Eastern Tropical Pacific Mean State and Variability During the Past 1000 Years

Abstract:

Dynamical changes in the tropical Pacific are hypothesized to have exerted an important influence on climate of the past millennium. However, direct proxy evidence in support of this hypothesis from the Eastern Tropical Pacific remains sparse. Here we present a unique 1000+ year continuous record of oceanic mixed layer temperature and its variability from a sediment multi-core collected in 2009 near the Galápagos Islands. The study location sits in the center of action of the El Niño Southern Oscillation (ENSO), a major driver of tropical and global climate variability. We analyzed Mg/Ca ratios of multiple specimens of the mixed-layer dwelling foraminifera Globigerinoides ruber to reconstruct mean sea surface temperature (SST) over the past 1000 years. We also analyzed δ¹⁸O from individual specimens of G. ruber from the same samples to assess mixed layer temperature variability during the same period. Both the multi-shell Mg/Ca and single-shell δ¹⁸O reveal statistically significant and systematic changes during the past millennium. The Medieval Climate Anomaly (MCA, ~1100-1400 CE) features a sustained period of ~1°C cooler mean surface temperatures and reduced variability (by up to 35%) compared to the late 20th century. Little Ice Age (LIA) mean mixed layer temperatures were comparatively warmer and much more variable than the MCA. Intervals with greater variability than modern, as well as intervals with lower than modern variability are both present in the LIA. The most recent sediment interval corresponding to the period 1985-2009 CE has the highest mean SST of the past 1000 years, although it is within error of temperatures from ~1000 CE. An estimate of the zonal SST gradient of the tropical Pacific based on this Mg/Ca data and similar data from the western Pacific supports a pattern of enhanced zonal gradient during the MCA and reduced gradient during the LIA. We explore the implications of these results for tropical Pacific dynamics in the context of external forcing from volcanic and solar variability.