Holocene evolution of the Southern Hemisphere westerly winds in transient simulations with global climate models

Abstract

The Southern Hemisphere Westerly Winds (SWW) have been suggested to exert a critical influence on global climate through the wind-driven upwelling of deep water in the Southern Ocean and the potentially resulting atmospheric CO2 variations. The investigation of the temporal and spatial evolution of the SWW along with forcings and feedbacks remains a significant challenge in climate research. In this study, the evolution of the SWW under orbital forcing from the mid-Holocene (7 kyr BP) to pre-industrial modern times (250 yr BP) is examined with transient experiments using the comprehensive coupled global climate model CCSM3. In addition, a model inter-comparison is carried out using orbitally forced Holocene transient simulations from four other coupled global climate models. Analyses and comparison of the model results suggest that the annual and seasonal mean SWW were subject to an overall strengthening and poleward shifting trend during the course of the mid-to-late Holocene under the influence of orbital forcing, except for the austral spring season, where the SWW exhibited an opposite trend of shifting towards the equator.

Full text

Clim. Past, 8, 391–402, 2012
www.clim-past.net/8/391/2012/
doi:10.5194/cp-8-391-2012
© Author(s) 2012. CC Attribution 3.0 License.
Climate
of the Past
Holocene evolution of the Southern Hemisphere westerly winds
in transient simulations with global climate models
V. Varma1, M. Prange1,2, U. Merkel1, T. Kleinen3, G. Lohmann4, M. Pfeiffer4, H. Renssen5, A. Wagner2,4, S. Wagner6,
and M. Schulz1,2
1MARUM – Center for Marine Environmental Sciences, University of Bremen, 28334 Bremen, Germany
2Faculty of Geosciences, University of Bremen, 28334 Bremen, Germany
3Max Planck Institute for Meteorology, 20146 Hamburg, Germany
4Alfred Wegener Institute for Polar and Marine Research, 27568 Bremerhaven, Germany
5Department of Earth Sciences, Faculty of Earth and Life Sciences, VU University Amsterdam,
1081HV Amsterdam, The Netherlands
6HZG Centre for Materials and Coastal Research, 21502 Geesthacht, Germany
Correspondence to: V. Varma (vvarma@marum.de)
Received: 23 May 2011 – Published in Clim. Past Discuss.: 30 May 2011
Revised: 25 January 2012 – Accepted: 25 January 2012 – Published: 5 March 2012
Abstract. The Southern Hemisphere Westerly Winds
(SWW) have been suggested to exert a critical influence on
global climate through the wind-driven upwelling of deep
water in the Southern Ocean and the potentially resulting
atmospheric CO2variations. The investigation of the tem-
poral and spatial evolution of the SWW along with forcings
and feedbacks remains a significant challenge in climate re-
search. In this study, the evolution of the SWW under orbital
forcing from the mid-Holocene (7kyr BP) to pre-industrial
modern times (250yr BP) is examined with transient ex-
periments using the comprehensive coupled global climate
model CCSM3. In addition, a model inter-comparison is car-
ried out using orbitally forced Holocene transient simulations
from four other coupled global climate models. Analyses and
comparison of the model results suggest that the annual and
seasonal mean SWW were subject to an overall strengthen-
ing and poleward shifting trend during the course of the mid-
to-late Holocene under the influence of orbital forcing, ex-
cept for the austral spring season, where the SWW exhibited
an opposite trend of shifting towards the equator.
1 Introduction
Mid-latitude westerly winds belong to the prominent features
of the global tropospheric circulation. The present-day posi-
tions of the Southern Hemisphere Westerly Winds (SWW)
during austral summer (December-January-February) and
winter (June-July-August) are illustrated in Fig. 1. The
SWW dominate climate dynamics and influence the precip-
itation patterns between ∼30◦S and 70◦S (e.g. Thresher,
2002; Shulmeister et al., 2004). Changes in strength and
position of the SWW may affect the large-scale Atlantic hy-
drography and circulation through the impact on the Indian-
Atlantic Ocean water exchange by Agulhas leakage (Sijp and
England, 2009; Biastoch et al., 2009). Furthermore, it has
been suggested that the SWW exert a crucial influence on the
global ocean circulation through wind-driven upwelling of
deep water in the Southern Ocean (Toggweiler and Samuels,
1995; Kuhlbrodt et al., 2007; Sijp and England, 2009). The
potentially resulting influence on atmospheric CO2varia-
tions on orbital timescales has been controversially discussed
(Toggweiler et al., 2006; Menviel et al., 2008; Tschumi et al.,
2008; Anderson et al., 2009; Moreno et al., 2010; d’Orgeville
et al., 2010). Therefore, understanding the variability and the
impact of various forcings on the SWW remains a significant
area of investigation.
Published by Copernicus Publications on behalf of the European Geosciences Union.

392 V. Varma et al.: Holocene evolution of the Southern Hemisphere westerly winds
Fig. 1. Present-day Southern Hemisphere zonal wind climatology
at 850 hPa for (a) austral summer (DJF) and (b) austral winter (JJA),
based on NCEP/NCAR reanalysis data (1968–1996; Kalnay et al.,
1996). Overlaid isotherms (contours) represent the climatological
sea surface temperatures (◦C) for the corresponding seasons based
on the NODC World Ocean Atlas (Levitus et al., 1998). During
DJF, the northern margin of the zonal wind shows a more southward
confined pattern, while during JJA, it extends further to the north.
In general, the surface westerly winds cover the region between
∼30◦S and 70◦S, with the present-day strongest wind centred at
around ∼50◦S.
The variability of the SWW on glacial-interglacial
timescales has been discussed in some earlier works, in
which contradicting results regarding the direction of the
meridional shift of the mean wind were presented. While
some climate modelling studies suggested a poleward shift
in storm tracks and SWW during the Last Glacial Maximum
(Valdes, 2000; Wyroll et al., 2000; Kitoh et al., 2001; Shin
et al., 2003), other models simulated an equatorward (Kim et
al., 2003) or no latitudinal displacement, but rather an inten-
sification (Otto-Bliesner et al., 2006) of the mean westerlies.
Likewise, proxy-based reconstructions of the glacial SWW
provided contradictory views with claims of a poleward dis-
placement (e.g. Markgraf, 1987; Markgraf et al., 1992) in
contrast to evidence of an equatorward shift (e.g. Heusser,
1989; Lamy et al., 1998, 1999; Shulmeister et al., 2004)
compared to pre-industrial conditions. Lamy et al. (2010)
suggested that past variations in the SWW were not only
characterized by latitudinal shifts but also by expansions and
contractions of the wind belt. For the deglacial peak warmth
in Antarctica (∼12–9kyr ago), they provided evidence for
a minimal latitudinal extent of the belt, analogous to its
present-day summer configuration.
An important forcing of global climate on longer time
scales is accomplished by changes in the seasonal insolation
caused by the varying Earth orbital parameters. This astro-
nomical forcing is generally regarded as a dominant factor
for glacial-interglacial climate changes (Milankovitch, 1941;
Hays et al., 1976; Berger, 1978; Imbrie et al., 1992). Al-
though the climate of the Holocene is generally considered
as relatively stable compared to the last glacial (e.g. Grootes
Fig. 2. Latitudinal distribution of insolation in Wm−2at the
top-of-the-atmosphere for 7 kyr BP minus present-day, calculated
according to Berger (1978), through the year.
and Stuiver, 1997), it has also been suggested that there have
been long-term trends in the spatial and temporal patterns of
surface temperature during the Holocene (e.g. Battarbee and
Binney, 2008). A considerable variation in the seasonal and
latitudinal distribution of insolation, especially a decrease in
austral winter-spring insolation accompanied by an increase
in austral summer-fall insolation, can be observed between
7 kyr BP and present-day (Fig. 2). These changes in sea-
sonal insolation might have caused long-term variations in
the structure, position, and intensity of the SWW on multi-
millennial timescales (e.g. Markgraf et al., 1992; Lamy et al.,
2001, 2010; Jenny et al., 2003). The aim of this study is to
analyze the response of the SWW to the changes in insolation
during the mid-to-late Holocene using transient experiments
with the comprehensive global climate model CCSM3. In
addition, we compare this simulated Holocene evolution of
the SWW under orbital forcing with transient experiments
from a range of other global climate models. These analyses
will lead us to the suggestion that the annual and seasonal
mean SWW experienced a poleward shifting trend in general
– except for the austral spring season – during the course of
the Holocene under orbital forcing, consistently in all climate
models used for this inter-comparison.
2 Methods
2.1 Experimental setup for CCSM3
To study the Holocene evolution of SWW under the influence
of orbital forcing, transient experiments have been carried
out using the comprehensive global climate model CCSM3
(Community Climate System Model version 3). NCAR’s
(National Center for Atmospheric Research) CCSM3 is a
state-of-the-art fully coupled model, composed of four sepa-
rate components representing atmosphere, ocean, land and
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V. Varma et al.: Holocene evolution of the Southern Hemisphere westerly winds 393
sea-ice (Collins et al., 2006). Here, we employ the low-
resolution version described in detail by Yeager et al. (2006).
In this version the resolution of the atmospheric component
is given by T31 (3.75◦transform grid), with 26 layers in
the vertical, while the ocean has a nominal resolution of 3◦
(like the sea-ice component) with refined meridional reso-
lution (0.9◦) around the equator and a vertical resolution of
25 levels.
From a pre-industrial equilibrium simulation (Merkel et
al., 2010), the model was integrated for 400 yr with condi-
tions representing 9kyr BP orbital forcing to reach a new
quasi-equilibrium. After this spin-up, transient experiments
were carried out by applying an acceleration (by a factor
of 10) to the orbital forcing year until present-day. The
underlying assumptions for the application of this acceler-
ation technique are that orbital forcing operates on much
longer timescales (>millennia) than those inherent in the at-
mosphere and surface mixed layer of the ocean (months to
years), and that climate changes related to long-term vari-
ability of the thermohaline circulation during the time pe-
riod considered are negligible in comparison with orbitally-
driven surface temperature variations (Lorenz and Lohmann,
2004; Lorenz et al., 2006). Climate trends of the last 9000 yr,
imposed by the external orbitally driven insolation changes,
are represented in the experiments with only 900 simula-
tion years with the application of acceleration by a factor of
10. Thus, it was possible to conduct three Holocene tran-
sient experiments with different initial conditions within the
available computer resources. While the first transient run
was initialized with the quasi-equilibrated 9kyr BP state,
the second and third transient runs used the 8.9 and 8.8 kyr
BP climates from the first transient run as initial conditions
at 9 kyr BP. Throughout the Holocene experiments, green-
house gas concentrations as well as aerosol and ozone dis-
tributions were kept at pre-industrial values as prescribed
by the protocol of the Paleoclimate Modelling Intercompar-
ison Project (PMIP), Phase II (Braconnot et al., 2007). Be-
sides, variations in the Sun’s output of energy and changes
in continental ice-sheets were ignored such that variations in
the orbital parameters were the sole external forcing in the
model simulations.
2.2 Model inter-comparison
In addition to our CCSM3 experiments, results from five
other Holocene transient climate model simulations are an-
alyzed here in order to study the evolution of the SWW
under insolation changes. These models are ECHO-G
(Lorenz and Lohmann, 2004; Wagner et al., 2007), COS-
MOS (Sect. 2.2.3), ECBilt-CLIO-VECODE (Renssen et al.,
2009) and CLIMBER2-LPJ (Kleinen et al., 2010). As in the
CCSM3 transient runs, all these models have been forced by
orbital variations only, keeping greenhouse gas concentra-
tions constant at their pre-industrial levels. A short and very
general overview of these simulations is given below and
detailed descriptions are available from the given references.
2.2.1 ECHO-G (I)
Holocene climate has been simulated using the cou-
pled atmosphere-ocean general circulation model ECHO-G
(Legutke and Voss, 1999). The atmospheric part of this
model is the fourth generation of the European Centre at-
mospheric model of Hamburg (ECHAM4, Roeckner et al.,
1996). The prognostic variables are calculated in the spec-
tral domain with a triangular truncation at wave number 30
(T30), which corresponds to a Gaussian longitude–latitude
grid of approximately 3.8◦. The vertical domain is repre-
sented by 19 levels. The ocean model includes a dynamic-
thermodynamic sea-ice model and is defined on a grid with
approximately 2.8◦resolution (with increased meridional
resolution of 0.5◦in the tropics to allow a more realistic
representation of the ENSO phenomenon) and 20 irregularly
spaced levels in the vertical. Acceleration by a factor of 10
has been applied to the orbital forcing in these experiments to
produce a two-member ensemble of transient Holocene runs
covering the last 7000yr (Lorenz and Lohmann, 2004).
2.2.2 ECHO-G (II)
Model and forcing are identical to ECHO-G (I), except for
the fact that there is no acceleration applied on the orbital
forcing for the Holocene transient run (Wagner et al., 2007).
Comparing the results of the non-accelerated ECHO-G (II)
experiment with those from the accelerated ECHO-G (I) al-
lows an assessment of the effect of orbital acceleration on the
Holocene simulation of the SWW.
2.2.3 COSMOS
The core of COSMOS consists of the atmosphere model
ECHAM5 (Roeckner et al., 2003) and the ocean model MPI-
OM (Marsland et al., 2003). For long-term integrations, a
low- resolution version of this model is applied with spectral
T31 (3.75◦transform grid) resolution in the atmosphere and
approximately 3◦horizontal resolution in the ocean. In the
vertical, atmosphere and ocean model grids are defined on
19 and 40 levels, respectively. The ocean model includes a
dynamic-thermodynamic sea-ice model with viscous-plastic
rheology. A dynamic vegetation module is coupled to the
land surface model JSBACH allowing an interactive adapta-
tion of the terrestrial biosphere to varying climate conditions
(Brovkin et al., 2009). Orbital acceleration with a factor of
10 has been applied to simulate the past 8000yr.
Besides the simulations with coupled general circulation
models described above, two Holocene runs with Earth sys-
tem Models of Intermediate Complexity (EMICs) are also
included in this study, they being, ECBilt-CLIO-VECODE
and CLIMBER2-LPJ.
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394 V. Varma et al.: Holocene evolution of the Southern Hemisphere westerly winds
Table 1. Brief summary of the climate models used for inter-comparison.
Model name Orbital acceleration Resolution
CCSM3 by a factor of 10; 3 member ensemble T31 – Atmosphere & Land: 3.75◦; 26 layers
Ocean & Ice: 3.6◦x1.6◦; 25 layers
ECHO-G (I) by a factor of 10; 2 member ensemble T30 – Atmosphere & Land: 3.8◦; 19 layers
Ocean & Ice: 2.8◦; 20 layers
ECHO-G (II) non-accelerated; 1 simulation T30 – Atmosphere & Land: 3.8◦; 19 layers
Ocean & Ice: 2.8◦; 20 layers
COSMOS by a factor of 10; 1 simulation T31 – Atmosphere & Land: 3.75◦; 19 layers
Ocean & Ice: 3◦; 40 layers
ECBilt-CLIO-VECODE non-accelerated; 1 simulation T21 – Atmosphere: 5.6◦; 3 layers
Ocean: 3◦; 20 layers
Atmosphere: 51◦x10◦
CLIMBER2-LPJ non-accelerated; 1 simulation Ocean: zonally averaged, with 2.5◦latitudinal
resolution; 11 layers
26
730
Figure 3. Trend in the annual mean low-level zonal wind in a) CCSM3, b) ECHO-G (I), c) 731
ECHO-G (II), d) COSMOS, e) ECBilt-CLIO-VECODE, and f) CLIMBER2-LPJ for the 732
period 7 kyr BP to 250 yr BP. All polar stereographic plots represent the Southern 733
Hemisphere, with latitudes placed at 10° intervals, starting from the equator to 90°S. 734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
Fig. 3. Trend in the annual-mean low-level zonal wind in (a) CCSM3, (b) ECHO-G (I), (c) ECHO-G (II), (d) COSMOS, (e) ECBilt-CLIO-
VECODE, and (f) CLIMBER2-LPJ for the period 7 kyr BP to 250yr BP. All polar stereographic plots represent the Southern Hemisphere,
with latitudes placed at 10◦intervals, starting from the equator to 90◦S.
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V. Varma et al.: Holocene evolution of the Southern Hemisphere westerly winds 395
2.2.4 ECBilt-CLIO-VECODE
The first EMIC transient run was carried out with version 3
of ECBilt-CLIO-VECODE. The atmospheric component is
ECBilt, a quasi-geostrophic model with 3 layers in the ver-
tical and T21 (∼5.6◦) horizontal resolution (Opsteegh et al.,
1998). CLIO is the oceanic component and consists of a free-
surface, primitive-equation ocean general circulation model
coupled to a dynamic-thermodynamic sea-ice model (Goosse
and Fichefet, 1999). CLIO is defined on 20 levels in the ver-
tical and has a 3◦horizontal resolution. VECODE interac-
tively simulates the dynamics of trees and grasses (Brovkin et
al., 2002). Orbital forcing without acceleration was applied
to simulate the past 9000yr.
2.2.5 CLIMBER2-LPJ
The second EMIC used in this inter-comparison is
CLIMBER2-LPJ (Petoukhov et al., 2000). This model con-
sists of a 2.5-dimensional statistical-dynamical atmosphere
with a resolution of approximately 51◦(longitude) by 10◦
(latitude), a zonally averaged ocean resolving three basins
with a latitudinal resolution of 2.5◦, and a sea-ice model.
CLIMBER2-LPJ also contains dynamic vegetation, oceanic
biogeochemistry, a model for marine biota, and a sediment
model (Archer, 1996; Brovkin et al., 2002, 2007). The
transient simulations were carried out with non-accelerated
orbital forcing for the past 8000 yr, keeping greenhouse
gas forcing fixed as in the other model experiments to
pre-industrial levels.
The model descriptions are summarized in Table 1. The
spatial distribution of the annual-mean SWW averaged over
the period 7 kyr BP to 250yr BP represented in various
models is given in Fig. 1 of the Supplement.
3 Results
In this section, we present the simulated insolation-forced
SWW Holocene trends for all climate models used for the
inter-comparison. As the strength and position of the SWW
are strongly related to sea-surface temperature (Brayshaw et
al., 2008; Lu et al., 2010; Chen et al., 2010), we will also
analyse the modelled trends in surface temperature. In order
to have a time period of comparison which is common for
all model simulations, all analyses have been done for the
period 7 kyr BP to 250yr BP. For CCSM3 and ECHO-G (I)
we have used the three-member and two-member ensemble
means, respectively.
3.1 Annual and seasonal mean trends in SWW
The spatial distribution of Holocene trends in the annual-
mean low-level zonal wind in the Southern Hemisphere for
the period 7 kyr BP to 250yr BP for all models is represented
in Fig. 3. The zonal wind trends are plotted at 850hPa for
CCSM3, ECHO-G (I and II) and COSMOS, and at the low-
ermost model level for ECBilt-CLIO-VECODE (800hPa)
and CLIMBER2-LPJ. All models exhibit a general trend of
strengthening in the southern and central SWW region and
a weakening trend in the northern part of the SWW belt,
which can be interpreted as a poleward displacement of the
annual-mean westerly circulation during the course of the
mid-to-late Holocene (Fig. 3). This spatio-temporal wind
pattern resembles a long-term trend of the Southern Annu-
lar Mode (or Antarctic Oscillation) towards its positive phase
(e.g. Thompson and Wallace, 2000; Sen Gupta and England,
2006). Strengthening of the SWW in the latitudinal belt be-
tween about 40◦S and 60◦S (i.e. the SWW core region) is
most intense and continuous in ECHO-G (I and II), followed
by CCSM3. While COSMOS shows a pronounced strength-
ening of the SWW in the region between ∼50◦S and 70◦S,
ECBilt-CLIO-VECODE simulates a less annular pattern,
but, with respect to the zonal mean, a strengthening in the
core SWW latitude belt is seen. CLIMBER2-LPJ produces
the weakest trends, probably due to its simplified dynamics
that does not explicitly simulate eddy momentum transports.
The simulated temporal evolution of the annual-mean
SWW in all models used for inter-comparison is represented
by an index and is displayed in Fig. 4. The index is defined as
the difference of the zonally averaged zonal low-level winds
between the latitudes 55◦S and 35◦S and is a measure for
latitudinal displacements of the SWW belt (Varma et al.,
2011). An evident trend observed in all the models is the
strengthening of the low-level winds towards 55◦S during
the course of the Holocene (Fig. 4). The strongest changes
occur during the mid-Holocene (4000 to 6000 yr BP) in al-
most all the models. Again, ECHO-G (I) and ECHO-G (II)
are very similar, CLIMBER2-LPJ follows the deterministic
insolation, CCSM3 and COSMOS show pronounced internal
variability for the last 3000yr.
The zonally averaged simulated Holocene trends in low-
level zonal winds separately for each season are represented
in Fig. 5 (see Figs. 2–5 of the Supplement for the maps of
seasonal trends in Southern Hemisphere zonal winds). For
the March-April-May (MAM) season, all models show the
most pronounced southward shift and strengthening of SWW
in the latitudinal belt between about 40◦S and 60◦S. Dur-
ing the June-July-August (JJA) season, CCSM3, ECHO-G
(I and II) and CLIMBER2-LPJ sustain the pattern of SWW
strengthening in that latitudinal belt, whereas ECBilt-CLIO-
VECODE exhibits a weakening in this region. The most
striking feature in Fig. 5 is the SWW behaviour during the
September-October-November (SON) season. This season
shows the trend of a SWW weakening (between the latitudes
∼40◦S and 60◦S) and a northward shift in all the models,
i.e. opposite to the annual-mean trend.
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396 V. Varma et al.: Holocene evolution of the Southern Hemisphere westerly winds
Fig. 4. Temporal evolution of annual-mean SWW position during the period 7kyr BP to 250yr BP in (a) CCSM3, (b) ECHO-G (I), (c)
ECHO-G (II), (d) COSMOS, (e) ECBilt-CLIO-VECODE, and (f) CLIMBER2-LPJ, defined in terms of the difference between the latitudes
55◦S and 35◦S (southern and northern parts of the SWW belt respectively) of the zonally averaged low-level zonal winds (black curves).
The time axis is plotted against the anomaly of the mean wind position. A 1000 yr boxcar smoothing with respect to the orbital year has been
applied to all the time series except for CLIMBER2-LPJ. Linear regression lines for the unsmoothed time series are shown in red. Note the
different ordinate scales.
Fig. 5. Zonally averaged seasonal and annual mean trends in the low-level zonal wind in (a) CCSM3, (b) ECHO-G (I), (c) ECHO-G (II), (d)
COSMOS, (e) ECBilt-CLIO-VECODE, and (d) CLIMBER2-LPJ for the Southern Hemisphere for the period 7kyr BP to 250 yr BP. Note
the different ordinate scales.
3.2 Annual and seasonal mean trends in surface tem-
perature
The spatial distributions of Holocene trends in the Southern
Hemisphere annual-mean surface temperature for the period
7 kyr BP to 250yr BP is shown in Fig. 6. The most notice-
able trend pattern in all models relates to an intense cool-
ing in the southern high latitudes especially around Antarc-
tica. In low latitudes, the temperature trend patterns are more
heterogeneous among the different models. For instance,
CCSM3 exhibits a large-scale (albeit weak) tropical warming
trend, while ECHO-G (II) shows more of a tropical cooling
(Fig. 6c).
The seasonal response pattern of Holocene surface tem-
perature trends in the Southern Hemisphere caused by vari-
ations in orbital forcing is more entangled. The zonally av-
eraged trends in the surface temperature on a seasonal basis
as simulated by the different models are displayed in Fig. 7
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V. Varma et al.: Holocene evolution of the Southern Hemisphere westerly winds 397
29
795
796
Figure 6. Trend in the annual mean surface temperature in a) CCSM3, b) ECHO-G (I), c) 797
ECHO-G (II), d) COSMOS, e) ECBilt-CLIO-VECODE, and f) CLIMBER2-LPJ for the 798
period 7 kyr BP to 250 yr BP. 799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
Fig. 6. Trend in the annual-mean surface temperature in (a) CCSM3, (b) ECHO-G (I), (c) ECHO-G (II), (d) COSMOS, (e) ECBilt-CLIO-
VECODE, and (f) CLIMBER2-LPJ for the period 7 kyr BP to 250 yr BP.
(see Figs. 6–9 of the Supplement for the Southern Hemi-
sphere maps of seasonal trends in surface temperature). Aus-
tral summers (DJF) experience a lower-than-present insola-
tion during the early Holocene (Fig. 2) resulting in a gen-
eral warming trend in the Southern Hemisphere during the
course of the Holocene, which is most pronounced over the
continents (Fig. 8 in the Supplement). By contrast, the aus-
tral winter season (JJA) shows strong cooling trends over
the Southern Hemisphere continents as a direct response
to decreasing insolation (Fig. 6 in the Supplement). The
MAM and SON seasons exhibit the most uniform trends on
a hemispherical scale over both Southern Hemisphere land
and ocean in all the models (Figs. 5 and 7 in the Supple-
ment). Among all the seasons, the austral spring (SON)
shows the strongest seasonal cooling trend, whereas the aus-
tral fall (MAM) exhibits the strongest seasonal warming
trend (Fig. 7) as a result of insolation changes in combination
with a delayed response of the climate system by 1–3 months
owing to the thermal inertia of the surface ocean (cf. Renssen
et al., 2005). However, even during the MAM season, the
Southern Ocean regions around Antarctica show a cooling
trend, opposite to what would be expected from the local in-
solation trend (Fig. 2). This regional cooling trend has been
attributed to the long memory of the Southern Ocean through
the storage of late winter-spring surface temperature anoma-
lies in the deep upper-ocean winter layer in combination with
sea ice-albedo and ice-insulation feedbacks (Renssen et al.,
2005). While the study of Renssen et al. (2005) is based on
a single coupled model, our multi-model inter-comparison
supports their results and reveals that this is a robust fea-
ture captured by all models. As a result, all models show
a year-round Holocene cooling trend in the Southern Ocean
(Figs. 6, 7).
4 Discussion
The Southern Hemisphere surface westerlies mainly result
from the convergence of transient eddy momentum fluxes
acting against losses by surface friction. The eddies, in
turn, are driven by the potential energy available from tro-
pospheric temperature gradients (e.g. Lorenz, 1955; Lindzen
and Farrell, 1980) that are ultimately caused by the merid-
ional gradient in incoming solar radiation. Almost 70 % of
the shortwave radiation that enters the atmosphere and is not
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398 V. Varma et al.: Holocene evolution of the Southern Hemisphere westerly winds
Fig. 7. Zonally averaged seasonal and annual mean trends in the surface temperature in (a) CCSM3, (b) ECHO-G (I), (c) ECHO-G (II), (d)
COSMOS, (e) ECBilt-CLIO-VECODE, and (d) CLIMBER2-LPJ for the Southern Hemisphere for the period 7kyr BP to 250 yr BP. Note
the different ordinate scales.
Fig. 8. Zonally averaged annual-mean trends in the Southern Ocean
upwelling (based on ocean vertical velocity at 30m depth) for
ECHO-G (II) (blue), COSMOS (red) and CCSM3 (green) for the
period 7 kyr BP to 250 yr BP. Positive trends indicate strengthening
of upwelling.
reflected back to space is absorbed at the surface (e.g. Kiehl
and Trenberth, 1997). Varying insolation has therefore a di-
rect effect on SST which, in turn, may influence strength and
position of the SWW by affecting baroclinic eddy growth
and momentum flux convergence through changes in tropo-
spheric meridional temperature gradients and static stability
(Brayshaw et al., 2008; Lu et al., 2010; Chen et al., 2010).
The model results presented in our study consistently sug-
gest that the annual and seasonal mean SWW exhibit an
overall strengthening and poleward shifting trend during the
course of the mid-to-late Holocene under the influence of
orbital forcing, except for the austral spring season (SON),
where the SWW exhibit an opposite trend of shifting towards
the equator (Fig. 5). During the SON season, the trend in
insolation-forcing (Fig. 2) leads to a global SST cooling trend
which may explain the equatorward displacement of the mid-
latitude winds. By means of general atmospheric circulation
modelling and scaling arguments, it has recently been shown
that a decrease in the global surface temperature reduces the
latitudinal extent of the Hadley cell (Frierson et al., 2007)
and shifts the eddy-driven westerlies towards the equator (Lu
et al., 2010).
However, the most pronounced surface temperature pat-
tern that could be noted in the annual-mean of the Holocene
simulations is the strong cooling trend in the southern high
latitudes, especially around Antarctica (Fig. 6). Ice cores in-
deed provide evidence for a widespread Antarctic Holocene
cooling trend (Masson et al., 2000) underpinned by palaeo-
climate reconstructions from the Ross Sea (Steig et al., 1998)
and the Palmer Deep (Domack et al., 2001). The south-
ern high-latitude cooling trend results in a steepening of the
pole-to-equator surface temperature gradient. Theoretical
and modelling studies have shown that an enhanced merid-
ional surface temperature gradient affects the position of the
westerlies (Brayshaw et al., 2008; Lu et al., 2010; Chen et
al., 2010). An increasing meridional SST gradient – espe-
cially in the mid-latitudes between 40◦and 50◦S – results
in a poleward shift of the eddy-driven zonal winds (Chen et
Clim. Past, 8, 391–402, 2012 www.clim-past.net/8/391/2012/

V. Varma et al.: Holocene evolution of the Southern Hemisphere westerly winds 399
al., 2010). This may explain the overall poleward shifting
trend of the SWW that prevails in the annual-mean in all the
models. During the MAM season, increasing insolation dur-
ing the austral late summer with highest values in low lati-
tudes (i.e. increasing meridional insolation gradient; Fig. 2)
in combination with the 1–3 months time lag, owing to the
thermal inertia of the surface climate system (e.g. Renssen et
al., 2005) leads to a further increase in the meridional tem-
perature gradient (in particular in the mid-latitudes between
40◦and 50◦S; Fig. 7) as well as to a global SST warming
trend. Therefore, the poleward shifting trend of the SWW is
strongest during the austral fall.
Our findings for a poleward shift of SWW from the early-
mid Holocene to the present are largely consistent with a
recent study by Rojas and Moreno (2010), who analyzed
a multi-model-mean of PMIP2 simulations for 6 kyr BP.
They found an enhanced annual-mean westerly flow between
∼35◦S and 45◦S and a weakening south of ∼45◦S for the
mid-Holocene time slice relative to the present.
In order to assess the influence of the simulated Holocene
SWW trends on Southern Ocean upwelling, we analyzed the
trends in annual-mean upper-ocean vertical velocity in the
comprehensive climate models (the EMICs were not consid-
ered here because of their relatively weak SWW trends). Fig-
ure 8 shows that the annual-mean poleward shifting SWW
trend in all the models is accompanied by a positive trend in
upwelling south of ∼55◦S. Whether the simulated trends in
Southern Ocean upwelling had the potential to significantly
affect atmospheric CO2concentrations through degassing of
the deep ocean (cf. Moreno et al., 2010) remains unclear
without implementation of carbon cycle models. We further
note that the degree of realism to which non-eddy resolv-
ing ocean models simulate the upwelling response to SWW
changes is under debate (e.g. B¨
oning et al., 2008; Meredith
et al., 2012).
Validating the model results with reconstructions of the
paleo-SWW proves still to be elusive, as there is a substan-
tial incongruity between different proxy records. For the
SWW core region around 51◦–53◦S, for instance, terrestrial
ecosystem proxy records from western Patagonia (Moreno et
al., 2010) suggest a trend of increasing SWW strength dur-
ing the past 7000 yr that is not supported by sedimentological
and pollen-based reconstructions of South Patagonian pre-
cipitation by Lamy et al. (2010). As a cautionary note, we
emphasize again that the model simulations suggest opposite
Holocene trends in SWW strength and position for different
seasons (Fig. 5) which may affect the proxy records and their
interpretation.
In view of a substantial incongruity between different
SWW simulations for the Last Glacial Maximum (Rojas et
al., 2009), the agreement among the different models with
respect to Holocene SWW trends is encouraging. One rea-
son for this outcome may be the relatively simple forcing
(insolation only) in the Holocene experiments, whereas the
forcing for the Last Glacial Maximum simulations addition-
ally includes atmospheric greenhouse gases and continental
ice-sheets with potentially opposing effects on the SWW via
tropospheric and middle atmosphere temperatures and tem-
perature gradients, static stability, tropopause height, ocean
circulation, etc. (e.g. Shindell and Schmidt, 2004; Lorenz
and deWeaver, 2007; Toggweiler and Russell, 2008; Lu et
al., 2010; Lee et al., 2011).
5 Conclusions
The investigation of the temporal and spatial evolution of
the SWW along with forcings and feedbacks remains a sig-
nificant challenge in climate research. In this study, we
examined the Holocene evolution of SWW under the in-
fluence of orbital forcing with transient experiments using
the state-of-the-art comprehensive coupled global climate
model CCSM3. In addition, a model inter-comparison has
been conducted using Holocene transient simulations from
four other coupled global climate models, namely, ECHO-G,
COSMOS, ECBilt-CLIO-VECODE and CLIMBER2-LPJ.
Analyses and comparison of the model results suggest that
the annual and seasonal mean SWW were subject to an over-
all strengthening and poleward shift during the course of the
mid-to-late Holocene under the influence of orbital forcing,
except for the austral spring season, where the SWW exhib-
ited an opposite trend of shifting towards the equator. The
magnitude of the SWW shift is much smaller in the EMICs
compared to the comprehensive general circulation models
such that the potential feedbacks in their climate/carbon cy-
cle simulations may be underestimated. The comparison be-
tween an accelerated and a non-accelerated ECHO-G exper-
iment revealed that the simulation of the analyzed trends is
unaffected by the orbital acceleration technique employed in
some of the transient runs.
Whether the simulated shifts in the SWW had the poten-
tial to significantly affect Holocene atmospheric CO2con-
centrations through degassing of the deep ocean via changes
in wind-driven upwelling in the Southern Ocean (Moreno
et al., 2010) remains elusive for the time being. Moreover,
the effect of increasing greenhouse gases from the mid to
the late Holocene (e.g. Raynaud et al., 2000) is not included
in the orbital-forced model simulations presented here, al-
though there is strong evidence for a CO2-induced strength-
ening and poleward shift of the SWW over the past four
decades (e.g. Arblaster and Meehl, 2006; Toggweiler and
Russell, 2008). In future studies, the combined effects of
orbital and greenhouse gas forcing should be explored us-
ing comprehensive climate models in order to put the South-
ern Hemisphere circulation changes of the last decades into
a long-term context.
www.clim-past.net/8/391/2012/ Clim. Past, 8, 391–402, 2012

400 V. Varma et al.: Holocene evolution of the Southern Hemisphere westerly winds
Supplementary material related to this
article is available online at:
http://www.clim-past.net/8/391/2012/
cp-8-391-2012-supplement.pdf.
Acknowledgements. We would like to thank the two anonymous
reviewers for their constructive comments and suggestions. This
work was funded through the DFG (Deutsche Forschungsgemein-
schaft) Priority Programme “INTERDYNAMIK” and through
the DFG Research Center/Excellence Cluster “The Ocean in the
Earth System”. CCSM3 simulations were performed on the SGI
Altix supercomputer of the Norddeutscher Verbund f¨
ur Hoch- und
H¨
ochstleistungsrechnen (HLRN). We also acknowledge the use of
the NCAR Command Language (NCL) and NOAA/PMEL’s Ferret
in our data analysis and visualization herein.
Edited by: M. Siddall
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