Under this award the PIs will use large corals heads primarily of the species Diploria labyrinthiformis to reconstruct multicentury long records of SST variability in the western subtropical North Atlantic. This should enable exploration of the ocean's role in climate change on decadal and longer time scales, specifically the North Atlantic Oscillation. The available instrumental record of SST extends back little more than a century. However, biases and sparseness of data render the older part of the SST record unreliable and the demonstrated association between NAO and mid-latitude SST anomalies is based exclusively on data gathered since 1950. The PIs will extend the SST record for the mid-latitude North Atlantic using long-lived brain corals collected on Bermuda, a site well placed to index the subtropical gyre. Three independent paleothermometers ( 18O, Sr/Ca and skeletal growth) will be applied to the skeletons of two massive colonies to construct a 500 year-long multiproxy record of SST variability at seasonal resolution. In addition the PIs will sample, date and analyze four massive non-living colonies already identified as potentially old enough to extend the 500 year-long proxy record back to 1000 AD. This multicentury long record will enable key, unresolved issues in North Atlantic climate to be addressed. In particular, estimates of the sensitivity of the mid-latitude atmosphere to changes in surface boundary conditions requires seasonally-resolved records of SST that span the significant climatic events of the late Holocene. The PIs will examine possible secular changes in SST related to global warming and the connection with the NAO anomaly of the past 30 years. Other key questions relate to the sensitivity of SST to solar forcing and volcanic eruptions, and the existence of decadal through centennial-scale periodicity in surface ocean conditions.
