Decadal surface ocean variability in the lower Gulf of California: Records for the past 300 years

Abstract

Recent observational and theoretical studies of the Gulf of California have produced some new ideas and models about its circulation patterns and the controls on interannual to decadal timescales, that call on the importance of the tropical and equatorial Pacific as the main region of forcing. The available instrumental record for this region is of short duration, reaching back only into the early 1980s, which coupled with the poor resolution for previous years limits the validation of these models on longer timescales. To overcome the shortness of this record we resort to a reconstruction of SST variability for the last two centuries with the aid of the oxygen stable isotopic composition of calcitic shells preserved in the high-resolution laminated sediments that accumulate on the sea-floor of the La Paz basin, off the Baja California Peninsula eastern margin. From the analysis of the instrumental record we found that winter sea surface temperatures (SSTs) show greater variability from year to year than summer SSTs. Longer records further show a change in cold season temperatures with a trend towards warmer values at the turn of the last century, while the warm season SSTs show no corresponding trend, which underlines the sensitivity of cooler months to interannual SST variability in this region. We further present an extended SST record for the past three centuries based on a reconstruction from the stable oxygen isotopic composition of the planktic foraminifer Globigerina bulloides (d'Orbigny), shown to have its maximum production in the lower Gulf of California during the winter-spring season. This paleothermometer shows mean winter SSTs similar to the instrumentally recorded ones for the last 170 years while for the period between 1700 to 1830 shows cooler winters alternating with relatively warmer ones. The variability in winter SSTs on interannual to decadal timescales seem to be modulated by low-frequency ENSO variability, possibly through higher frequency and intensity of El Niño/La Niña events that temporarily lock surface ocean temperatures into warm/cold decadal periods, thus tracking tropical oceanographic variability on decadal timescales for the past two centuries through ocean driven feedbacks.