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A strong Pacific Walker Circulation during the Little Ice Age
Anya V Hess1, Shawn Wang1,2, Caroline C Ummenhofer1
1Woods Hole Oceanographic Institution, Woods Hole, MA, USA.
Abstract:
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.
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