Multi-Decadal Rainfall Variability under the South Pacific Convergence Zone from 1570-2005

The Pacific Ocean Basin undergoes natural climate variability on decadal to multi-decadal timescales. Ocean-atmosphere feedbacks drive a complex cycle in tropical and extra-tropical winds and zonal sea-surface temperatures (SST) resulting in basin-scale climate reorganizations. The Pacific Ocean zonal SST gradient responds to upwelling of cold waters near the equator transported by shallow Pacific Meridional Overturning Circulation (PMOC), which in turn responds to changes in winds. However, the frequency, timing, and magnitude of past decadal changes in the Pacific are largely unknown. To address this question, we developed a U-Th dated, 430-year stable oxygen isotope (δ18O) cave record of past precipitation in Vanuatu, a location whose climate is heavily influenced by variability in the South Pacific Convergence Zone (SPCZ). The δ18O-based precipitation record shows repeating cycles that are ~25-50 years in duration and correspond to ~115 mm/month of rainfall change that can occur in as fast as 5 years - a change larger and quicker than the 1976 regime shift. The stalagmite δ18O to rainfall amount conversion is based on the "amount effect" observed at similar tropical sites. The "Little Ice Age" (LIA) section of the record exhibits no trend in precipitation, hence no change in the strength or position of the SPCZ over Vanuatu. This result suggests a disconnection between changes in the SPCZ and the Intertropical Convergence Zone (ITCZ), which does exhibit a southerly shift during the LIA period. Changes in PMOC via ocean-atmosphere feedbacks most likely explain the decadal shifts in SPCZ strength through alteration of the tropical Pacific zonal SST gradient and subsequent trade wind response. Periods of high paleo-rainfall in Vanuatu (strong PMOC, increased zonal SST gradient and increased trade winds) correlate with historical accounts of little to no El Niño activity implying that Pacific Basin mean state regulates interannual variability.