A strong Pacific Walker Circulation during the Little Ice Age.

Reconstructions of climate over the past millennium provide context for disentangling anthropogenic forcing from natural multidecadal variability. However, the strength of the east-west tropical Pacific atmospheric circulation (the Pacific Walker Circulation) during the Little Ice Age (LIA; approximately 1450 to 1850 CE), our most recent cool period, remains a subject of debate. Hydroclimate reconstructions suggest strengthening during the LIA, whereas a decrease in tropical Pacific zonal sea-surface temperature gradient suggests weakening. We present a third line of evidence, the surface-to-thermocline temperature difference in the Indonesian Seas, which was smaller during the LIA than during the Medieval Climate Anomaly (approximately 950 to 1250 CE), suggesting a deeper thermocline, in agreement with hydroclimate evidence for a stronger LIA Walker Circulation. Our finding that the Walker Circulation strengthened in association with LIA cooling is consistent with model projections of a weaker Walker Circulation in a warming world and with the expectation that the recent strengthening trend will ultimately reverse.