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Historical trends in the jet streams
Cristina L. Archer
1
and Ken Caldeira
1
Received 12 February 2008; revised 10 March 2008; accepted 14 March 2008; published 18 April 2008.
[1] Jet streams, the meandering bands of fast winds located
near the tropopause, are driving factors for weather in the
midlatitudes. This is the first study to analyze historical
trends of jet stream properties based on the ERA-40 and the
NCEP/NCAR reanalysis datasets for the period 1979 to
2001. We defined jet stream properties based on mass and
mass-flux weighted averages. We found that, in general, the
jet streams have risen in altitude and moved poleward in
both hemispheres. In the northern hemisphere, the jet stream
weakened. In the southern hemisphere, the sub-tropical jet
weakened, whereas the polar jet strengthened. Exceptions to
this general behavior were found locally and seasonally.
Further observations and analysis are needed to confidently
attribute the causes of these changes to anthropogenic
climate change, natural variability, or some combination of
the two.
Citation: Archer, C. L., and K. Caldeira (2008),
Historical trends in the jet streams, Geophys. Res. Lett., 35,
L08803, doi:10.1029/2008GL033614.
1. Introduction
[2] Jet streams are narrow bands of fast, meandering air
currents that flow around the globe near the tropopause
level in both hemispheres. They are often classified in two
categories: sub-tropical jets, found at the poleward margin
of the upper branch of the Hadley circulation, and polar jets,
located above the polar-frontal zone, a region of sharp
thermal contrast between cold polar air and warm tropical
air [Holton, 1992; Bluestein, 1993].
[
3] Jet streams are important bec ause synoptic scale
disturbances tend to form in the regions of maximum jet
stream wind speed and to propagate downstream along
storm t racks that follow the jet axes [Holton, 1992].
Changes in jet stream location, intensity, or altitude can
therefore cause variations in frequency and intensity of
storms. Also, jet streams inhibit formation and development
of hurricanes, which preferentially develop in low-shear
regions of the atmosphere [Gray, 1968; Vecchi and Soden,
2007]. They affect air transport not only because of their
high winds, but also because of the clear-air turbulence
associated with jet cores [Bluestein , 1993].
2. Data
[4] To study if and how the jet streams have changed in
the past few decades, we used two reanalyses of historical
weather data: ERA-40 [Uppala et al., 20 05], from the
European Centre for Medium-Ra nge Weather Forecasts,
which covers 44 years from 1958 to 2001, and NCEP/
NCAR [Kalnay et al., 1996; Kistler et al., 1999], from the
National Centers for Environmental Protection and the Na-
tional Center for Atmospheric Research, covering the years
from 1948 to 2006. Monthly averages of zonal and merid-
ional (u and v respectively) wind velocity components were
available at 2.5 degrees hori zontal resolution and with
6 vertical levels between 400 and 100 hPa. Whereas
conventional observations (e.g., upper-air winds, tempera-
ture, and humidity from radiosondes; surface data from
various land and buoy networks; ocean wave heights) were
assimilated in both datasets throughout the entire periods,
satellite-borne observations (e.g., infrared and microwave
radiances; total and column ozone; surface-pressure and
winds over ocean) were only assimilated from 1979 on.
For this reason, this study will focus only on the period
1979-2001. Despite limita tions [Pawson and Fiorino ,
1999], the ERA-40 and NCEP/NCAR datasets are the best
sets of reanalyzed weather data available [Uppala et al.,
2005].
3. Methods
[5] In bot h hemispheres, the jet streams are located
between the 400 and the 100 hPa levels. The Northern
Hemisphere (NH) jet has a single-band spiral-like structure,
generally beginning south of the Canary Islands and ending
one eastward circumnaviga tion later over England. The
Southern Hemisphere (SH) jet has a more c oncentric
structure, with a persistent ring around Antarctica, hereafter
referred t o as Southern Hemisphere Polar (SHP) jet, and a
seasonally varying second ring at about 30 S, hereafter
referred to as the Southern Hemisphere sub-Tropical (SHT)
jet [Koch et al., 2006].
[
6] Jet streams are not continuous, but rather fragmented,
meandering, and with notable wind speed and elevation
variations. As such, the task of clearly identifying jet stream
boundaries at a given time can be difficult and ambiguous
[Koch et al., 2006]. To overcome this problem, we define jet
stream properties via integrated quantities, which are more
numerically stable and less grid-dependent than are simple
maxima and minima.
[
7] First, for each horizontal grid point in the reanalyses,
we define the mass weighted average wind speed between
400 and 100 hPa (WS) as:
WS
i; j
¼
P
k¼100hPa
k¼400hPa
m
k
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
u
2
i; j; k
þ v
2
i; j; k
q
P
k¼100hPa
k¼400hPa
m
k
; ð1Þ
where u
i,j,k
and v
i,j,k
are the monthly-average horizontal
wind components at grid point (i,j,k), and m
k
is the mass at
level k. Figure 1a shows the 23-year average of WS,
GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L08803, doi:10.1029/2008GL033614, 2008
Click
Here
for
Full
A
rticl
e
1
Department of Global Ecology, Carnegie Institution of Washington,
Stanford, California, USA.
Copyright 2008 by the American Geophysical Union.
0094-8276/08/2008GL033614$05.00
L08803 1of6
hereafter referre d to as the jet stream wind speed,
obtained from the ERA-40 dataset for all grid points
(the pattern of WS from NCEP/NCAR is almost identical
and is shown in the auxiliary material Figure S1).
1
Figure
1a shows wind maxima to the East of the continents in
the Northern Hemisphere, and to the East of Australia
and to the South of Africa in the Southern Hemisphere,
as observed [Koch et al., 2006]. Seasonal differences
between the two hemispheres were also found (Figures 1c
and 1e). The NH jet forms a nearly continuous band
between northern Africa and Hawaii in DJF (Figure 1),
but it shifts northward, fragments, and weakens in JJA. In
the SH, the Polar jet is omnipresent in all seasons, with
wind speed maxima south of Africa; the SH sub-Tropical
jet is only present in JJA as a nearly continuous band
between Australia and South America.
[
8] Given mass and wind speed at each pressure level z
between 400 and 100 hPa, for each horizontal grid cell, the
mass-flux weighted pressure P is defined as:
P
i; j
¼
P
k¼100hPa
k¼400hPa
m
k
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
u
2
i; j; k
þ v
2
i; j; k
q
p
k
P
k¼100hPa
k¼400hPa
m
k
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
u
2
i; j; k
þ v
2
i; j; k
q
; ð2Þ
where p
k
is the pressure at level k. P represents the average
pressure of flows near the tropopause, and therefore the
average altitude of these flows. Because jet streams are
found near the tropopause, we can use P to characterize the
height of both the jet streams and the tropopause. In both
hemispheres, the jet streams are lower (and closer to the
poles) in the summer than they are in the winter, but the NH
jets are generally lower than the SH jets (Figures 2a, 2c, and
2e for the ERA-40 and auxiliary material Figure S2 for the
NCEP/NCAR). In the NH, the jets are lowest downwind of
their wind speed maxima, whereas in the SH the jets are
lowest where they are fastest.
[
9] Given the total mass-flux between 400 and 100 hPa,
we calculate the mass-flux weighted latitude in the NH for
each longitude i in the gridded reanalysis fields as follows:
L
NH
i
¼
P
j¼70N
j¼15N
P
k¼100hPa
k¼400hPa
m
k
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
u
2
i; j; k
þ v
2
i; j; k
q
8
i; j
P
j¼70N
j¼15N
P
z¼100hPa
k¼400hPa
m
k
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
u
2
i; j; k
þ v
2
i; j; k
q
ð3Þ
where f
i, j
is the grid cell latitude. We use this integrated
value L
NH
to characterize the latitude of the NH jet stream at
each longitude i. The following latitude bands will be used
in equation (3), and in the rest of the paper, to define the
three jet streams: 15N-70N for the NH, 40S-15S for the
SHT, and 70S-40S for the SHP jets. These intervals were
chosen based on zonal mean wind speeds (not shown).
[
10] Because these jet stream properties are, by design,
weighted averages over large volumes (i.e., all grid cells
between the 400 and 100 hPa levels within a given latitude
band worldwide), it is possible that trends occurring in a
sub-volume are partially masked by the lack of trends in other
sub-volumes. Trends in jet stream properties calculated in
1
Auxiliary materials are available in the HTML. doi:10.1029/
2008GL033614.
Figure 1. Jet stream wind speed from the ERA-40 reanalyses in 1979 – 2001: (a, c, e) annual, DJF, and JJA averages (m/s);
(b, d, f ) linear regression trends (m/s/decade), hatched where statistically significant (P-value <0.15), taking auto-correlation
into account.
L08803 ARCHER AND CALDEIRA: HISTORICAL TRENDS IN THE JET STREAMS L08803
2of6
this study are therefore generally expected to be conservative
lower-bound estimates.
4. Annual and Global Trends
[11] Using these jet stream properties, simple scalar
metrics for the entire globe (or for regions of interest) can
be obtained. We calculated global-average latitude, wind
speed, and altitude for the three jet streams (NH, SHT, and
SHP) for the period 1979-2001 from both the ERA-40 and
the NCEP/NCAR datasets. Since the temporal evolution of
the annual and seasonal averages from the ERA-40 and the
NCEP/NCAR reanalyses were nearly identical, only the
former are shown in Figure 3, whereas the latter are
available in the auxiliary material Figure S3. Parameters
of the linear regression against a linear trend from both
reanalyses are listed in Table 1. The auto-correlation of the
time series, when larger than 0.15, was taken into account to
correct the P-value of the linear regression.
[
12] We found that all three jet streams moved poleward
during the period 1979–2001, at rates varying from 0.06–
0.11 degrees/decade in the SHT, to 0.07–0.10 degrees/
decade in the SHP, and to 0.17–0.19 degrees/decade in
the NH jet. The SHP jet, however, shifted equatorward in
the austral winter, but not enough to compensate for the
poleward shift in the austral summer (Figure 3, left panels).
A poleward shift of the jet streams i s consistent with
numerous other signals of global warming found in previ-
ous studies, such as the expansion of the Hadley cell, the
poleward shift of the storm tracks, the widening of the
tropical belt, and the cooling of the stratosphere. However,
this is the first study to examine jet stream latitude trends in
the reanalyses.
[
13] Lorenz and DeWeaver [2007] looked at climate
projections from 15 models under the Intergovernmental
Panel for Climate Change (IPCC) A2 scenario (‘‘business as
usual’’ in the 21st century) and found qualitatively consis-
tent jet shifts poleward in both hemispheres. Kushner et al.
[2001] found that strong anthropogenic greenhouse gas
emissions can cause a similar poleward shift of the SHP
jet (by 0.08 degrees/decade) in a climate model. The
connection of poleward expansion of the Hadley cell with
global warming was identified by Frierson et al. [2007], in
idealized simulations with increased mean temperature, and
by Lu et al. [2007], who used climate models in a strong
greenhouse gas emission scenario. Both studies found
similar expansion rates, between 0.2 and 0.6 degrees/K
(degrees of latitude per degree K of warming), equivalent
to a poleward shift of the jet streams by 0.02–0.06 degrees/
decade (given a 0.5 K of warming in 1979–2005), smaller
than the values found in this study. On the other hand, other
studies have reported larger poleward shifts of parameters
related to the jets. Fu et al. [2006] and Hu and Fu [2007]
estimated a widening of the Hadley cell of 2–4.5 degrees in
1979–2005, corresponding to a shift of the jets by 0.37–
0.86 degrees/decade. Also, a poleward (and upward) shift in
the storm tracks in both hemispheres ( 0.6 degrees/decade
from the plots) was found by Yin [2005] in a 15-member
ensemble of 21st century climate models in a moderate
green house gas emission scenario. Seidel et al. [2008]
reported several estimates of the widening of the Tropics
varying between 1 and 8 d egrees during 1979 – 200 5,
corresponding to poleward shifts of 0.2–1.6 degrees/decade
for the jets. Finally, Williams [2006] and Haigh et al. [2005]
showed that stratospheric warming would cause a lowering
Figure 2. Jet stream pressure (as a measure for altitude) from the ERA-40 reanalyses in 1979 – 2001: (a, c, e)
annual, DJF, and JJA averages (hPa); (b, d, f ) linear regression trends (hPa/decade), hatched where statistically significant
(P-value <0.15), taking auto-correlation into account. At an altitude of 10 km, the pressure is approximately 265 hPa in
the standard atmosphere and a pressure change of 1 hPa represents an altitude change of about 26 m.
L08803 ARCHER AND CALDEIRA: HISTORICAL TRENDS IN THE JET STREAMS L08803
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of the tropopause and a shift of the jets equatorward,
therefore confirming that the strato spheric cooling expected
from anthropogenic emissions would cause a poleward (and
upward) shift of the jets.
[
14] An increase in jet stream altitude implies a negative
change in pressure and is reflected in a negative pressure
trend in Table 1. All jet streams had negative pressure trends
(except for a non-significant positive trend in the SHT jet
Figure 3. Annual, DJF, and JJA: anomalies from the 23-year (1979–2001) average latitude, average pressure (hPa, as a
measure of altitude), and average wind speed (m/s) of the Northern Hemisphere (NH), Southern Hemisphere sub-Tropical
(SHT), and Southern Hemisphere Polar (SHP) jet streams from the ERA-40 reanalyses. Positive latitude trends in the
northern hemisphere indicate a poleward shift in the jet stream, as do negative trends in the southern hemisphere.
Table 1. Statistical Regression Analysis of Annual Averages of Latitude, Altitude, and Wind Speed for Each Jet Stream From Both the
ERA-40 and the NCEP/NCAR Datasets in 1979 – 2001
a
Parameter Jet Stream Dataset Slope Per Decade Correlation Coefficient P-value
Latitude, deg NH ERA-40 0.165 0.363 0.096
NCEP 0.185 0.399 0.064
SHT ERA-40 0.063 0.268 0.215
NCEP 0.111 0.445 0.033
SHP ERA-40 0.073 0.222 0.308
NCEP 0.101 0.279 0.195
Pressure, hPa NH ERA-40 0.419 0.545 0.009
NCEP 0.036 0.066 0.765
SHT ERA-40 0.412 0.458 0.027
NCEP 0.017 0.022 0.920
SHP ERA-40 0.832 0.746 <0.001
NCEP 0.410 0.506 0.013
Wind speed, m/s NH ERA-40 0.156 0.287 0.183
NCEP 0.182 0.337 0.115
SHT ERA-40 0.365 0.381 0.214
NCEP 0.422 0.429 0.153
SHP ERA-40 0.237 0.251 0.336
NCEP 0.404 0.429 0.045
a
NH = Northern Hemisphere, SHT = Southern Hemisphere sub-Tropical, and SHP = Southern Hemisphere Polar. Bold values indicate P-values that have
been corrected to take into account the auto-correlation of the series, if greater than 0.15.
L08803 ARCHER AND CALDEIRA: HISTORICAL TRENDS IN THE JET STREAMS L08803
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for the NCEP/NCAR reanalyses). Statistically significant
pressure decreases occurred in the SHP jet (0.41 to
0.83 hPa/decade, corresponding to +11 to +22 m/decade
in the standard atmosphere with 26.3 m/hPa altitude
decrease with pressure). The NH jet showed a decrease in
pressure in both datasets ( 0.04 to 0.42 hPa/decade or
about +2 to +11 m/decade). Because of their highly aver-
aged nature, these estimated trends in jet stream pressure are
lower than those found in previous studies of tropopause
height from climate models or observations. The increase in
altitude of the jets occurred mostly in the boreal winter
(Figure 3, center panels).
[
15] Santer et al. [2003] reported an average decrease in
tropopause pressure of 1.45 hPa/decade (+60 m/decade)
for the 1979 – 1999 period. In a climate model under a
‘‘business as usual’’ CO
2
emission scenario, Kushner et al.
[2001] found a rising of the SHP by up to 3 hPa/decade
(+79 m/decade in standard atmosphere), and Lorenz
and DeWeaver [2007] an average tropopause rising of
40 m/decade (1.52 hPa/decade in standard atmosphere ).
Seidel and Randel [2006] used radiosonde data to estimate
that the tropopause rose by 1.6 hPa/decade (+64 m/decade)
during 1980-2004.
[
16] The strength, or wind speed, decreased in the NH
(0.16 to 0.18 m/s/decade) and in the SHT jet (0.37 to
0.42 m/s/decade), and increased in the SHP jet ( +0.25 to
+0.42 m/s/decade). Whereas no significant seasonal differ-
ences were found for the wind speed decrease in the SHT
jet, the strengthening of the SHP jet occurred mostly in DJF,
as did the weakening of the NH jet (Figure 3, right panels).
[
17] The increasing trend in the SHP jet winds is consis-
tent with findings by Russell et al. [2006], who reported an
increase in the westerlies over the Southern Ocean of 20%
in the past 20 years, by Kushner et al. [2001], who found
smal l increases (+0.08 m/s/decade) due to green house
gases alone, and by Thompson and Salomon [2002], who
related wind speed increases to photochemical ozone losses.
5. Seasonal and Spatial Trends
[18] More details about changes in jet stream properties
can be obtained by looking at two-dimensional and seasonal
maps of trends from the ERA-40 dataset (the corresponding
trends from the NCEP/NCAR dataset are available in the
auxiliary material, Figures S1 and S2, right panels). In
general, the SH trends are more horizontally homogeneous
than in the NH, due to the greater ocean extent.
[
19] The jet stream shift towards the poles can be detected
in Figure 1, where the areas of high average wind speed (left
panels) are generally bounded by areas with statistically-
significant positive trends on their poleward sides, and by
areas of statistically-significant negative trends on their
equatorward sides (right panel s). In the NH in D JF
(Figure 1d), the jet wind speed has been increasing over a
band covering northern Europe, central Asia, and the
northern Pacific, to the north of the jet core (Figure 1c),
accompanied by a second band with negative trends over
southern Asia and the Pacific, to the south of the jet core.
This pattern in the NH is also evident in the boreal summer
(JJA, Figure 1f ). In the SH during the austral summer
(DJF), strong wind speed increases were found all around
Antarctica and strong wind speed decreases were found
further north along a band centered at about 40S (Figure 1d),
again consistent with a poleward shift of the jets. In the SH
during the austral winter (JJA), however, the SHP jet tended
to shift equatorward (Figure 1f ), but not enough to com-
pensate for the poleward shift observed in the SH austral
summer. This seasonal difference, with a strong poleward
shift in the SH summer, was found in a climate model by
Kushner et al. [2001], who related it to anthropogenic
emissions.
[
20] Figure 2b further confirms that jet stream pressure
trends are overall negative on average, to indicate that they
are rising in altitude. In both hemispheres, the jets have
risen more in the summer than in the winter. For example, in
DJF (Figure 2d), statistically-significant negative pressure
trends in the SH are larger and more wide spread than in
JJA (Figure 2f ). In both hemispheres during their respective
winters (DJF for NH and JJA for SH), there is a strong
correspondence between wind speed and pressure trends
(Figures 1d and 2d for the NH, Figures 1f and 2f for the
SH). This suggests that, in both hemispheres in their
respective winters only, jet stream wind speed e ither
increases as the jet lowers (e.g., over central Asia), or
decreases as the jet rises (e.g., over the Southern Ocean
between Africa and Australia).
6. Conclusions
[21] Global warming is expected to affect the distribution
of mass (and thus pressure) in the atmosphere and therefore
affect the strength and location of the jet streams. Because
of the complex nature of jet streams, which are discontin-
uous in time and space, meandering, and with notable wind
speed and elevat ion vari ations, it i s important to use
objective and numerically stable metrics to define their
properties. We introduced mass and mass-flux weighted
averages of wind speed, pressure, and latitude of the jets
in both hemispheres and used these quantities to study jet
stream trends from a subset (1979 –2001) of the ERA-40
and the NCEP/NCAR reanalyses. We found that, on aver-
age, the jets are generally moving poleward in both hemi-
spheres. The northern hemisphere jet is weakening. In the
southern hemisphere, the sub-tropical jet is also weakening,
whereas the polar jet is strengthening. However, seasonal
and local trends may differ from this general behavior. For
example, the northern hemisphere jet strengthened while
decreasing in altitude in the boreal winter over Asia.
[
22] In general, trends of jet stream properties found in
this study are consistent in sign, but smaller in magnitude,
with those found in previous studies. This suggests that the
weighted averages over large volumes used to characterize
the jet streams in this study correctly capture the jet
properties, but they must be considered conservative
lower-bound estimates for larger trends that may manifest
themselves in subsets of these volumes.
[
23] These changes in jet stream latitude, altitude, and
strength have likely affected, and perhaps will continue to
affect, the formation and evolution of storms in the mid-
latitudes and of hurricanes in the sub-tropical regions.
Further observations and analyses are needed to confidently
attribute the causes of these changes to anthropog enic
L08803 ARCHER AND CALDEIRA: HISTORICAL TRENDS IN THE JET STREAMS L08803
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climate change, natural variability, or some combination of
the two.
References
Bluestein, H. B. (1993), Synoptic-Dynamic Meteorology in Midlatitudes,
vol. 2, Observations and Theory of Weather Systems, 594 pp., Oxford
Univ. Press, New York.
Frierson, D. M. W., J. Lu, and G. Chen (2007), Width of the Hadley cell in
simple and comprehensive general circulation models, Geophys. R es.
Lett., 34, L18804, doi:10.1029/2007GL031115.
Fu, Q., C. M. Johanson, J. M. Wallace, and T. Reichler (2006), Enhanced
mid-latitude tropospheric warming in satellite measurements, Science,
312, 1179, doi:10.1126/science.1125566.
Gray, M. W. (1968), Global view of the origin of tropical disturbances and
storms, Mon. Weather Rev., 96, 10,669 – 10,700.
Haigh, J. D., M. Blackburn, and R. Day (2005), The response of tropo-
spheric circulation to perturbations in lower-stratospheric temperat ure,
J. Clim., 18, 3672 – 3685.
Holton, J. R. (1992), An Introduction to Dynamic Meteorology, 511 pp.,
Elsevier, New York.
Hu, Y., and Q. Fu (2007), Observed poleward expansion of the Hadley
circulation since 1979, Atmos. Chem. Phys. Disc., 7, 9367 – 9384.
Kalnay, E., et al. (1996), The NCEP/NCAR 40-year reanalysis project, Bull.
Am. Meteorol. Soc., 77, 437 – 471.
Kistler, R., et al. (1999), The NCEP-NCAR 50-year reanalysis, Bull. Am.
Meteorol. Soc., 82, 247 –267.
Koch, P., H. Weirnli, and H. C. Davies (2006), An event-based jet-stream
climatology and typology, Int. J. Climatol., 26, 283 – 301, doi:10.1002/
joc.1255.
Kushner, P. J., I. M. Held, and T. L. Delworth (2001), Southern Hemisphere
atmospheric circulation response to global warming, J. Clim., 14, 2238 –
2249.
Lorenz, D. J., and E. T. DeWeaver (2007), Tropopause height and zonal
wind r esponse to global warming in the IPCC scenario integrations,
J. Geophys. Res., 112, D10119, doi:10.1029/2006JD008087.
Lu, J., G. A. Vecchi, and T. Reichler (2007), Expansion of the Hadley cell
under global warming, Geophys. Res. Lett., 34, L06805, doi:10.1029/
2006GL028443.
Pawson, S., and M. Fiorino (1999), A comparison of reanalyses in the
tropical stratosphere. part 3: Inclusion of the pre-satellite data era, Clim.
Dyn., 15, 241 – 250.
Russell, J. L., K. W. Dixon, A. Gnanadesikan, R. J. Stouffer, and
J. R. Toggweiler (2006), The Southern Hemisphere westerlies in a
warming world: Propping open the door to the deep ocean, J. Clim.,
19, 6382– 6390.
Santer, B. D., et al. (2003), Contributions of anthropogenic and natural
forcing to recent tropopause height changes, Science, 301, 479– 483,
doi:10.1126/science.1084123.
Seidel, D. J., and W. J. Randel (2006), Variability and trends in the global
tropopause esti mated from radiosonde data, J. Geophys. Res ., 111,
D21101, doi:10.1029/2006JD007363.
Seidel, D. J., Q. Fu, W. J. Randel, and T. J. Reichler (2008), Widening of
the tropical belt in a changing climate, Na ture Geosci. , 1,21–24,
doi:10.1038/ngeo.2007.38.
Thompson, D. W. J., and S. Salomon (2002), Interpretation of recent South-
ern Hemisphere climate change, Science, 296, 895 – 899.
Uppala, S. M., et al. (2005), The ERA-40 re – analysis, Q. J. R. Meteorol.
Soc., 131, 2961 –3012, doi:10.1256/qj.04.176.
Vecchi, G. A., and B. J. Soden (2007), Increased tropical Atlantic wind
shear in model projections of global warming, Geophys. Res. Lett., 34,
L08702, doi:10.1029/2006GL028905.
Williams, G. P. (2006), Circulation sensitivity to tropopause height,
J. Atmos. Sci., 63, 1954 – 1961.
Yin, J. H. (2005), A consistent poleward shift of the storm tracks in simula-
tions of 21st century climate, Geophys. Res. Lett., 32, L18701,
doi:10.1029/2005GL023684.
C. L. Archer and K. Caldeira, Department of Global Ecology, Carnegie
Institution of Washington, Stanford, CA 94305, USA. (lozej@stanford.edu)
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