ENSO's Shrinking Twentieth‐Century Footprint Revealed in a Half‐Millennium Coral Core From the South Pacific Convergence Zone

Abstract

A 492‐year‐long, continuous δ18O time series from a massive Porites coral colony in Ta'u, American Samoa, records contrasting responses to different types of El Niño‐Southern Oscillation (ENSO) through a mixed sea surface temperature and salinity signal. Currently, conventional El Niño (La Niña) events generate cold and salty (warm and fresh) anomalies at Ta'u, while Modoki El Niño (La Niña) events warm (cool) the waters at Ta'u. Over the course of the twentieth century, the Ta'u δ18O record underwent a polarity shift in its response to conventional ENSO: A warm and fresh (cool and salty) response to El Niño (La Niña) was replaced by the opposite pattern. We interpret this as evidence for the movement of the Eastern Pacific ENSO null zone, the narrow band of the surface ocean where sea surface temperature variability is not on average correlated with ENSO. This movement appears to be related to overall shrinking of the ENSO footprint over the twentieth century. We infer no such trend in the Modoki footprint. The five‐century‐long Ta'u record shows dramatic, century‐scale changes in ENSO‐band variability. Comparisons with other ENSO reconstructions lead to conflicting interpretations: The Ta'u coral may have recorded changes in the strength of ENSO or in its spatial footprint. Changes in the spatial footprint manifest as a changing sensitivity to ENSO at any given location, presenting challenges to established methods of ENSO reconstruction.

Coral carbon isotope sensitivity to growth rate and water depth with paleo-sea level implications

“…Approximately 20 mol m −2 of CO 2 has been taken up by the upper ocean at each of these study sites

Fig. 3South Pacific coral δ 13 C and skeletal extension rate: coral annual skeletal extensions rates (in blue) and annual average skeletal δ 13 C (black open dots) and δ 13 C data with the average atmospheric 13 C Suess effect removed (solid black dots) in Porites lutea corals from: a Fiji core AB 53 , b Fiji core 1F 52 , c Tonga core TNI2 54 , d Rarotonga core 2R 51,52 , and e American Samoa core Ta’u-1 55,56 . Extension rate and δ 13 C significantly correlate to varying degrees (see R values in Table 1) up until the mid-20th century when coral δ 13 C abruptly shifts ~1.0‰ lower at all of these sites.

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“…In November 2011, we cored a large colony of P. lutea on the western side of the island of Ta’u located at S 14° 15′ 33.74″: W 169° 30′ 01.61″ (or S 14 15.566, W 169 30.027) on an exposed outer reef in 7.5 m of water (water depth to top of coral) 55,56 . Prior to slabbing, the longest core, Tau-1, was scanned by X-ray computer automated tomography (CT) to reveal growth bands and to determine the optimal cutting planes (CT collage of coral cores in Tangri et al 56 ). The core was then slabbed on a two-bladed tile saw to produce 5 mm thick slabs.…”

“…The final chronology for Ta’u-1 based annual banding and annual δ 18 O cycles extends from 1521 to 2011 CE with an average extension rate of 1.2 cm yr −1 . The core preparation and isotopic analytical methods of the Ta’u-1 analyses were previously published 55,56 .…”

A new record for a massive Porites colony at Ta’u Island, American Samoa

“…of 19.0 mm year −125 in comparable latitudes and adjusted for an average temperature of 28.4 °C near Ta'u 26 , suggest that the two largest corals found on Ta'u are approximately 368 years and 420 years old, respectively. Another age estimate can be generated using the 490 annual density bands measured by Tangri et al 15 www.nature.com/scientificreports/ between estimates of over 200 years for both corals is noteworthy and illustrates the variability associated with determining ages for long-lived species. One of the most interesting aspects of this study is that multiple massive Porites colonies were documented on three sides of the island, where they are subjected to differing environmental regimes (e.g., wave exposure, wind exposure, ocean current strength, and direction) that may affect growing conditions.…”

“…In some studies, the colonies were reported simply as a matter of record for their unusual size [e.g., 13,18]. In other cases, colonies were sampled as part of a larger effort to examine growth rates 17 , temperature records coupled with calcification rates 24 , and climate reconstruction 15 . To our knowledge, none of the reported colonies have exceeded the size of the largest colonies measured from Ta'u.…”