Pacific Rainfall over Millennial Scales (PROMS)

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The Tropical South Pacific (TSP) is a vast area of ocean containing over 5000 small islands supporting a vulnerable human population of c.10 million of which 57% live within 1 km of the coast. All life on these islands depends on reliable precipitation for the provision of freshwater and food security, meaning that changes in the location and intensity of rainfall can result in damaging and costly societal impacts via droughts and flooding, with historical evidence of island abandonment and population migration.Within the TSP, over 10 million people and sensitive ecosystems on the 5700 isolated Pacific islands depend on rainfall from the convective processes generated in the South Pacific Convergence Zone (SPCZ), which is the largest convergence zone in the Southern Hemisphere, and influences regional, hemispheric and global climate, and in turn is influenced by ENSO and other remote modes of climate variability. Movement and strength of SPCZ convergence results in seasonal, interannual and longer term change in precipitation which are linked to severe droughts, cyclone paths resulting in impacts on Pacific island nations ability to meet Sustainable Development Goals, and in the past drove periods of mass human migration.In the last 1000 years, sustained periods of lower rainfall are thought to have contributed to the abandonment of some Atoll islands and the decision to migrate east into the Pacific. In other islands, changes in rainfall led to the development of different styles of agricultural system. However, compared to other parts of the earth, there is comparatively limited information about past precipitation changes in the TSP, in part because of a historical lack of suitable proxies and archives for producing high resolution, continuous records. Proxies for precipitation reconstructed from sediments or stalagmites are limited to marginal areas to the SPCZ and are also short (<600yrs) or have gaps in the timeseries.Current evidence indicates that decadal-centennial changes in SPCZ/TSP precipitation are linked to global modes of remote climate variability, particularly in the Atlantic, the rest of the Pacific, and the Southern Ocean, but our data to test theories or to validate GCMs is relatively short (<100 years). To address the lack of data and to create a step change in our understanding of the processes by which change is generated in the SPCZ by external modes of climate variability we will generate new network of high resolution (10-50yr) quantitative precipitation records from the whole SPCZ region that will capture its movement and change in strength over a 3500 period when we know large changes have occurred. We will utilise new multiproxy syntheses of palaeoclimate records alongside reconstructions of candidate modes of variability such as the Interdecadal Pacific Oscillation (IPO) and Atlantic Multidecadal Variability (AMV) and Southern jet stream index (SAM) to test for their relationship to SPCZ rainfall. Using a global climate/circulation model we will run a suite of experiments using plausible scenarios of modes of climate variability, either in isolation, or combined with each other, and informed by existing and new proxy data. These experiments will allow us to understand the processes by which different modes of climate variability drive SPCZ rainfall, and thus create fundamental new understanding of the decadal-scale SPCZ changes likely under different past and future modes of climate variability. We can achieve this research now due to the timely collaboration between the team currently leading quantitative hydroclimate reconstruction in the SPCZ and Tropical South Pacific (Sear, Sachs, Langdon, Ladd, Croudace), and the team currently leading the mechanistic understanding and modelling of the SPCZ over a range of time scales (seasonal - multidecadal) (Joshi, Matthews, Roberts, Brown, Osborn).