Contraction of the Western Pacific Tropical Rain Belt and Weakening of the Walker Circulation: Future Lessons From the Past Two Interglacials.

The western Pacific warm pool (WPWP) is the heat engine of the global climate system delivering vast amounts of heat and moisture to the atmosphere. Controls on regional convection, however, are numerous, making it difficult to simulate past and future changes in WPWP hydroclimate with confidence. Here, we synthesize new and previously available precipitation sensitive records from the WPWP spanning the last and present interglacial periods. We find two primary modes of rainfall variability, both driven by precession forcing, that are common to both interglacial periods: (a) a contraction of the tropical rain band across the interglacial and (b) a mid‐interglacial strengthening of the Pacific Walker Circulation (PWC). We further demonstrate that while the amplitude of the change in seasonal insolation across the Holocene is far lower than during the LIG due to the low eccentricity state of Earth's orbit, the response of regional rainfall is comparable during both interglacials, indicating a nonlinear response to the insolation forcing. Finally, we suggest an enhanced sensitivity of the PWC to non‐insolation climate forcing, including greenhouse gases and sea level change, under strongly reduced boreal fall insolation as observed during the late Holocene and late LIG. Plain Language Summary: The western Pacific warm pool (WPWP) contains the warmest surface water in the global oceans and is therefore responsible for transferring large amounts of heat and moisture to the atmosphere and fueling high rates of rainfall. The intensity and distribution of this rainfall is modulated by complex interactions between regional monsoon systems and the cross‐equator and equatorial Pacific zonal temperature gradients, which makes it difficult to assess how regional rainfall will respond to past and future climate change. In this paper, we produce a new record of rainfall from the WPWP spanning the last two warm periods in Earth history. We compare this new record to previously published reconstructions of rainfall from the WPWP. We find that variations in rainfall in the WPWP during warm periods are dominantly controlled by changes in the cross‐equator contrast in incoming solar radiation driven by changes in orbital precession leading to two responses: (a) a north‐south expansion and contraction and (b) an east‐west shift in the region of maximum precipitation. These results provide insight to the sensitivity of rainfall in the western Pacific to climate forcing. Key Points: Precession driven expansion and contraction dominates variability in the Western Pacific tropical rain band in the last two interglacialsModes of variability and rainfall intensity are insensitive to the amplitude of seasonal insolation changeHeightened sensitivity of the Pacific Walker Circulation to greenhouse gases and sea level under low boreal fall insolation [ABSTRACT FROM AUTHOR]