Effects of the 2013–2016 Northeast Pacific warm anomaly on physical and biogeochemical variables off northwestern Baja California, derived from a numerical NPZD ocean model

Abstract Interannual climatic anomalies have important physical and biological consequences for the southern part of the California Current System (CCS). Two examples of these anomalies are “The Blob” and El Nino phenomena. The Blob was an anomalously warm water mass that formed in the Gulf of Alaska during fall 2013 and then affected most of the eastern North Pacific, including the waters off Baja California, Mexico. In this work we use a biogeochemical nutrient-phytoplankton-zooplankton-detritus (NPZD) model coupled to a three-dimensional hydrodynamic model, which allowed the simulation of some of the physical and biological consequences of the anomalous warm 2013–2016 period (The Blob was observed in 2014 and El Nino by mid-2015). A model-observation comparison showed that in some cases the model solution deviates substantially from the observed vertical thermal structure, but the temporal evolution of the spatially-averaged variables diagnosed by the model shows good agreement with the observations, except for the period from late 2015 to mid-2016, when the sea surface temperature (SST) is underestimated and the model failed to reproduce the correct sign of the surface chlorophyll-a (sCHL) anomalies. An EOF analysis of the 15-year (2003–2017) record of monthly mean modeled variables allowed the identification of two main contributions to the model variability: one mode mostly correlated with the Multivariate El Nino-Southern Oscillation (ENSO) Index (MEI), explaining ∼ 50 % of the variance of temperature (T) and dissolved nitrogen (N) and ∼ 10 % of the variance of chlorophyll-a (CHL), and another mode mostly correlated with the Coastal Upwelling Index (CUI) anomaly, explaining ∼ 50 -70% of variance of CHL and only ∼ 10 -20% of variance of T and N. In the 2013–2016 period, an El Nino signal diagnosed by the MEI induced a remarkable warming off northwestern Baja California. As a consequence, the thermocline and the nutricline deepened during this period and caused a deepening and weakening of the subsurface CHL maximum (SCM). Around the southern region of the study area (near Punta Baja) these effects were more intense at the surface over the shelf, but in the northern region (south of Southern California Bight) the effects were more intense subsurface seaward of the shelf break. This ENSO-modulated subsurface warming tends to weaken the Southern California Eddy during El Nino events, and intensify it during La Nina episodes. During the 2013–2015 period there was also a weakening of the upwelling-favorable winds, which caused a prolonged negative anomaly of the wind forcing, reducing the nutrient flux and causing negative CHL anomalies. Finally, in addition to the partial relation between the ENSO and the warming observed along the Baja California coast, the anomalies were observed in this area 4–6 months before the onset of the El Nino conditions (MEI > 0.5 ) period, when the Blob was affecting the southern part of the CCS.