CORAL EVIDENCE FOR ABRUPT CHANGES IN OCEAN-ATMOSPHERE DYNAMICS IN THE WESTERN PACIFIC SINCE 1565 AD

Global and hemispheric proxy-temperature composites consistently demonstrate that the 20^th century was significantly warmer than the four preceding centuries. A fundamental question is whether these reconstructions are of global climate variability considering only sparse palaeorecords are available from the tropics and the Southern Hemisphere. Key indicators of the tropical ocean-atmosphere system are sea surface temperature (SST), which sets climate boundary conditions, and sea surface salinity (SSS), which provides a measure of energy transfer through the evaporation-precipitation balance.

We present multi-proxy palaeoenvironmental records from eight massive Porites coral colonies, spanning 120 to 420 years of continuous growth, collected from the central Great Barrier Reef, Australia. Stable isotopes (d¹⁸O and d¹³C), Sr/Ca, U/Ca and Ba/Ca ratios were measured in 5-year increments. By replicating the measurements between colonies we demonstrate how faithfully corals record changes in their environment over decadal-to-centennial timescales. We construct composite records in a manner analogous to dendrochronology and confidence intervals for each proxy.

The coral palaeothermometers Sr/Ca and U/Ca ratios, measured in tandem with d¹⁸O, allow the separation of SST changes from changes in seawater d¹⁸O, thereby resolving SSS. The composite Sr/Ca and U/Ca are in excellent agreement back to 1565, and capture the 20th century warming trend, up to the 1980s when the cores were collected.  The most remarkable feature of the 420-year record is that SSTs were consistently as warm as the second half of the 20^th century from the early 18^th and through most of the 19^th centuries. Changes in the evaporation-precipitation balance dominate the d¹⁸O record. A striking 0.2 permil shift from the 1850s to modern values in the 1870s, indicates an abrupt freshening, which is common to coral d¹⁸O records throughout the SW Pacific. We investigate the scenario that a strengthened latitudinal temperature gradient may have prevailed during the 'Little Ice Age', with evidence for an intensified Hadley circulation impacting on evaporation rates and the surface-ocean circulation in the SW Pacific. The mid-19th century demise of the 'Little Ice Age' coincides with cooling and abrupt freshening of the SW Pacific.