Atlantic Ocean circulation at the Last Glacial Maximum : inferences from data and models

Abstract

This thesis focuses on ocean circulation and atmospheric forcing in the Atlantic Ocean atthe Last Glacial Maximum (LGM, 18-21 thousand years before present). Relative to thepre-industrial climate, LGM atmospheric CO2concentrations were about 90 ppm lower,ice sheets were much more extensive, and many regions experienced significantly coldertemperatures. In this thesis a novel approach to dynamical reconstruction is applied tomake estimates of LGM Atlantic Ocean state that are consistent with these proxy recordsand with known ocean dynamics.Ocean dynamics are described with the MIT General Circulation Model in an Atlanticconfiguration extending from 35◦S to 75◦N at 1◦resolution. Six LGM proxy types areused to constrain the model: four compilations of near sea surface temperatures from theMARGO project, as well as benthic isotope records ofδ18O andδ13C compiled by Marchaland Curry; 629 individual proxy records are used. To improvethe fit of the model tothe data, a least-squares fit is computed using an algorithm based on the model adjoint(the Lagrange multiplier methodology). The adjoint is usedto compute improvements touncertain initial and boundary conditions (the control variables). As compared to previousmodel-data syntheses of LGM ocean state, this thesis uses a significantly more realisticmodel of oceanic physics, and is the first to incorporate sucha large number and diversityof proxy records.A major finding is that it is possible to find an ocean state thatis consistent with all sixLGM proxy compilations and with known ocean dynamics, givenreasonable uncertaintyestimates. Only relatively modest shifts from modern atmospheric forcing are required tofit the LGM data. The estimates presented herein succesfullyreproduce regional shifts inconditions at the LGM that have been inferred from proxy records, but which have not beencaptured in the best available LGM coupled model simulations. In addition, LGM benthicδ18O andδ13C records are shown to be consistent with a shallow but robustAtlantic merid-ional overturning cell, although other circulations cannot be excluded.