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ATMOSPHERIC AND OCEANIC SCIENCE LETTERS, 2010, VOL. 3, NO. 5, 288292
Has the Drake Passage Played an Essential Role in the Cenozoic Cooling?
ZHANG Zhong-Shi
1,2
, YAN Qing
2
, and WANG Hui-Jun
2
1
Bjerknes Centre for Climate Research, UniResearch, Bergen N-5007, Norway
2
Nansen-Zhu International Research Center, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
Received 29 August 2010; revised 12 September 2010; accepted 12 September 2010; published 16 September 2010
Abstract The Drake Passage is the seaway between
South America and Antarctica. It is widely believed that
the thermal isolation effects caused by the opening of the
Drake Passage played an important role in the abrupt
cooling that occurred at the Eocene-Oligocene boundary
in the Cenozoic. These effects are also thought to be in-
dependent of the geometry of the passage. Here, the au-
thors demonstrate that the climate impacts of the Drake
Passage depend on the passage geometry by comparing
the climate’s sensitivity to the opening of the Drake Pas-
sage under the present and the Early Eocene land-sea
configurations. These experiments show that the thermal
isolation effects caused by the passage are much stronger
under the present land-sea configuration. In comparison,
under the Early Eocene land-sea configuration, the weak
anomalies in heat transport caused by the opening of the
narrow and shallow Drake Passage are not strong enough
to thermally insulate Antarctica. The climate effects of the
Drake Passage on the Cenozoic cooling have been over-
estimated in previous sensitivity studies carried out using
the present land-sea configuration. Thus, it is unlikely that
the opening of the Drake Passage played an essential role
in the abrupt Cenozoic cooling, especially in the abrupt
cooling at the Eocene-Oligocene boundary.
Keywords: Drake Passage, land-sea configuration, Ce-
nozoic cooling
Citation:
Zhang, Z.-S., Q. Yan, and H.-J. Wang, 2010:
Has the Drake Passage played an essential role in the Ce-
nozoic Cooling? Atmos. Oceanic Sci. Lett., 3, 288–292.
1 Introduction
The Drake Passage is the seaway between the southern
tip of South America at Cape Horn, Chile, and the South
Shetland Islands of Antarctica. At present, the passage is
more than 800 km wide and greater than 4000 m deep.
About 60 million years (Ma) ago, the Drake Passage
had not yet opened, and South America was still con-
nected with Antarctica. The Drake Passage opened later
and gradually expanded in the Cenozoic. There is evi-
dence indicating an early shallow opening in the Early
Eocene (Livermore et al., 2005) and subsequent deepen-
ing at ~ 41 Ma in the Middle Eocene (Scher and Martin,
2006). However, the age estimates for the opening of the
Drake Passage are still under debate (Barker, 2001).
The opening of the Drake Passage is thought to have
been essentially important in the Cenozoic cooling. In the
Cenozoic, the Early Eocene (~50 Ma) was an extremely
Corresponding author: ZHANG Zhong-Shi, zhongshi.zhang@uni.no
warm period (Zachos et al., 2001). The temperature in the
deep ocean was above 10C (Lear et al., 2000). The level
of warmth in the Arctic is thought to have been similar to
that in today’s subtropical region (Sluijs et al., 2006).
Later, the earth gradually developed from a greenhouse to
an icehouse climate, with abrupt cooling and Antarctic
glaciation occurring at the Eocene-Oligocene boundary
(~34 Ma, Zachos et al., 2001). The opening of the Drake
Passage has been suggested to have enabled the develop-
ment of the Antarctic Circumpolar Current (ACC), which
thermally insulated Antarctica through reduced southward
heat transport (Toggweiler and Bjornsson, 2000) and
caused significant cooling over Antarctica at the Eo-
cene-Oligocene boundary (Kennett, 1977). The thermal
isolation effects of the opening of the Drake Passage have
been analyzed by many modeling studies based on the
present land-sea configuration (e.g., Nong et al., 2000;
Sijp and England, 2004). These effects have also been
thought to be independent of the geometry of the Drake
Passage (Toggweiler and Bjornsson, 2000).
However, the role of the Drake Passage has been chal-
lenged by an increasing number of studies. Geological
studies (Livermore et al., 2005; Scher and Martin, 2006)
indicate that the opening of the Drake Passage might have
occurred much earlier than the Eocene-Oligocene bound-
ary. A modeling study has shown that the ACC becomes
sluggish with a narrow and shallow Drake Passage in the
Eocene land-sea configuration (Huber and Sloan, 2001).
The reduction in ocean heat transport caused by the open-
ing of the Drake Passage has been suggested to have had
a smaller effect than that expected from decreasing levels
of atmospheric CO
2
in the transition from the greenhouse
to the icehouse climate (DeConto and Pollard, 2003;
Huber and Nof, 2006). The tropical seaways may have
also played a more important role than the Drake Passage
in understanding the abrupt cooling events of the Ceno-
zoic (Zhang et al., 2009).
It is obvious that the above modeling studies, carried
out with different land-sea configurations, show varying
degrees of importance for the opening of the Drake Pas-
sage, which brings uncertainty to the above debate. In this
study, using the climate model FOAM (the Fast Ocean
Atmosphere Model), we compared the climate’s sensitivity
to the opening of the Drake Passage under two land-sea
configurations. One is the present land-sea configuration,
and the other is the Early Eocene land-sea configuration.
2 Model and experiment design
Version 1.5 of FOAM is a fully coupled General Cir-
NO. 5 ZHANG ET AL.: WAS THE DRAKE PASSAGE IMPORTANT TO THE CENOZOIC COOLING? 289
culation Model (GCM) run without flux corrections
(Jacob et al., 2001). The atmospheric component in
FOAM is run with a horizontal resolution of R15 (4.5 ×
7.5 degrees) and 18 vertical levels. The ocean component
is a finite-difference z-coordinate ocean model with a
horizontal resolution of 1.4 × 2.8 degrees, 24 vertical lev-
els, and a free surface. This version of FOAM provides a
good simulation of the mean and variability of the modern
climate (e.g., Liu et al., 2000). It also has been widely
used in paleoclimate studies (e.g., Donnadieu et al., 2006;
Zhang et al., 2009).
We designed two groups of experiments. The first
group was based on the present land-sea configuration
and bathymetry condition, but with (experiment PRE,
present) or without (experiment PRENDP, present without
the Drake Passage) the Drake Passage open. The Drake
Passage was about 1400 km wide (ten model grids) with a
maximum depth of 3000 m in experiment PRE. The sec-
ond group was based on the Early Eocene land-sea con-
figuration and bathymetry condition established by Zhang
et al. (2009), again with the Drake Passage open (experi-
ment EOC, Eocene) or closed (experiment EOCNDP,
Eocene without the Drake Passage). The Drake Passage
was about 700 km wide (five model grids) with a maxi-
mum depth of about 150 m in experiment EOC.
In these four experiments, the solar constant and orbital
parameters were set to present conditions. The vegetation
on land was prescribed as shrubland/grassland. The con-
centration of atmospheric CO
2
was set to eight times the
pre-industrial level, following estimates for the Early Eo-
cene (Royer, 2006).
Experiments PRE and EOC were spun up for 1500
years from the same preset initial conditions. Then, these
two experiments were continued for 500 years. Starting
from the end of the 1500-year spin ups of experiments
PRE and EOC, respectively, experiments PRENDP and
EOCNDP were carried out for 500 years. All of the results
reported here are the averages of the last 100 years of
each experiment.
3 Climate sensitivities
The model, FOAM, simulated a reasonable strength for
the ACC. For the present climate with atmospheric CO
2
level set at 350 ppmv, the simulated strength of the ACC
was about 160 Sv. In experiment PRE, with the atmos-
pheric CO
2
level set at 2240 ppmv, the strength of the
ACC was about 25 Sv. The weak ACC in experiment PRE
resulted from the extremely warm conditions, with deep
ocean temperatures above 10C in the experiment. After
the Drake Passage was opened under the present land-sea
configuration, the ACC occurred around Antarctica in
experiment PRE (Fig. 1a). In comparison, the strength of
the ACC was significantly weaker, about 4 Sv, in experi-
ment EOC. No clear development of the ACC was ob-
served with the opening of the Drake Passage in experi-
ment EOC (Fig. 1b).
With the development of the ACC, thermal isolation
effects were observed in experiment PRE. Significant
cooling occurred at the surface of the South Atlantic and
the Southern Ocean after the Drake Passage was opened
in experiment PRE (Fig. 2a). Although the cooling around
Western Antarctica was stronger, almost the whole South-
ern Ocean and Antarctica were cooled. The mean SST
decreased by about 1.5C at the surface of the Southern
Ocean. The SST changes in the Southern Ocean were
similar to those in the simulations carried out by Sijp and
England (2004). Warming appeared in the Northern
Hemisphere. This pattern of SST change, with strong
cooling in the Southern Hemisphere but warming in the
Northern Hemisphere, agrees well with earlier studies
(e.g., Nong et al., 2000; Sijp and England, 2004).
However, the cooling effect caused by the opening of
the Drake Passage was significantly weaker under the
Figure 1 ACC anomalies (cm s
–1
) after the Drake Passage was opened under (a) the present and (b) the Early Eocene land-sea configurations. Ar-
rows show current directions for current speeds larger than 0.5 cm s
–1
. The color scale shows current speeds. Areas with current speeds larger than 1
cm s
–1
are shaded.
290 ATMOSPHERIC AND OCEANIC SCIENCE LETTERS VOL. 3
Figure 2 SST (C) and SSS (psu) anomalies after the Drake Passage was opened. (a) SST anomalies and (b) SSS anomalies under the present
land-sea configuration; (c) SST anomalies and (d) SSS anomalies under the Early Eocene land-sea configuration; Comparison of zonal mean (e) SST
anomalies and (f) SSS anomalies in the Atlantic, respectively. Only SST or SSS changes with confidence levels greater than 95% are illustrated.
Early Eocene land-sea configuration (Fig. 2c). Only small
changes of SST were observed at the surface of the Atlan-
tic and the global ocean. A cooling center occurred in the
South Atlantic in experiment EOC. However, SST de-
creased by only about 0.1–0.4C in the cooling center.
The response of sea surface salinity (SSS) to the open-
ing of the Drake Passage was also larger under the present
land-sea configuration than under the Early Eocene
land-sea configuration (Figs. 2b and 2d). After the Drake
Passage was opened in experiment PRE, SSS decreased
by 3–4 psu in the South Atlantic. In comparison, SSS de-
creased by only 0.3–0.5 psu in the South Atlantic in ex-
periment EOC.
The opening of the Drake Passage results in more heat
transport from the Southern Hemisphere to the Northern
Hemisphere in the models. The modification of heat
transport can be clearly observed, especially in the Atlan-
tic basin (Fig. 3). However, the modification caused by
the opening of the Drake Passage is much stronger under
the present land-sea configuration than under the Early
Eocene land-sea configuration (Figs. 3e and 3f). As a re-
sult, significant SST anomalies occur after the Drake
Passage is opened in experiment PRE, but weak SST
anomalies occur in experiment EOC.
4 Implications for the Cenozoic cooling
It should be noted that FOAM might have underesti-
mated the strength of the ACC in experiment PRE and
experiment EOC, although the extremely warm condi-
tions are one of the reasons for the sluggish simulation of
the ACC. These simulations of the ACC need to be re-
peated using other models. However, the above simula-
tions demonstrate that the development of the ACC de-
pends on the geometry of the Drake Passage. The strength
of the ACC is about five times larger in experiment PRE
under the present land-sea configuration than in experi-
ment EOC under the Early Eocene land-sea configuration.
The thermal isolation effects caused by the opening of
the Drake Passage also depend on the passage geometry
associated with the land-sea configuration. These results
do not support the early study carried out with a highly
idealized model (Toggweiler and Bjornsson, 2000), which
showed that the channel geometry of the Drake Passage
was relatively unimportant to its climate effects. It should
be noted that this earlier study used a highly idealized
bathymetry, a “water planet” with two islands located at
the two poles and one land barrier connecting the two
islands. With this idealized bathymetry, the importance of
NO. 5 ZHANG ET AL.: WAS THE DRAKE PASSAGE IMPORTANT TO THE CENOZOIC COOLING? 291
Figure 3 Heat transports (PW) examined in the experiments. Heat transport in the global ocean basin under (a) the present land-sea configuration
and (c) the Early Eocene land-sea configuration; Heat transport in the Atlantic Ocean Basin under (b) the present land-sea configuration and (d) the
Early Eocene land-sea configuration; Comparison of (e) global heat transport anomalies and (f) Atlantic heat transport anomalies after the Drake
Passage was opened under the two land-sea configurations, respectively. Heat transport was calculated by integrating the meridional velocity and
temperature.
the land-sea configuration and bathymetry was com-
pletely ignored. Hence, the salinity response to the open-
ing of the Drake Passage was overestimated. The salinity
response is highly related to fresh water input associated
with the land-sea configuration and bathymetry conditions.
As our simulations demonstrate, only weak SSS anoma-
lies occur after the Drake Passage is opened under the
Early Eocene land-sea configuration (Fig. 2d). These
weak changes in salinity could not have caused a large
reorganization of ocean circulation resulting in a climate
response.
In the earlier sensitivity experiments (e.g., Nong et al.,
2000; Sijp and England, 2004), the cooling effect of the
Drake Passage on the Cenozoic cooling was overesti-
mated because they were carried out with the present to-
pography and bathymetry conditions. It is likely that the
Drake Passage opened in the Early Eocene (Livermore et
al., 2005), and it might have reached a depth of 1000 m in
the Middle Eocene (Scher and Martin, 2006). Therefore,
the opening was much earlier than the abrupt cooling
event at the Eocene-Oligocene boundary, about 34 Ma.
Although the age estimates for the opening of the Drake
Passage are still under debate (Barker, 2001), there is no
doubt that the Drake Passage was narrow and shallow
following its initial opening. As the present study demon-
strates, a narrow opening of the Drake Passage could not
have caused strong cooling, even if the opening happened
at the Eocene-Oligocene boundary. Thus, it is unlikely
that the thermal isolation effects caused by the opening of
the Drake Passage played an important role in the dete-
rioration from a greenhouse to icehouse climate in the
Cenozoic. There must be other more important factors
that contributed to the abrupt cooling and Antarctic glaci-
ation at the Eocene-Oligocene boundary.
However, the impact of the expansion of the Drake
Passage on the long-term Cenozoic cooling trend should
not be neglected, although the model simulations pre-
sented here and the recent studies of DeConto and Pollard
(2003), Huber et al. (2004), Huber and Nof (2006), and
Zhang et al. (2009) indicate that the Drake Passage plays
a minor role in cooling the Antarctic continent. With the
gradual expansion and deepening of the Drake Passage,
the increased strength of the ACC could be expected to
contribute to the observed long-term global cooling in the
Cenozoic.
5 Summary
In summary, we compared the climate’s sensitivity to
the opening of the Drake Passage under the present and
292 ATMOSPHERIC AND OCEANIC SCIENCE LETTERS VOL. 3
the Early Eocene land-sea configurations with four nu-
merical experiments. These experiments show that the
thermal isolation effects caused by the opening of the
passage are much stronger under the present land-sea
configuration. The opening of the Drake Passage under
the present land-sea configuration causes more heat
transport to the Northern Hemisphere and reduces heat
transport to Antarctica, hence leading to a significant
cooling around Antarctica. In comparison, the climate
sensitivity to the opening of the Drake Passage is much
weaker under the Early Eocene land-sea configuration.
The opening of a narrow and shallow Drake Passage only
causes weak anomalies in heat transport and global sur-
face temperature, which are not strong enough to ther-
mally insulate Antarctica. These experiments demonstrate
that the climate impact of Drake Passage depends on the
passage geometry. In earlier sensitivity experiments that
were carried with the present topography and bathymetry
conditions, the cooling effect of the Drake Passage has
been overestimated in explaining the Cenozoic cooling. It
is unlikely that the opening of the Drake Passage played
an important role in the Cenozoic cooling, especially in
the abrupt cooling at the Eocene-Oligocene boundary.
Acknowledgements. This work was supported by the National
Natural Science Foundation of China under Grant 40902054 and the
Earth System Model Modeling project supported by Statoil, Nor-
way.
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