Solar and Orbital Forcing of the El Niño Southern Oscillation System in the Holocene

Proxy evidence and model experiments suggest that the El Niño Southern Oscillation (ENSO) system varied on both orbital and millennial timescales during the late Quaternary. A leading explanation for this variability is a mechanism called the ocean dynamical thermostat, whereby increased radiative energy invigorates atmospheric circulation resulting in cooling in the eastern part of the Pacific Ocean similar to that observed during the La Niña (cold) phase of ENSO. Proxy records of Mg/Ca-derived sea surface temperature (SST) and climate models show an increased temperature gradient across the Pacific corresponding to a more La Niña-like state during the early to mid Holocene, driven by enhanced insolation during Boreal summer/fall. Previous measurements on the planktic foraminiferal species Globigerina bulloides from the Soledad Basin off the coast of Baja California show orbital scale cooling during the early to mid Holocene (10-4 ka), consistent with this scenario. Millennial scale cold intervals between 11-7 ka correspond to solar maxima, suggesting that the ocean dynamical thermostat also operates on millennial timescales. However G. bulloides lives during the spring upwelling season, potentially tracking a history of local upwelling rather than ENSO. We explore two other foraminiferal species, Neogloboquadrina incompta and Globigierinoides ruber, to confirm an ENSO-like response of the upper ocean to both orbital and solar forcing. N. incompta lives during the spring upwelling season (like G. bulloides) but at greater depths, and is therefore less sensitive to upwelling. G. ruber lives during the late summer to early fall season when the surface ocean is warmest and most stratified, and experiences minimal upwelling. All three species show an ENSO-like response to millennial scale solar variability during the early to mid Holocene. On orbital timescales we see the competing/amplifying influence of seasonal insolation on dynamic cooling. Our results confirm that the ocean dynamical thermostat operates on millennial timescales, and they are consistent with an orbital scale operation. The Mg/Ca proxy appears to be sensitive to both the season and depth at which the species calcified