Long-term variability in sea surface temperature and chlorophyll-a concentration in the Pacific region off Baja...

The Pacific region off Baja California (PBC) is part of the California Current System (CCS), its high energy flux and transition zone between mid-latitude and tropical ocean conditions make it one of the most productive regions in the world. Given the variability of sea surface temperature (SST) and chlorophyll-a (Chl-a) in this area, it is necessary to establish regions for their independent study and the relationships between them. We used satellite imagery and cluster analysis to group stations with greatest homogeneity in these variables and regionalize the PBC. We evaluated the seasonal and interannual variability of SST and Chl-a from 2000 to 2022 and analyzed the associations of interannual variability with El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). Results revealed seven bioregions characterized by strong latitudinal gradients in SST and Chl-a, with northern bioregions showing higher productivity than the southern ones. All bioregions showed strong seasonal and interannual variation in SST and Chl-a. Interannual (3-8 years) and decadal (10-12 years) cycles associated with ENSO and PDO were observed. A period (2000-2012) of cold-eutrophic conditions was observed, followed by a second period (2013-2022) of warm-oligotrophic waters. In this second period, "the Blob", a very strong El Niño and a positive PDO phase had combined effects on the study area, significantly increasing SST and decreasing Chl-a, with no subsequent recovery to pre-2013 values after these three major events ceased. Additionally, all bioregions moved northwards, the southern ones covering larger areas, suggesting water tropicalization linked to global warming.