrations during El Niño, and increased nutrient and plankton concentrations during La Niña. However, the anomalous model response in temperature, chlorophyll, and zooplankton is generally much weaker than observed and includes a 1–2 month delay compared to observations. We also highlight the asymmetry in the model CCS response, where composite model La Niña events are stronger and more significant than model El Niño events, which is a feature previously identified in observations of CCS SST as well as in tropical Pacific Niño-4 SST where atmospheric teleconnections associated with ENSO are forced. These physical–biological composites provide a view of some of the limitations to the potentially predictable impacts of ENSO teleconnections on the CCS within the modeling framework of CESM-POP2-BEC.