Chapter 10 EVOLUTION OF THE INDO-PACIFIC WARM POOL AND HADLEY-WALKER CIRCULATION SINCE THE LAST DEGLACIATION

The Indo-Pacific warm pool (IPWP), East Pacific cold tongue, and deep overturning atmospheric Hadley (meridional) and Walker (zonal) circulations form a tightly coupled system. In this chapter, we explore the concept of the Hadley circulation as the fundamental driver of changes in this system, and examine its possible impact on global climates of the past. Recent modeling studies indicate that the Hadley circulation is sensitive to Milankovitch forcing, dominated by the precession cycle (22,000 years) in the tropics. It is well established that the increasing Northern Hemisphere summer insolation during the post-glacial transition enhanced northern summer monsoon rainfall, particularly across the Asian landmass. Based on the results of modeling studies, it is probable that the northward asymmetry in tropical heating led to asymmetrical intensification of the Hadley circulation during the early Holocene. The response of the tropical ocean to the intensification of the Hadley circulation is given by foraminiferal Mg/Ca and coral Sr/Ca sea surface temperature (SST) reconstructions, which show that oceanatmosphere feedbacks drove the tropical Pacific into a westwardconcentrated La Niña–like state (warming in the west, cooling in the east) between ~11,000 and ~4,000 years ago. At the same time, air temperatures reconstructed from Southern Hemisphere high-altitude tropical ice cores also equal or exceed late Holocene values. The widespread warming of the tropical middle troposphere during the early Holocene suggests that the additional flux of water vapor and heat from the warmer IPWP during the La Niña state overwhelmed any atmospheric cooling brought about by the expansion of the East Pacific cold tongue. However, the expanded cold tongue area could also play a role in the early Holocene warming through enhanced