George H. Scott

Changes in the position of the Subtropical Front south of New Zealand since the last glacial period

This study fills an important gap in our understanding of past changes in the Southern Subtropical Front (S-STF) in the southwest Pacific Ocean. Paleo-sea surface temperatures (SST) were estimated from planktic foraminiferal census counts from cores straddling the modern S-STF in the Solander Trough, south of New Zealand. The estimated SST were compared for 6 time slices; glacial period (25-21 ka), Last Glacial Maximum (LGM; 21-18 ka), early deglaciation (18-16 ka), late deglacial/early Holocene period (14-8 ka), mid-Holocene period (8-4 ka), and late Holocene period (4-0 ka). The position of the S-STF was determined by two methods: (1) the location of the 10 degrees C isotherm and (2) the location of the highest SST gradients. These new results suggest that the S-STF was not continuous between east and west of New Zealand during the glacial period. Steep SST gradients indicate that a strong S-STF rapidly shifted south during the LGM and early deglaciation. During the late deglacial and Holocene periods the position of the S-STF differs between the two methods with reduced SST gradients, suggesting amore diffuse S-STF in the Solander Trough at this time. The glacial SST data suggest that the S-STF shifted north to the west of New Zealand, while to the east there was a stronger SST gradient across the front. This was possibly the result of an increased wind stress curl, which could have been caused by stronger, or more northerly Southern Hemisphere westerly winds (SHWW), or a merging of the SHWW split jet in this region.

Evidence for a Holocene Climatic Optimum in the southwest Pacific: A multiproxy study

The early Holocene sea surface temperature (SST) gradient across the subtropical front (STF) to the east of New Zealand was ~2°C (measured between core sites MD97-2121 and MD97-2120): considerably less than the ~6°C modern gradient between the two core sites. We document the surface ocean temperatures east and south of New Zealand during the early and middle Holocene, to test and expand upon this reconstruction. This new study samples a latitudinal transect of seven sediment cores from 37°S to 60°S in the southwest Pacific from subtropical waters north of New Zealand to polar waters in the Southern Ocean. Our compilation of SST proxies consists of 525 SST estimates from five different methods and includes 243 new data points. We confirm that an early Holocene warm peak in this region was mostly restricted to the area immediately south of the STF, which resulted in a lower temperature gradient across the STF than in modern times. However, there is no change in Holocene SST south of the polar front. Faunal assemblages suggest an early Holocene meridional expansion of fauna characteristic of the modern subtropical front in the Bounty Gyre. We suggest that such an expansion could be achieved by a reduced inflow of Subantarctic Surface Water into the Bounty Gyre. Results from a modern-analog matching platform called the Past Interpretation of Climate Tool (PICT) suggest that the early Holocene SST is most consistent with reduced westerly winds in the New Zealand sector of the Southern Ocean.