A shift in South Pacific hydroclimate over the last 1500 years driven by tropical Pacific variability

The South Pacific Convergence Zone (SPCZ) is a critical region of intense precipitation, particularly during the austral summer (November—March), situated in the tropical Pacific Ocean. This region is known to be highly sensitive to variations in sea surface temperatures (SSTs) across the Pacific. Previous research has demonstrated that decadal-scale shifts in the SPCZ are influenced by SST variability, but our understanding of longer-term changes—spanning multi-decadal to millennial timescales—remains limited. These constraints arise from the scarcity of high-resolution hydroclimate proxy records and systemic biases in even state-of-the-art coupled climate models.

To address this knowledge gap, we combine newly developed hydroclimate proxies (from Nuku Hiva and Tahiti, French Polynesia) with previously established proxies and novel climate model simulations to explore SPCZ dynamics over the last 1500 years. Our findings provide new insights into centennial- and millennial-scale precipitation variability and its potential drivers.

Our proxy data indicate a spatial shift in SPCZ precipitation patterns from 1000 yrs BP to 200 yrs BP. During this interval, the eastern SPCZ experienced a significant increase in precipitation, whilst the western SPCZ underwent notable drying. This north-eastward shift in precipitation was likely driven by changes in Pacific SST gradients, as shown by our climate model simulations and proxy SST reconstructions. Our modelling results show that a previously hypothesised weakening of the tropical Pacific zonal SST gradient is consistent with our new proxy reconstructions and offer a plausible mechanism for the observed hydroclimatic shifts.

An intriguing corollary of our study is the coincidence of this millennial-scale hydroclimatic changes with key events in human history. The eastward shift in SPCZ precipitation overlaps temporally with the Polynesian colonization of the eastern SPCZ region, including islands such as the Marquesas, and eventually Hawaii. This suggests a potential linkage between climate-driven changes in the SPCZ and patterns of human migration and settlement. Enhanced precipitation in the eastern SPCZ would have likely improved freshwater availability, agricultural potential, and overall habitability of these islands, potentially facilitating successful colonization. Conversely, drier conditions in the western SPCZ may have influenced resource pressures, encouraging exploration and eastward movement.

Ultimately, these findings emphasize the need for continued development of high-resolution proxies and improvements in coupled climate models to deepen our understanding of long-term Pacific region climate dynamics and their societal impacts.