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GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L03803, doi:10.1029/2008GL036076, 2009

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ArticleCause of the widening of the tropical belt since 1958
Jian Lu,1,2,3 Clara Deser,1 and Thomas Reichler4
Received 22 September 2008; revised 9 December 2008; accepted 30 December 2008; published 5 February 2009.

[1] Previous studies have shown that the width of thetropical belt has been increasing since at least the late 1970s
based on a variety of metrics. One such metric, thefrequency of occurrence of a high-altitude tropopause
characteristic of the tropics, is used here to show that theobserved widening of the tropics can be accurately
replicated by an atmospheric general circulation model
forced by the observed evolution of global SST and sea ice
distributions as well as the direct radiative effects from bothnatural and anthropogenic sources. Contrasting thissimulation with one forced by the observed SST and sea
ice distributions alone reveals that the widening trend can be
attributed entirely to direct radiative forcing, in particular
those related to greenhouse gases and stratospheric ozone
depletion. SST forcing causes no significant change in thewidth of the tropics, and even a contraction in someseasons. Citation: Lu, J., C. Deser, and T. Reichler (2009),
Cause of the widening of the tropical belt since 1958, Geophys.
Res. Lett., 36, L03803, doi:10.1029/2008GL036076.

1. Introduction[2] There is growing evidence from a variety of data
sources that the tropics have been widening, at least since
the beginning of the satellite era (1979). For example,
Hudson et al. [2007] used the gradient in total ozone
between the tropics and subtropics to track an expansionof the tropics in the northern hemisphere by 1.1° per
decade over the period 1979 –1991. A similar widening of
the tropics was found based on the height of the thermal
tropopause from radiosonde data [Seidel and Randel, 2007;
Reichler and Held, 2005] and NCEP/NCAR Reanalyses[Reichler and Held, 2005]. The tropical expansion is also
manifest in terms of tropospheric circulation and hydrology.
For example, the distance between the northern and southern hemisphere subtropical jets, a measure of the width of
the Hadley Cell, has grown by approximately 2° to 4.5°
since 1979 based on satellite-derived tropospheric temperature gradients [Fu et al., 2006]; direct estimate of the widthof the Hadley cell based on meridional mass streamfunction
using wind data from both ERA40 and NCEP/NCAR
Reanalyses [Hu and Fu, 2007] yields a widening of thesimilar order. As a consequence, the subtropical dry zone
inferred from satellite measurements of outgoing long-wave

1

National Center for Atmospheric Research, Boulder, Colorado, USA.
Advanced Study Program, NCAR, Boulder, Colorado, USA.
3
Center for Ocean-Land-Atmosphere Studies, Calverton, Maryland,
USA.
4
Department of Meteorology, University of Utah, Salt Lake City, Utah,USA.
2

Copyright 2009 by the American Geophysical Union.
0094-8276/09/2008GL036076$05.00

radiation (OLR) has also expanded poleward [Hu and Fu,
2007].
[3] Many aspects of the tropical expansion resemble
future climate projections from coupled models forced byincreasing greenhouse gases [Lu et al., 2007; Seidel et al.,2008], although the observed widening appears to haveoccurred faster than that simulated by the climate models[Johanson and Fu, 2008]. While the observed widening has
been speculated to be anthropogenic in origin, no studies (tothe best of our knowledge) have unambiguously identified a
precise mechanism. In this study, we make use of two setsof simulations with the GFDL atmospheric model AM2.1 toinvestigate the origin of the widening of the tropics. In one
ensemble simulation, the model is forced by the observed
global evolution of SST and sea ice over the 20th century;
while in the other ensemble simulation, the estimatedradiative forcing from both natural and anthropogenic
sources is specified along with the observed SST and sea
ice conditions. Our measure of tropical width is based on
the thermal tropopause metric of Seidel and Randel [2007],
namely the frequency of occurrence of a high-altitude
tropopause characteristic of the tropics. Using this definition, we find that only under radiative forcing can the model
replicate the observed tropical expansion, and that SST/sea
ice boundary forcing produces a contraction of the tropics.
By design, the radiative forcing includes both natural andanthropogenic components and does not allow direct attribution of the trend to anthropogenic causes. Nevertheless,our understanding of the distinctive physical signatures ofthe different radiative forcing agents enables us to suggestthat the anthropogenic component is the dominant cause of
the widening. The issue of statistical significance of thewidening trend in both annual mean and seasonal mean
fields is also addressed.

2. Data and Methods[4] Two sets of reanalysis data for the period 1958 – 2000
are used in this study: daily tropopause pressure data from
NCEP/NCAR [Kistler et al., 2001] and daily temperaturesfrom ERA40 [Uppala et al., 2005] from which we computedaily tropopause pressure. There are warnings against using
the reanalysis data prior to 1979 (the beginning of assimilation of satellite data into the reanalysis) for the study ofmultidecadal variability [Randel et al., 2000]. Thus, extra
caution has to be taken when examining trends since 1958.
We find that the tropopause probability density function(PDF) metric of Seidel and Randel [2007], which is also
used in this study (see below), is much less susceptible tothe effects of changes in instrumentation and observing
practices than the monthly mean tropopause height. Indeed,using this metric, we do not find any spurious jumps
between 1978 and 1979 in any of the derived time seriesof tropical width.

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L03803Figure 1. Climatological mean (1958 – 1999), zonally averaged annual PDF (unit is d/year) of tropopause pressure on 5hPa bins between 75 and 350 hPa for (a) ERA40 and (b) 10 ensemble (SST+RAD) simulations. The two vertical lines ineach panel demarcate the mean latitude of the tropical edges, which are 29.3°S and 30.0°N for ERA40 and 28.0°S and
29.2°N for the simulations.
[5] Two sets of ensemble simulations are conducted with
GFDL’s atmospheric model AM2.1 (see Delworth et al.
[2006] for model description). One set is forced by specifying the observed monthly evolution of SST and sea ice
between 1870 and 2000 (referred to as SST experiment) andanother by the SST/sea ice plus the estimated radiative
forcing from both natural (solar and volcanic aerosol) andanthropogenic sources (well-mixed greenhouse gases, tropospheric and stratospheric ozone, and tropospheric aerosols) over the same period (referred to as SST+RAD). The
radiative forcing is the same as that used for the 20th
century coupled model (CM2) simulations for the Fourth
Assessment Report of the Intergovernmental Panel on
Climate Change (IPCC AR4 [Hegerl et al., 2007]) at
GFDL. It is worth noting that under the same forcing, thefully coupled GFDL CM2.1, which uses AM2.1 as its
atmospheric component, can accurately replicate the globally and annually averaged lower stratospheric temperature
anomalies over 1979 – 2003 [Ramaswamy et al., 2006].
Each ensemble consists of ten members; each member starts
from a different initial condition taken from previous
AM2.1 simulations. Experiment SST+RAD simulates thepast climate with the best possible knowledge of the forcing
for the atmosphere, and provides the most comparable caseto the real world depicted by the reanalysis data. Assuming
linearity, the difference between SST+RAD and SST may be
regarded as the direct response to the radiative forcing. Notethat the methodology of specifying both the observed SST
and radiative forcing in the atmosphere-only model does not‘‘double count’’ the atmospheric radiative forcing because
only the direct effect is specified and the indirect effect isincluded in the prescribed SST forcing, and the two effects
are linearly additive in a similar experiment [Deser andPhillips, 2008].
[6] The daily tropopause pressure level is computed fromthe daily temperature data for the ERA40 reanalysis andeach individual member of the AM2.1 simulations. According to the World Meteorological Organization [1957] definition, the tropopause level is identified as the lowest
pressure level at which the lapse rate decreases to 2°C/km
and the average lapse-rate between this level and all higher
levels within 2 km does not exceed 2°C/km, using thealgorithm of Reichler et al. [2003]. The NCEP/NCAR
tropopause data also follows the WMO definition.
[7] For each grid point and each year/season, we computed the PDF of daily tropopause pressure by binning thedata into 5 hPa bins, from 50 hPa to 350 hPa. Figure 1
shows the climatological zonal mean of the PDFs for
ERA40 and the SST+RAD simulations based on the period
1958– 1999. The agreement between the two is encouraging, with both showing a dense concentration of low(<120 hPa, marked by the horizontal line in Figure 1)
tropopause pressure values in the tropics and a bimodal
distribution near the subtropics (centered at approximately
35° in each hemisphere). Following Seidel and Randel[2007], we associate tropopause pressures <120 hPa (orequivalently >15km) with tropical conditions and count the
number of tropical tropopause days at each latitude. From
this, we define the edge of the tropics as the latitude atwhich the number of tropical tropopause days exceeds 200
days per year. Defined this way, the mean edges of thetropics are located at the equatorward flanks of the subtropical bimodal distribution. These edges are 29.3°S (28.0°S)
and 30.0°N (29.2°N) for ERA40 (AM2.1 SST+RAD) as
indicated by the vertical dotted lines in Figure 1. The
resulting annual time series of the northern and southernedges of the tropics for the period 1958 – 1999 (1958 – 2007
for NCEP/NCAR Reanalysis) are shown in Figure 2 (thestarting year for our analysis is dictated by the availabilityof the ERA40 data set). Analogous time series for eachseason are provided in Figure S1 of the auxiliary material.1[8] We estimate the linear trend between 1958 and 1999
using standard linear regression analysis. The estimatedtrends and their 95% confidence intervals for each season
and the year as a whole are shown in Figure 3. The
significance test for the trend is conducted following themethod proposed by Santer et al. [2000]. In their proposed
Student’s t-test, the reduction of effective degrees of freedom due to the serial correlation of the regression residuals
is considered in both the estimation of the variance of theresiduals and indexing of the critical t value. An alternative
nonparametric estimate of the significance of the trend isprovided in Figure S2. Both tests lead to qualitatively
similar conclusions.

3. Results[9] Figure 2 shows the annual time series of the tropicaledges in both hemispheres for ERA40 (black) and NCEP/
1
Auxiliary materials are available in the HTML. doi:10.1029/
2008GL036076.

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Figure 2. Annual time series of the location of the tropicaledges, defined as the latitudes where days with tropopausepressures <120 hPa (heights > 15 km) exceed 200 days per
year. Estimates are from ERA40 (black) and NCEP/NCAR(dashed black) reanalysis data, and from the SST (green)
and SST+RAD (red) simulations. The years of major
volcanic eruptions are also indicated.
NCAR (dashed black) Reanalysis and the ensemble mean of
SST (green) and SST+RAD (red) simulations. We stress thatno large spurious discontinuity occurs between 1978 and1979 in this PDF-based measure of tropical width for either
Reanalysis. In each hemisphere, the mean latitude of thetropical edge is similar between SST+RAD simulation and
NCEP/NCAR Reanalysis, and displaced equatorward by
approximately 1° latitude relative to the ERA reanalysis.
Thus, on average, the total width of the tropics in the NCEP/
NCAR reanalysis and the ensemble mean SST+RAD simulation is about 2° latitude narrower than in the ERA40. The
simulations with SST+RAD forcing exhibit a significant
trend during 1958– 1999 in each hemisphere: 1.1° for thenorthern and 1.9° for the southern, equivalent to 0.71° per
decade for the total width of the tropical belt. This is similar
to that estimated from the Reanalyses (0.70° per decade for
ERA40 and 0.79° per decade for NCEP/NCAR). In contrast, the simulations forced by SST/sea ice alone tend to
produce a significant shrinking of the tropics (see Figure 3).
Similar results hold for the seasonal time series (Figure 3;
see also Figure S1 for the time series for each season).
These findings clearly point to the critical role of direct
radiative forcing in producing the widening in our measureof tropical width.
[10] Examining the seasonality of the tropical expansion,
we find that both Reanalyses products exhibit significant
trends in summer and fall in the northern hemisphere and inall seasons in the southern hemisphere (Figure 3). The
SST+RAD simulation reproduces the observed seasonality
reasonably well. Therefore, despite the uncertainties in thereanalyses and in the forcing of the SST+RAD simulation,
their consensus on the significance of the tropical expansion
makes a strong case for its reality. It is evident that the SST
experiments simulate a narrowing of the tropics in allseasons in both hemispheres, with significant narrowing
during winter, spring and fall in the northern hemisphereand fall in the southern hemisphere.

L03803[11] To further demonstrate the characteristics of thetropopause trends, using DJF as an example, we plot in
Figure 4 the linear trends superimposed on the climatological mean tropopause PDFs for ERA40 (Figure 4a),
SST+RAD simulation (Figure 4b), SST simulation(Figure 4c), and the difference between SST+RAD and
SST (Figure 4d). One striking feature in both ERA40 and
SST+RAD is the continuous dipole structure of the trends
throughout the tropics and southern hemisphere: Positive(negative) trends exist along the upper (lower) portion of themean tropical PDFs, indicative of a rising of the mean
tropopause in both the tropics and southern hemisphere
extratropics (a slight upward trend is also found at highlatitudes of the northern hemisphere). Decomposing thetrend into SST forced (Figure 4c) and direct radiatively
forced (Figure 4d) components, we find that both contributeto the rise of the tropical tropopause, while direct radiative
forcing dominates the rise of the extratropical southernhemisphere tropopause. Even though the rise of the tropical
tropopause is due to both SST and direct radiative forcing,the expansion of the tropical tropopause is due solely toradiative forcing as can be seen by the occurrence of
positive trends at the outer periphery of the mean tropical
PDF in Figure 4d.

4. Summary and Discussion[12] Simulations with GFDL’s AM2.1 atmospheric general circulation model driven by the our best knowledge offorcings for the 20th century, including the observed SST
and sea ice distributions as well as the direct radiative
effects from both natural and anthropogenic sources, reproduces qualitatively, and sometimes quantitatively, the observed widening of the tropics during 1958 – 1999 as
estimated from the ERA40 and NCEP/NCAR Reanalysis
data. This result confirms the findings of Seidel and Randel[2007] and corroborates the reality of the expansion of thetropical belt since 1958. By contrasting the results from two
sets of experiments, one forced by SST/sea ice alone and theother by SST/sea ice plus direct radiative effects, weFigure 3. Linear trend (1958 – 1999) in the latitude of thetropical edge for the (top) northern and (bottom) southernhemisphere and their corresponding 95% confidence
intervals (error bars). Results are based on ERA40 (solid
black) and NCEP/NCAR (dashed black) reanalysis data,and on the SST (green) and SST+RAD (red) simulations.

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L03803Figure 4. Linear trend (1958 –1999) (shading) (in d/year/decade) and climatological mean (contours) of the daily
tropopause pressure PDFs for DJF. Results are for (a) ERA40 reanalysis; (b) SST+RAD; (c) SST; and (d) (Figure 4b) –(Figure 4c). The two vertical lines indicate the mean latitudes of the tropical edges (33.4°S and 30.2°N for ERA40; 32.0°S
and 29.5°N for the simulations).
demonstrate that our measure of tropical expansion isentirely attributable to direct radiative forcing; SST forcing
alone causes no significant change in the width of thetropics, and even a contraction in some seasons. We
speculate that the SST-forced contraction might result fromthe dominance of the upward swing of the Pacific decadaloscillation (PDO) around 1976 – 77 [Deser et al., 2004;
Zhang et al., 1997] and/or the multidecadal variation in
Atlantic SSTs [Delworth and Mann, 2000], for the SST
pattern of the positive phase of the PDO resembles that of El
Nino, which is associated with a contraction of the tropics[Lu et al., 2008].
[13] Further examination of the temperature response in
the model simulations (not shown) reveals that the trends intropopause pressure and the consequent widening of thetropics is largely a result of stratospheric cooling, a feature
consistent with the radiative effects of increased GHGs andstratospheric ozone depletion and in alignment with thenotion that the tropopause rise over the past decades was
mainly associated with stratospheric cooling but less strongly with tropospheric warming [Seidel and Randel, 2006;
Austin and Reichler, 2008].
[14] In addition to well-mixed greenhouse gases andozone, the SST+RAD experiment also includes variations
in solar radiation, and direct scattering effects of anthropogenic and volcanic aerosols. Solar forcing has been shownto warm both the troposphere and stratosphere and should
thus exert a rather weak impact on tropopause height. Bothtypes of aerosols were shown to contribute to a downward
trend in tropopause heights by warming the lower stratosphere [Santer et al., 2003]. Further, we find from Figure 2
that the tropics tend to contract after major volcanic eruptions, thus providing no explanation for the widening.
Taken together, the most likely culprits for the wideningof the tropics since 1958 are increasing GHGs and strato-

spheric ozone depletion, both of which are of anthropogenic
origin. Identification of their separate roles would require
additional experiments with each radiative forcing agent
considered individually.
[15] Finally, we underline that our results are based on the
PDF of the tropopause to distinguish between tropics andextratropics. This is only one specific definition of tropical
width, and its connection to other approaches, for example
those based on tropospheric circulation or hydrologicalfeatures, remains to be established. Preliminary analysis
indicates that the connections between the different measures are complex and hence warrant further investigation.
[16] Acknowledgments. The authors thank David Schneider forreading the manuscript as NCAR internal reviewer. We are indebted to
Tom Delworth and Fanrong Zhang for designing and conducting theAM2.1 model experiments. JL is supported by the UCAR/NCAR Advanced Study Program. The National Center for Atmospheric Research issponsored by the National Science Foundation. TR is in part supported by
NSF grant ATM 0532280.

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