Physical and biogeochemical variability in Todos Santos Bay, northwestern Baja California, derived from a numerical NPZD model

Abstract A physical-biogeochemical Nitrate-Phytoplankton-Zooplankton-Detritus (NPZD) numerical model is used to study the variability of coastal phytoplankton biomass in northwestern Baja California and the Todos Santos Bay (TSB), a region of high socioeconomic importance located in the southern California Current System. The model reproduces adequately the most important oceanographic features of the study area, like the coastal chlorophyll- a (Chl-a) maxima and thermal gradients in the regions of enhanced coastal upwelling. The variability of Chl-a in the TSB is influenced by the activity of El Nino-Southern Oscillation (ENSO) and decadal modes of the Pacific, e.g. , the Pacific Decadal Oscillation (PDO) and the North Pacific Gyre Oscillation (NPGO). From de multi-year model simulation (2004–2011), this large-scale influence is remarkable in two contrasting anomalous years. The year 2006 was anomalously warm and with low Chl-a levels, associated with warm phases of ENSO and PDO and a weakening of the NPGO. These climatic anomalies caused a strong stratification and weak upwelling around the TSB, which induced a poor nutrient input into the Bay and a deep and weak subsurface Chl-a maximum (SCM) during summer. The year 2011, on the other hand, was a cold year with enhanced upwelling during the spring, associated with cold phases of ENSO and PDO and an intensification of the NPGO. These conditions also caused a weak stratification and an intense nutrient transport into the TSB and hence a shallower and stronger SCM.