M. Montoya, T.J. Crowley, H. von Storch
Temperatures at the last interglacial simulated by a coupled ocean-atmosphere model
Abstract
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.
Abstract
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
