Assessing El Nino Southern Oscillation variability during the past millennium

We present a reconstruction of El Nino Southern Oscillation (ENSO) variability spanning the Medieval Climate Anomaly (MCA, A. D. 800-1300) and the Little Ice Age (LIA, A. D. 1500-1850). Changes in ENSO are estimated by comparing the spread and symmetry of delta O-18 values of individual specimens of the thermocline-dwelling planktonic foraminifer Pulleniatina obliquiloculata extracted from discrete time horizons of a sediment core collected in the Sulawesi Sea, at the edge of the western tropical Pacific warm pool. The spread of individual delta O-18 values is interpreted to be a measure of the strength of both phases of ENSO while the symmetry of the delta O-18 distributions is used to evaluate the relative strength/ frequency of El Ni o and La Ni a events. In contrast to previous studies, we use robust and resistant statistics to quantify the spread and symmetry of the delta O-18 distributions; an approach motivated by the relatively small sample size and the presence of outliers. Furthermore, we use a pseudo-proxy approach to investigate the effects of the different paleo-environmental factors on the statistics of the delta O-18 distributions, which could bias the paleo-ENSO reconstruction. We find no systematic difference in the magnitude/strength of ENSO during the Northern Hemisphere MCA or LIA. However, our results suggest that ENSO during the MCA was skewed toward stronger/more frequent La Ni a than El Ni o, an observation consistent with the medieval megadroughts documented from sites in western North America.