Isotopic evidence for twentieth-century weakening of the Pacific Walker circulation

Abstract

The strength of the Pacific Walker circulation (WC) over the historical period is uncertain and strongly debated, largely due to the limited length of reliable instrumental records. Tropical Pacific corals provide continuous, highly-resolved records that can be used to overcome the limitations of the instrumental records. Here we combine coral δ18O records from the western Pacific Warm Pool (WPWP) and tropical central-eastern Pacific (CEP) to develop a coral WC index back to 1886. Our coral WC index is in good agreement with post-1950 instrumental records, justifying both a direct comparison with earlier instrumental records and an evaluation of twentieth century WC trends. Our results show that pre-1950 instrumental records appear to underestimate WC variability. The coral WC index reveals a significant weakening of the WC over the twentieth century in response to development of El Niño like SST pattern in the tropical Pacific and enhanced hydrological cycle over the CEP under greenhouse warming. Our results provide new evidence for twentieth century WC variability and offer an observational constraint for improving models and their prediction of future WC behavior.