Significant variations in late Holocene South Pacific Gyre strength

The Pacific Ocean subtropical gyres significantly influence the Earth's climate and ecosystems. Over the last decades, an increase in subtropical South Pacific Gyre (SPG) circulation has led to warming in the southwest Pacific up to 4x the global average. Are these changes anomalous or part of a natural cycle? Determining this requires long-term records of baseline ocean dynamics in the southwest Pacific. Unfortunately, SPG observations are sparse and instrumental records only span a few decades. Here, we reconstruct 3000 years of SPG dynamics at decadal resolution by measuring marine radiocarbon reservoir age deviations (∆R) in New Zealand black corals. We find that over the last millennium, century-scale ∆R variations around New Zealand are inversely correlated with reconstructions of the Southern Annular Mode (SAM) (r = -0.51, p < 0.001) from the eastern Pacific. We also observe a depression in mean ∆R between 2000-3000 year BP.

We propose that the subtropical SPG experiences modes of natural variability on multi-century and millennium timescales. Multi-century ∆R oscillations may represent varying ocean circulation strength and mixed-layer depth driven by changing westerly winds. This is supported by a correspondence between ∆R/SAM variations and reconstructions of New Zealand's synoptic meteorology and temperature over the last millennium. Our record also reveals a baseline shift in subtropical SPG mean ∆R that begins ca. 1900BP This observation mirrors a late Holocene SPG ∆R record showing a change in circulation over the 1900-3000BP interval, possibly forced by a period of strong and abrupt El Niño Southern Oscillation (ENSO) events.