Download Simulated changes in the extratropical Southern Hemisphere winds

GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L06701, doi:10.1029/2005GL025332, 2006
Simulated changes in the extratropical Southern Hemisphere winds
and currents
John C. Fyfe1 and Oleg A. Saenko1
Received 28 November 2005; revised 9 January 2006; accepted 30 January 2006; published 16 March 2006.
[1] The results from 12 global climate models show a
remarkably consistent strengthening and poleward shifting
of the zonal wind stress through the 20th and 21st centuries
at extratropical Southern Hemisphere latitudes. Changes in
the zonal circulation of the ocean in the region are broadly
consistent with the changes in zonal wind stress. In
particular, the climate models simulate a strengthening
and a poleward shift of the Antarctic Circumpolar Current.
The strengthening of the zonal wind stress also results in
intensifying northward Ekman transport across the Antarctic
Circumpolar Current which, in the unblocked latitudes of
Drake Passage, implies increasing southward geostrophic
transport in the ocean below about 2000 m. Zonal wind
stress changes such as these may be expected to enhance the
mesoscale eddy activity in the Southern Ocean.
Citation: Fyfe, J. C., and O. A. Saenko (2006), Simulated
changes in the extratropical Southern Hemisphere winds and
currents, Geophys. Res. Lett., 33, L06701, doi:10.1029/
2005GL025332.
1. Introduction
[2] With a volume transport of 130 – 140 Sv (1 Sv =
106 m3/s), the Antarctic Circumpolar Current (ACC) is the
most powerful current in the world ocean. It connects all the
major ocean basins, effectively exchanging heat, salt, and
other tracers between them, and has an important effect on
the Earth’s climate. The absence of land at the latitudes of
the Drake Passage means that the net northward ageostrophic mass transport can only be balanced by geostrophic
flows in the deep ocean, which in turn connects the
Southern Ocean to the other ocean basins around the world
[Gnanadesikan, 1999].
[3] In two recent studies [Fyfe and Saenko, 2005; Saenko
et al., 2005] we have examined the changes in the Southern
Ocean circulation simulated in response to increasing concentrations of atmospheric greenhouse gases (GHGs) using
the Canadian Centre for Climate Modelling and Analyses
(CCCma) global climate model (GCM). Our major conclusion was that as part of the simulated warming mid-latitude
zonal winds in the Southern Hemisphere (SH) strengthen
and shift poleward. This translates into changes in the
strength and position of the ACC, and in the mass exchange
across the ACC. Here we confirm and expand on these
results for an ensemble of GCMs.
1
Canadian Centre for Climate Modelling and Analysis, Meteorological
Service of Canada, Victoria, British Columbia, Canada.
Copyright 2006 by the American Geophysical Union.
0094-8276/06/2005GL025332$05.00
2. Data and Methodology
[4] We analyze decadal mean observed and simulated SH
zonal wind stress tx(l, f) and oceanic barotropic stream
function (l, f) (l is longitude and f is latitude). We
consider data from NCEP2 and ERA40 reanalyses, as well
as output from 12 GCMs. The model output was downloaded from an archive hosted by the Program for Climate
Model Diagnosis and Intercomparison (PCMDI). The models used are: CCCMA-CGCM3-1 (1), GFDL-CM2-0
(2), GFDL-CM2-1 (3), INMCM3-0 (4), MIROC3-2MEDRES (5), MRI-CGCM2-3-2A (6), CNRM-CM3
(7), IPSL-CM4 (8), MIUB-ECHO-G (9), MPI-ECHAM5
(10), NCAR-CCSM3-0 (11) and GISS-MODEL-E-R (12).
Documentation for the models is available on the PCMDI
website located at http://www-pcmdi.llnl.gov. We consider
pre-industrial control runs, 20th century runs (with historical GHG, aerosol, and in some cases, volcanic, and solar
forcing) and 21st century runs following the Intergovernmental Panel on Climate Change (IPCC) A2 emissions
scenario.
[5] The treatment of (l, f) is as follows. At each
longitude in the SH (l, f) is fitted to an error function
[Marquardt, 1963]. This leads to an approximation for the
vertically integrated zonal current U = @/@(Ref):
ðf FU ðlÞÞ2
U ðl; fÞ hU ðlÞ exp s2U ðlÞ
!
ð1Þ
where Re is the earth’s radius. In this way U(l, f) can
usefully be described in terms of it’s strength hU(l),
position
FU(l) and width sU(l). Similarly, after fitting
R
we obtain tx(l, f) ht(l)
f tx df to an error2 function
exp((f Ft(l)) /s2t(l)). Figure 1 shows the ERA40
climatological zonal wind stress before fitting (left) and
after fitting (right). The fitted field is a good approximation:
capturing the strength and position of the maxima in the
Indian Ocean (near 90E), south of New Zealand (near
180E) and off the tip of South America (near 90W).
[6] We first consider the extent to which these climate
models reproduce the present-day zonal wind stress climatology. Figure 2 shows the NCEP2, ERA40 and GCM-mean
climatological wind stress strength ht(l) (top) and position
Ft(l) (bottom). The GCM mean profile captures the maxima in the Indian Ocean and off the tip of South America
but misses the observed maximum south of New Zealand.
(The dark shading shows the 2.5% to 97.5% percentile
intervals assuming normally distributed GCM parameter
values.) The most notable model error is in the Pacific
sector where the wind stress strength and position are
significantly underestimated by most of the models. The
GCM mean (denoted with curly brackets) zonal mean
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Figure 1. Climatological (1991 – 2000) ERA40 zonal
wind stress tx (left) before fitting and (right) after fitting.
The outermost contour is 0.05 Pa and the contour interval in
0.025 Pa.
(denoted with square brackets) strength, position and width
are {[ht]} 0.18 ± 0.02 Pa, {[Ft]} 48.6 ± 1.6 and
{[st]} 11.5 ± 0.6 respectively. (Here, and henceforth the
interval will indicate the 2.5% to 97.5% percentile intervals
for the parameter values.) Clearly, the simulated climatological wind stress is systematically too weak (by about
10%), too equatorward (by about 4) and too narrow (by
about 1) relative to the reanalyses. We shall keep these
model biases in mind as we continue forward with the
analysis.
3. Results
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simulated zonal wind stress increases by about 25%. Despite minor local differences in the response, all of the
models predict a circumpolar strengthening and poleward
shifting of the mid-latitude zonal wind stress over the
Southern Ocean, as shown in Figure 3 for a subset of the
GCMs. The subset is for those GCMs for which control,
20th century and A2 scenario output is available for (l,
f). Locally, the zonal wind stress in the Pacific sector
displays particularly large changes: strengthening by about
40%, shifting poleward by 3.5 and narrowing by 1.5.
[ 8 ] In summary, the GCMs simulate a consistent
strengthening and poleward shifting of the zonal wind stress
over the Southern Ocean through the 20th and 21st centuries. We now consider to what degree these simulated
changes in the zonal wind stress translate into changes in
the Southern Ocean circulation.
3.2. Southern Ocean Zonal Circulation Changes
[9] A complete theory capable of predicting how the
ACC responds to changes in zonal wind stress is not
available. However, a simple theoretical argument based
on residual-mean theory [Rintoul et al., 2001; Karsten et al.,
2002; Marshall and Radko, 2003] can be made to suggest
that strengthening and poleward shifting of the zonal wind
stress over the Southern Ocean should result in strengthening and poleward shifting of the ACC [Fyfe and Saenko,
2005]. However, the simple theory is complicated by the
fact that in reality the pathway of the ACC is constrained by
ocean bottom topography. In addition, global warming
involves other changes to the surface climate (e.g., sea-ice
3.1. Zonal Wind Stress Changes
[7] From the pre-industrial period to the end of the 20th
century the simulated changes in the zonal wind stress
parameters are D{[ht]} 0.009 ± 0.004 Pa, D{[Ft]} 0.87 ± 0.27 and D{[st]} 0.17 ± 0.20, respectively.
In words, the GCMs indicate a modest, yet statistically
significant, wind stress strengthening and poleward shifting
over the 20th century. Toward the end of the 21st century, as
the concentration of atmospheric GHGs increases, these
changes are projected to become much more pronounced.
The simulated 20th through 21st century change in the
zonal wind stress parameters are D{[ht]} 0.038 ±
0.008 Pa, D{[Ft]} 2.8 ± 0.9 and D{[st]} 0.7 ±
0.5, respectively. In percentage terms, the strength of the
Figure 2. Climatological (1991 – 2000) (top) zonal wind
stress strength (ht(l)) and (bottom) position (Ft(l)). The
dark shading shows the 2.5% to 97.5% percentile intervals
assuming normally distributed GCM parameter values.
Figure 3. Simulated pre-industrial (1851– 1860) and endof-the 21st century (2091 – 2100) (left) profiles of zonal
wind stress strength (ht(l)) and (right) position (Ft(l)) for
a subset of the 12 GCMs. The subset is for those GCMs for
which control, 20th century and A2 scenario output is
available for (l, f). Figure 3 (left) shows that tx
strengthens so the pre-industrial profiles are below the
21st century profiles, with red indicating strengthening.
Figure 3 (right) shows that tx shifts southward so the preindustrial profiles are above the 21st century profiles, with
blue indicating poleward shifting. The encircled numbers on
the left correspond to the GCMs listed in the Introduction.
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Figure 4. As in Figure 3 but for (left) the depth integrated
zonal current strength (hU(l)) and (right) position (FU(l)).
melt) which may also affect the Southern Ocean circulation.
The theory also assumes steady state conditions while the
simulated climate system is evolving in time.
[10] Despite its limitations the residual-mean theory helps
explain the simulated response of the ACC to global
warming. In particular, as the theory suggests the GCM
mean vertically-integrated zonal current strengthens and
shifts poleward. Specifically, the 20th through 21st century
change in the GCM mean and zonal mean current strength,
position, and width are D{[hU]} 23.6 ± 8.5 m2/s,
D{[FU]} 1.0 ± 0.9 and D{[sU]} 1.8 ± 0.9,
respectively. We note the relatively small shift but pronounced narrowing. Figure 4 shows the pre-industrial and
the end-of-the 21st century profiles of zonal current strength
hU (left) and position FU (right) for the GCMs for which output is available. A comparison of Figure 4 with Figure 3
indicates that the changes in the depth-integrated zonal
current are broadly consistent with the changes in zonal
wind stress.
3.3. Southern Ocean Meridional Circulation Changes
[11] Simple dynamical considerations indicate that the
simulated changes in zonal wind stress must translate into
changes in the meridional circulation in the Southern Ocean.
We consider the time mean, zonally and vertically
integrated approximate momentum balance in the Southern
Ocean between meridional Ekman transport and geostrophic
transport [Munk and Palmen, 1951; Gille, 1997; Rintoul et
al., 2001]:
I
tx
dx ¼ ro f
¼
I Z
I Z
0
H
1 @p
dzdx
ro f @x
zonal pressure gradient), ro is the reference density of sea
water, f is the Coriolis parameter, and H is the depth of the
ocean. This balance links the Southern Ocean with the other
ocean basins, and is consistent with findings from highresolution ocean models over a broad range of southern
latitudes [Gille, 1997]. The GCM mean 20th through 21st
century change in the meridional Ekman transport (i.e., the
left-hand-side of (2)) along the path of maximum tx/rof
(located at Q = Ft s2t cot Q) is 4.3 ± 1.6 Sv. This increase
would be more than twice as large (i.e., to match the
25% increase in the zonal wind stress strength) if not for
the poleward shift of tx. To see this consider that the
Ekman transport M along a constant latitude path of
length L is ([tx]/ro f )L, from which it follows that DM/
M D[tx]/[tx] Df/f + DL/L. For a poleward shift Df/f
> 0 and DL/L < 0.
[12] From the point of view of wind-driven changes in
the deep ocean circulation, it is of particular interest to
consider the unblocked latitudes of Drake Passage (between
55 and 62). In this region, the zonally integrated
pressure gradient everywhere above the shallowest ocean
bottom topography is zero. This implies, according to
(2) and (3), that a strengthened northward Ekman transport
must eventually result in strengthened southward geostrophic currents in the deep ocean (i.e., below about
2000 m). From Figure 5, showing the GCM mean profiles
of ([tx]/ro f )L for pre-industrial and end-of-21st century
times, we conclude that the deep southward geostrophic
transport must increase in the darkly shaded (i.e.,
unblocked) latitudes by as much as 5 – 10 Sv. This increasing southward transport in the deep Southern Ocean is much
more robust than the decreasing southward transport simulated in the deep western boundary current of the North
Atlantic [e.g., Cubasch et al., 2001]. This, of course, does
not mean that there is any contradiction between these two
results. The water for the intensified deep southward transport in the Southern Ocean need not be supplied from the
North Atlantic alone.
[13] High resolution ocean models (R. Hallberg and
A. Gnanadesikan, The role of eddies in determining the
structure and response of the wind-driven Southern Hemisphere overturning: Results from the Modeling Eddies in the
Southern Ocean project, submitted to Journal of Physical
Oceanography, 2005) indicate that zonal wind stress
changes such as these may affect the oceanic mesoscale
eddy activity. Indeed, changes in the eddy-induced circulation could potentially negate the direct response of the
residual overturning circulation on isopycnal surfaces.
ð2Þ
0
vg dzdx
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ð3Þ
H
where p is pressure, vg is meridional geostrophic velocity
(i.e., the portion of the meridional velocity explained by the
Figure 5. GCM mean profiles of Ekman transport
([tx]/rof)L for pre-industrial times (1851– 1860: blue), the
end-of-the 21st century times (2091 –2100: red) and their
difference (black). The dark-shading indicates the unblocked latitudes of Drake Passage. Note that this
calculation is for the original unfitted profiles of tx.
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However, a satisfactory estimate of these meso-scale eddy
processes is unobtainable in the GCMs considered here, all
of which parameterize the effects of the mesoscale eddies on
the large-scale circulation.
4. Summary and Discussion
[14] An ensemble of 12 global climate models simulate a
consistent strengthening, poleward shift and narrowing of
the zonal wind stress over the Southern Ocean through the
20th and 21st centuries. However, as a group the GCMs
produce a present-day zonal wind stress climatological
distribution which is too weak, too equatorward and too
narrow relative to reanalysis. Simulated changes in the
Southern Ocean zonal circulation which are associated with
the changes in zonal wind stress include a strengthening,
poleward shift and narrowing of the Antarctic Circumpolar
Current. We infer, based on balance considerations, that
intensifying northward Ekman transport across the Antarctic
Circumpolar Current is balanced by increasing southward
transport in the deep ocean below about 2000 m. Recent
high-resolution ocean model results suggests that zonal
wind stress changes such as reported here enhance the
mesoscale eddy activity in the Southern Ocean. Finally,
we note the importance that changing Southern Hemisphere
winds and eddies may have on the oceanic uptake of
anthropogenic carbon [Mignone et al., 2006].
[15] Acknowledgments. We acknowledge the modeling groups for
providing their data, the PCMDI for collecting and archiving the data, the
JSC/CLIVAR Working Group on Coupled Modelling (WGCM) and their
Coupled Model Intercomparison Project (CMIP) and Climate Simulation
Panel for organizing the analysis activity, and the IPCC WG1 TSU for
technical support. The IPCC Data Archive at Lawrence Livermore National
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Laboratory is supported by the Office of Science, U.S. Department of
Energy. We also thank George Boer and Ken Denman for their insightful
comments.
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J. C. Fyfe and O. A. Saenko, Canadian Centre for Climate Modelling and
Analysis, Meteorological Service of Canada, University of Victoria,
Victoria, P.O. Box 1700, BC,Canada V8W 2Y2. ([email protected])
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