Enhanced Multidecadal Variability in the 17th Century From a Comparison of Coral Isotope Records in the Western Tropical Indian Ocean

Decadal climate variability in the tropical Indo-Pacific is becoming clearly recognized from paleoclimatic and instrumental data, but its spatiotemporal signature remains poorly characterized. Here we use new stable isotope records from the slow-growing coral Diploastrea heliopora to describe substantial changes in the dominant frequencies of climate variability in the western tropical Indian Ocean between the 20th and the 17th centuries. Sampling methodology tests reveal isotopic offsets within skeletal elements of this species, which were avoided by sampling exclusively in the columella. The modern record demonstrates coherency with relevant instrumental and proxy time series, supporting its use in reconstructing past climate. However, the 20th century record appears to lose fidelity in the most recent decades, likely a result of multiple environmental stresses at this site. We observe sea surface temperature warming over much of the 20th century and significant power at ENSO periodicities. The 17th-century record, in contrast, lacks any trend, exhibits interannual variance slightly weaker than modern, and contains a pronounced interdecadal signal not evident in the 20th century. The 17th-century multidecadal signal correlates with other large-scale indices of climate variability, including reconstructions of ENSO, Pacific decadal variability and northern hemisphere temperature. We infer a large-scale mode of multidecadal variance that explicitly includes the tropical oceans, perhaps analogous to a stronger version of today's IPO/PDO. The 17th and 20th centuries represent times of markedly different solar irradiance. We find no clear evidence of solar irradiance influence on interannual (ENSO) variability across the tropical Indo-Pacific during these intervals. These results highlight the substantial natural variability in dominant modes of tropical variability and the significant power that resides in multidecadal periods of tropical ocean variability, particularly before the 20th century.