Abstract
Understanding the variations of the Walker Circulations during the Last Interglacial (LIG, ~ 130–115 ka BP) may advance our knowledge of climate change in an orbitally induced warmer world. Here we investigate the variations and the underpinning mechanisms of the Pacific Walker Circulation (PWC) and the Indian Ocean Walker Circulation (IWC) during the LIG, using the lig127k experiments from the Paleoclimate Modeling Intercomparison Project Phase 4. Compared to the preindustrial, annual PWC is broadened by 3° and strengthened by 3.5% during the LIG, whereas annual IWC is narrowed by 3° and weakened by 17.2%. The antiphase change between annual PWC and IWC during the LIG is dominated by the change in boreal warm seasons (June–October), which is in turn closely tied to the changes in sea surface temperatures between the Indian Ocean and the western Pacific Ocean. Specifically, the models predict a general warming over the tropical Indian Ocean but cooling over the tropical western Pacific in boreal warm seasons during the LIG. This interbasin contrast results in enhanced sea-level pressure over the tropical western Pacific and decreased sea-level pressure over the tropical Indian Ocean, together with opposite changes in precipitation. The west-to-east sea-level pressure gradient contributes to anomalous low-level easterlies in the deep tropics. These changes result in an enhancement and broadening of the PWC, whereas a reverse change for the IWC. The proposed dominating role of interbasin contrast in temperature anomaly is further confirmed by a linear baroclinic model and an atmospheric general circulation model.