M. Montoya, T.J. Crowley, H. von Storch

Temperatures at the last interglacial simulated by a coupled ocean-atmosphere model

The last interglacial (Eemian, 125,000 years ago) is generally considered the warmest time period in the last 200,000 years. This interval has sometimes been used as a reference point for greenhouse projections and also validation of climate model behavior under altered boundary conditions. Herein we report results from a coupled ocean-atmosphere model of the surface temperature response to changes in the radiative forcing during the last interglacial. Although the model generates the expected summer warming in the Northern Hemisphere, winter cooling of comparable magnitude occurs over large areas of Africa and tropical Asia. The net effect of these changes is that the global annual temperature for the Eemian run is 0.3oC cooler than the control run, which is representative of mid-late 20th century values. This conclusion receives support from validation of model sea surface temperatures (SSTs) with those estimated by the CLIMAP Project. Our results imply that, contrary to expectations that Eemian global temperatures would occur at some point in the future, they had already been reached by the mid 20th century. Comparison of greenhouse projections with other Pleistocene warm periods furthermore suggests that by the early part of the next century temperatures may exceed any reasonable reference time interval of the last two million years.

Paleooceanography 13, 170-177, 1998

M. Montoya, H. von Storch, T.J. Crowley

Climate simulation for 125,000 years ago with a coupled ocean-atmosphere General Circulation Model.

Paleoclimate simulations provide a tool to test general circulation models (GCMs) under boundary conditions independent from those under which models are built. In this line, we have used the ECHAM-1 T21/LSG coupled ocean-atmosphere GCM to simulate climatic conditions at the last interglacial maximum (Eemian, 125,000 years ago). This was a time period of extreme differences from present in the seasonal cycle of insolation in the Northern Hemisphere. The results reflect in great part the expected surface temperature changes (with respect to the control run) due to the amplification (reduction) of the seasonal cycle of insolation in the Northern (Southern) Hemisphere. A number of simulated features agree with previous results from atmospheric GCM simulations and with the evidence from the geological record (e.g. enhanced warming in northwest Europe, (e.g. intensified summer south west monsoons) except in the Northern Hemisphere poleward of 30oN, where dynamical feedbacks in the North Atlantic and North Pacific increase zonal temperatures about 1oC above what would be predicted from simple energy balance considerations. As this is the same area where most of the geological data originate, this result suggests that previous estimates of Eemian global average temperature might have been biased by sample distribution. Although the Northern Hemisphere summer monsoon is intensified, globally averaged precipitation over land is within about 1% of the present, contravening some geological inferences but not the deep-sea delta13C estimates of terrestrial carbon storage changes. Winter circulation changes in the northern Arabian Sea, driven by strong cooling on land, are as large as summer circulation changes that are the usual focus of interest. This result suggests that interpreting variations in the Arabian Sea sedimentary record solely in terms of the summer monsoon response could sometimes lead to errors. A smaller monsoonal response over northern South America suggests that interglacial paleo-trends in this region were not just due to El Niño variations. Overall the results suggest that fully dynamical models provide new insights into the last interglacial climate, but the conclusions need to eventually be checked against other coupled models and more geological data.

J. Climate (in press)

The papers may be ordered from montoya@dkrz.de or storch@gkss.de

3 June 1999