Low-latitude western north atlantic climate variability during the past millennium : insights from proxies and models

Abstract

Estimates of natural climate variability during the past millennium provide a frame of reference in which to assess the significance of recent changes. This thesis investigates new methods of reconstructing low-latitude sea surface temperature (SST) and hydrography, and combines these methods with traditional techniques to improve the present understanding of western North Atlantic climate variability. A new strontium/calcium (Sr/Ca) -SST calibration is derived for Atlantic Montastrea corals. This calibration shows that Montastrea Sr/Ca is a promising SST proxy if the effect of coral growth is considered. Further analyses of coral growth using Computed Axial Tomography (CAT) imaging indicate growth in Siderastrea corals varies inversely with SST on interannual timescales. A 440-year reconstruction of low-latitude western North Atlantic SST based on this relationship suggests the largest cooling of the last few centuries occurred from -1650-1730 A.D., and was -I°C cooler than today. Sporadic multidecadal variability in this record is inconsistent with evidence for a persistent 65-80 year North Atlantic SST oscillation. Volcanic and anthropogenic radiative forcing are identified as important sources of externally-forced SST variability, with the latter accounting for most of the 2 0 th century warming trend. An 1800-year reconstruction of SST and hydrography near the Gulf Stream also suggests SSTs remained within about I°C of modern values. This cooling is small relative to other regional proxy records and may reflect the influence of internal oceanic and atmospheric circulation. Simulations with an atmospheric general circulation model (AGCM) indicate that the magnitude of cooling estimated by proxy records is consistent with tropical hydrologic proxy records. AcknowledgementsI thank my advisors Anne Cohen and Delia Oppo for their time and support. Listening to their ideas has been inspirational, and their willingness to listen to mine has been appreciated. They will be role models to me throughout my scientific career. The generous and selfless contributions of my thesis committee, including Bill Curry, Ping Chang, Ed Boyle and Jeff Donnelly are also greatly appreciated. My only regret is not taking greater advantage of their collective knowledge.I also express my appreciation to the members of WHOI's Paleoceanography group, within which the analytical efforts of Rindy Ostermann, Scot Birdwhistell, Simon Thorrold, Darlene Ketten, Julie Arruda, Kathyrn Rose and Luping Zou deserve special recognition. The support of the Academic Programs Office is unparalleled and is greatly appreciated. I also thank the Geology and Geophysics administrative staff, the MIT Education Office and WHOI's Facilities staff for their efforts to make daily operations run smoothly.I am grateful to know my fellow Joint Program students and I thank them for their encouragement, shop talk and countless dinner parties. Thanks also to the larger Woods Hole community. I will miss sharing a smile and a wave as I ride past. Finally, to Em....