Delta Oxygen-18 and SEA SURFACE TEMPERATURE collected from KNORR in Equatorial Pacific Ocean from 0862-01-01 to 2009-01-01 (NCEI Accession 0142201)

Tropical Pacific Ocean dynamics during the Medieval Climate Anomaly (MCA) and Little Ice Age (LIA) are poorly characterized due to lack of evidence from the eastern equatorial Pacific. We reconstructed sea surface temperature, El Niño– Southern Oscillation (ENSO) activity, and the tropical Pacific zonal gradient for the past millennium from Galápagos ocean sediments. We document a “Mid - Millennium Shift” (MMS) in ocean - atmosphere circulation ~1500 -1650 CE, from a state with strong zonal gradient and dampened ENSO to one with weak gradient and amplified ENSO. The MMS coincided with deepest LIA cooling and was likely caused by southward shift of the Intertropical Convergence Zone. Peak MCA (900 - 1150 CE) was a warm period in the eastern Pacific, contradicting the paradigm of a persistent La Niña pattern.