Influence of Last Glacial Maximum boundary conditions on the global water isotope distribution in an atmospheric general circulation model

T Tharammal,D Noone,U Merkel,A Paul

doi:10.5194/cp-9-789-2013

Abstract

Abstract. To understand the validity of δ18O proxy records as indicators of past temperature change, a series of experiments was conducted using an atmospheric general circulation model fitted with water isotope tracers (Community Atmosphere Model version 3.0, IsoCAM). A pre-industrial simulation was performed as the control experiment, as well as a simulation with all the boundary conditions set to Last Glacial Maximum (LGM) values. Results from the pre-industrial and LGM simulations were compared to experiments in which the influence of individual boundary conditions (greenhouse gases, ice sheet albedo and topography, sea surface temperature (SST), and orbital parameters) were changed each at a time to assess their individual impact. The experiments were designed in order to analyze the spatial variations of the oxygen isotopic composition of precipitation (δ18Oprecip) in response to individual climate factors. The change in topography (due to the change in land ice cover) played a significant role in reducing the surface temperature and δ18Oprecip over North America. Exposed shelf areas and the ice sheet albedo reduced the Northern Hemisphere surface temperature and δ18Oprecip further. A global mean cooling of 4.1 °C was simulated with combined LGM boundary conditions compared to the control simulation, which was in agreement with previous experiments using the fully coupled Community Climate System Model (CCSM3). Large reductions in δ18Oprecip over the LGM ice sheets were strongly linked to the temperature decrease over them. The SST and ice sheet topography changes were responsible for most of the changes in the climate and hence the δ18Oprecip distribution among the simulations.

Highlights

Ocean ScienceThe Last Glacial Maximum (LGM, about 19 000–23 000 yr before present) marks the peak of the last glacial period between about 110 000 and 10 000 yr before present
We suppressed the feedbacks from the ocean and sea ice by using fixed sea surface temperature (SST) derived from previous simulations of a coupled atmosphere–ocean model, which allowed the response to the LGM SST to be considered as a separate factor
This methodology had the advantage of largely isolating the effect of the individual forcing factors on the atmosphere without the response being dominated by the SST feedback

Summary

Introduction

Ocean ScienceThe Last Glacial Maximum (LGM, about 19 000–23 000 yr before present) marks the peak of the last glacial period between about 110 000 and 10 000 yr before present. During the LGM, a large portion of North America and northern Eurasia was covered by ice sheets, the sea level was reduced by about 120 m (Fairbanks, 1989; LambdeifcfkeraenndceCshianpvpeeglle,t2a0tTi0oh1n)e.aTndhCesronyielwotylsyppeexhipmoepsraeecdteladndthaeresausrfaancde albedo. The Laurentide ice sheet covering most of Canada and a large portion of the northern United States had a firstorder impact on the large-scale atmospheric circulation in the Northern Hemisphere via topographic and thermal forcing as demonstrated in global circulation models A reduced atmospheric concentration of CO2 (185 ppmv, Petit et al, 1999) compared to the pre-industrial (PI, approximately 1800 AD) value of 280 ppmv influenced climate, as reduced greenhouse gas (GHG) levels have a direct effect on radiative forcing (Shin et al, 2002)

Objectives

Findings

Discussion

Conclusion