ClimaTea Lecture: "The North Atlantic Subpolar Front over the Past 15 kyrs as Inferred from Sequential Methods"

Speaker: Olivier Marchal (WHOI)

Abstract: The role of North Atlantic variability in climate change remains poorly understood due in large part to the limited span of instrumental records. This situation invites researchers to consult other sources of information, in particular records of sea surface temperature (SST) estimated from the analysis of marine sediments. The interpretation of SST records from deep-sea sediment cores, however, is a delicate problem, as the SST estimates are generally scarce, inaccurate, and sometimes inconsistent. Here we discuss the interpretation of a small number of North Atlantic SST records characterized by (generally) centennial resolution and spanning the past 15,000 years, a time interval of the last glacial-interglacial transition that comprises episodes of abrupt climate change, such as the Younger Dryas. An attempt is made to infer the time-evolving SST field in the North Atlantic from the quantitative combination of the SST records with an advective mixed layer model using sequential methods of optimal estimation theory (an extended Kalman filter and a fixed-interval smoother). The error covariance of the time-evolving SST field is computed from the error statistics of the SST records, the model dynamics, and estimates of model errors. We find that the climatologic position of the 10-degree Celsius surface isotherm, which today coincides approximately with the North Atlantic Current and the Subpolar Front, pivoted recurrently around a region near the Tail of the Grand Banks: during cold climate intervals, the isotherm assumed a more zonal and southern position compared to today, whereas during warm climate intervals it was generally oriented SW-NE, such as today. In our presentation, emphasis will be placed on the statistical, oceanographic, and climatic significance of this result.