The climate response of the Indo-Pacific warm pool to glacial sea level

Growing climate proxy evidence suggests that changes in sea level are important drivers of tropical climate change on glacial--interglacial time-scales. These paleodata suggest that rainfall patterns over the Indo-Pacific Warm Pool (IPWP) are highly sensitive to the landmass configuration of the Maritime Continent, and that lowered sea level contributed to large-scale drying during the Last Glacial Maximum (LGM, ca. 21,000 years before present). Using the Community Earth System Model Version 1.2 (CESM1) we investigate in detail the mechanisms by which lowered sea level influenced the climate of the IPWP during the LGM. The CESM1 simulations show that, in agreement with previous hypotheses, changes in atmospheric circulation are initiated by the exposure of the Sunda and Sahul shelves. Ocean dynamical processes amplify the changes in atmospheric circulation by increasing the east-west sea-surface temperature (SST) gradient along the equatorial Indian Ocean. The coupled mechanism driving this response is akin to the Bjerknes feedback, and results in a large-scale climatic reorganization over the Indian Ocean with impacts extending from east Africa to the western tropical Pacific. Exposure of Sahul shelf and the associated changes in surface albedo play a key role because it is more effective at exciting the positive feedback than exposure of the Sunda shelf. This mechanism could explain the pattern of dry (wet) eastern (western) Indian Ocean identified in climate proxies and LGM simulations. However, this response also requires a strengthened SST gradient along the equatorial Indian Ocean, and it is currently unclear whether this is detectable in marine paleoreconstructions. Strategies to resolve this issue are discussed.