Late-Holocene Climate Variability in Southern New Zealand: A reconstruction of regional climate from an annually laminated sediment sequence from Lake Ohau

This research aims to improve understanding of synoptic climate systems influencing southern New Zealand and document changes in the intensity and frequency of these systems beyond the historical record by analyzing a 1,350-year annually laminated sediment sequence recovered from Lake Ohau, South Island, New Zealand (44.234°S, 169.854°E). Climatological patterns originating in both the tropics (El-Niño-Southern Oscillation (ENSO), Interdecadal Pacific Oscillation (IPO)) and in the Antarctic (Southern Annular Mode (SAM)) influence year-to-year variability in New Zealand’s climate (e.g. temperature and precipitation). However, the range of natural variability of these systems in the southwest Pacific over time is poorly known because the instrumental record is short (~100 years). The high-resolution record from Lake Ohau offers a unique opportunity to investigate changes in regional hydrology and climate, and to also explore connections to large-scale climate patterns over the last millennium.  Hydrodynamic and hydroclimatic processes that influence and control the production, transport, and deposition of sediment within the Lake Ohau catchment are examined and constrained in order to develop a robust climate record. A key aim is to determine the role that meteorology and climate play in controlling sediment flux. The physical properties and facies of a 5.5-meter-long Lake Ohau sediment core are analyzed using thin-sections, high-resolution X-radiographs scans, and particle-size analyses. Time-series analysis is used to establish links between varve facies, hydroclimate variability and regional synoptic climate types over the instrumental record. Utilizing this climate-proxy relationship, inflow conditions are reconstructed over the last 1,350 years and compared with regional temperature reconstructions to generate a Western South Island paleo-atmospheric circulation index. Relationship between this paleocirculation index and other proxy reconstructions show significant variability in the relative forcing of tropical (ENSO) and Southern Hemisphere highlatitude (SAM) synoptic climate drivers on New Zealand and southwest Pacific climate.  Overall, this work demonstrates that: a) the laminated sediments from Lake Ohau are varves and the formation of the annual stratigraphy is strongly controlled by lake hydrodynamics, in particular, thermal lake stratification; b) sediment stratigraphy reflects changes in austral warm period (December-May) inflow, enabling a highresolution reconstruction of hydroclimate over the last 1,350 years and; c) the generation of a paleocirculation index for the Western South Island points to significant changes between northerly or southerly dominated atmospheric conditions in southern New Zealand, particularly over the ‘Little Ice Age’ (1385-1710 AD). During this time, the strength of tropical teleconnections weakened and a strong negative phase SAM persisted. Comparison with high-resolution regional proxy records from Antarctica and the Central Pacific point to significant regional coherence with a strong negative phase SAM acting as a primary driver of the onset of Little Ice Age conditions across the South Pacific.