Climate Variability in the California Current System from a 31-year (1980-2010) Historical Analysis Computed using the ROMS 4D-Var Data Assimilation System

Climate variability in the California Current System (CCS) is investigated using circulation estimates based on a 31-year (1980-2010) historical analysis of the CCS calculated using the Regional Ocean Modeling System (ROMS) 4-dimensional variational (4D-Var) data assimilation system. The leading 3-dimensional multivariate empirical orthogonal functions (3D EOFs) of the CCS circulation were computed and provide a complete view of low-frequency circulation variability within the CCS. The leading 3D EOFs are found to capture much of the variability in the CCS associated with the El Niño Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO) and the North Pacific Gyre Oscillation (NPGO). Variability in the coastal circulation on time scales associated with ENSO and the NPGO was further quantified by computing the co-varying power between time series of climate indices and time series of the circulation using wavelet analysis. Cross-wavelet spectra reveal the nature of the variability in the coastal circulation associated with ENSO and the NPGO and indicate significant latitudinal dependence. The coastal circulation response to ENSO varies considerably from event to event, while for the NPGO the coastal circulation response is most pronounced during the 1990s and is apparently tied to coastal promontories. A similar analysis of circulation estimates computed from a forward run of the model using similar forcing and boundary conditions, but without data assimilation, indicates that 4D-Var has a significant impact on estimates of the CCS circulation and variability.