Ocean-color variability in the Gulf of California: scales from days to ENSO

Time series of surface chlorophyll a concentration (Csat) and phytoplankton net primary production (NPP) in the Gulf of California were derived using satellite data from OCTS, SeaWiFS, MODIS, AVHRR and the VGPM primary productivity model. The 6-year (1997–2003) time series showed variability at a multitude of scales. The annual cycle was the dominant mode in Csat variability in the entire gulf, except just south of the midriff islands where the semiannual cycle dominated. The semiannual cycle has Csat maxima during the spring and fall transition periods when the general circulation is switching between cyclonic in the summer and anticyclonic in the winter and is less developed, therefore allowing a more efficient tidal mixing. The spring and fall maxima often consisted of multiple peaks of about 10 days. A significant peak at about 1 month was often present in the short-term Csat variability, especially in areas near the midriff islands, suggesting the influence of tidal mixing. The interannual variability was dominated by the 1997–98 El Niño and the following La Niña. During the El Niño period NPP decreased by 30–40% in the southern part of the gulf (by approximately 1 Tg C month−1), but the changes in the central and northern parts were less evident.