Assessing ENSO variability over the past millennium: a western tropical Pacific perspective

We present a reconstruction of El Niño Southern Oscillation (ENSO) variability for the last millennium using stable oxygen and carbon isotope data obtained by analyzing samples from a marine sediment core (core MD98-2177) collected in the Indonesian Seas, at the edge the western Pacific warm pool. At this location the δ18O of calcite produced by thermocline-dwelling planktonic foraminifers reflect the temperature and salinity variability that accompanies both eastern as well as central Pacific types of ENSO. Changes in ENSO variability have been inferred from the spread and symmetry of distributions in oxygen isotope (δ18O) values within a sample population of individual Pulleniatina obliquiloculata. The individual isotopic values document monthly subsurface temperatures and salinities that accompanied the seasonal cycle, changes in the range of calcification depths, and interannual variability, including ENSO. Changes in the range of calcification depths were differentiated from changes in ENSO variability by comparing the spread and symmetry of the δ18O distributions with their δ13C counterparts. In contrast to previous studies, we use robust and resistant statistics to quantify the moments of the δ18O and δ13C distributions; an approach motivated by the relatively small sample size and the presence of outliers. We applied this technique to 16 depth horizons in core MD98-2177, corresponding to the 20th century, Medieval Climate Anomaly (MCA), and Little Ice Age (LIA) sections of the core. Our results indicate that ENSO variability during the late 20th century was lower than during any other sampled intervals over the past millennium. There is no systematic difference in variability associated with warmer climatic conditions during the MCA or cooler climatic conditions of the LIA. ENSO variability at the peak of the Northern Hemisphere MCA was similar to that of the early 20th century but intensified between the 13th and 14th century, reaching a maximum at ~1400 A.D. These results agree with a recent reconstruction of El Niño and La Niña frequency from the eastern tropical Pacific. The MCA was also characterized by decades of stronger/more frequent La Niñas, in agreement with the relatively cooler conditions in the eastern and central tropical Pacific during this time period observed in other records. These decades also correspond, within age model uncertainty, to the medieval megadroughts in Western North America reconstructed from a network of drought-sensitive trees.