Western North Pacific Monsoon Variability since the Last Glacial Maximum

Our study analyzes the response of the Western North Pacific Summer Monsoon (WNPSM), a division of the Asian-Australian monsoon system, to internal climate variability, to abrupt climate change, and/or to changes in external forcing. Here, we combine new hydroclimate reconstructions based on the d18O composition of stalagmites that were collected from sites across the Philippines with published proxy data and compare the compilations with climate model output to understand the mechanisms that drive changes in the monsoon system from the Last Glacial Maximum (LGM) to present. Over the last millennium, stalagmite d18O in the WNPSM exhibits a gradual trend towards drier conditions. While nearby proxy data agree on the hydroclimate trend in the region, output from transient climate model runs forced by solar and volcanic changes do not agree with the data - suggesting that the hydroclimate trends might be reflecting internal climate variability. During the abrupt climate change event known as the Younger Dryas (YD) a direct relationship between the East Asian Summer Monsoon, as recorded in Chinese stalagmites, is also observed in the WNPSM: both records get drier during the YD cold interval. However, the tropical hydroclimate response occurs more gradually than the abrupt change in the Greenland ice cores, and climate models suggest that the difference in the timing is most likely due to sea ice feedbacks around Greenland. Over the Holocene, we expected Philippine stalagmite d18O records to have a similar response to changing summer insolation and hence, a trend of decreasing monsoon rainfall over the Holocene. However, the Holocene trend in two partially replicated stalagmite d18O records is opposite to that expected: inferred rainfall increases in the WNPSM over the Holocene, despite the decrease of summer insolation over the Holocene. Climate models suggest that the rainfall anomalies are due to the land-ocean thermal response over the Holocene, which drives the atmospheric reorganization. During the LGM, stalagmite d18O in the Philippines suggests drier conditions in the WNPSM, in agreement with other proxy data as well as climate model output. Climate models attribute the drying in the western tropical Pacific to ice sheet albedo and elevation, as well as exposure of the Sunda and Sahul shelves due lower sea level.