A Principal Oscillation Pattern Analysis of the Circulation Variability in the California Current System associated With the El Nino Southern Oscillation

A linear inverse model of the circulation of the California Current System was constructed from a 31-year sequence of circulation estimates based on a data assimilative configuration of the Regional Ocean Modeling System. Principal oscillation pattern (POP) analysis of the linear inverse model has identified several distinct dynamical modes, one with an oscillation period of 3.6 years that describes much of the observed circulation variability of the California Current System associated with the El Nino Southern Oscillation (ENSO). This POP captures not only the circulation changes due to large amplitude ENSO events but also that associated with many of the weak-to-moderate events as well. The POP describes both nearshore and offshore variability and captures many of the previously identified phase relations between different circulation variables, unifying them as part of a coherent three-dimensional time-evolving mode of the circulation. The connection between the POP and the ocean surface forcing was also explored revealing their spatiotemporal connections to the ENSO circulation variability. Analysis of the dynamical balances of POP circulation anomalies reveals fundamentally different dynamical regimes in nearshore and offshore regions and sheds light on the possible role of wind-induced baroclinic instability in modulating eddy kinetic energy during large ENSO events.