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Global and Regional Climate Response to Late Twentieth-CenturyWarming over the Indian Ocean
JAMES J. LUFFMAN, ANDREA S. TASCHETTO, AND MATTHEW H. ENGLAND
Climate Change Research Centre, University of New South Wales, Kensington, New South Wales, Australia
(Manuscript received 10 February 2009, in final form 15 October 2009)
ABSTRACT
The global and regional climate response to a warming of the Indian Ocean is examined in an ensemble of
atmospheric general circulation model experiments. The most marked changes occur over the Indian Ocean,
where the increase in tropical SST is found to drive enhanced convection throughout the troposphere. In the
extratropics, the warming Indian Ocean is found to induce a significant trend toward the positive phase of the
northern annular mode and also to enhance the Southern Hemisphere storm track over Indian Ocean longitudes
as a result of stronger meridional temperature gradients. Convective outflow in the upper levels over the warming
Indian Ocean leads to a trend in subsidence over the Indian and Asian monsoon regions extending southeastward
to Indonesia, the eastern Pacific, and northern Australia. Regional changes in Australia reveal that this anom-
alous zone of subsidence induces a drying trend in the northern regions of the continent. The long-term rainfall
trend is exacerbated over northeastern Australia by the anomalous anticyclonic circulation, which leads to an
offshore trend in near-surface winds. The confluence of these two factors leads to a drying signal over north-
eastern Australia, which is detectable during austral autumn. The rapid, late twentieth-century warming of the
Indian Ocean may have contributed to a component of the observed drying trend over northeastern Australia in
this season via modifications to the vertical structure of the tropical wind field.
1. Introduction
The Indian Ocean plays a fundamental role in deter-
mining regional climate. Changes in sea surface tem-
perature (SST) in the Indian Ocean have been linked
to a significant proportion of rainfall variability across
Asia (Ashok et al. 2001), Africa (Giannini et al. 2003;
Washington and Preston 2006), and Australia (e.g.,
Nicholls 1989; Drosdowsky and Chambers 2001; England
et al. 2006). Modes of variability unique to the Indian
Ocean have recently been identified and are now under-
stood as important drivers of regional climate variability.
The Indian Ocean dipole (IOD) was described by Saji
et al. (1999) as a temporary modification to the east–
west structure of the tropical wind field, which can be
measured as the difference between SST in the western
and eastern tropical Indian Ocean. The IOD exhibits
significant correlations with regional rainfall, including
Africa, southern Asia (Saji and Yamagata 2003), and
western and southern Australia (Ashok et al. 2003;
Ummenhofer et al. 2008, 2009; Cai et al. 2009).
Despite the relatively large number of studies of
seasonal-to-interannual Indian Ocean variability, few
studies have been conducted on the impact of Indian
Ocean warming on regional climate. This is the central goal
of the present study. The warming trend has been signifi-
cant across most of the basin, with only a small area in the
southwest showing a decreasing trend (Fig. 1a). Despite
prominent interannual to decadal variations (Fig. 1b), the
most notable feature of the temperature record is the
long-term trend seen during 1970–2005. The basin-
averaged Indian Ocean SST has increased since 1970
at a rate of 0.938C century21, with local rates as high as
2.928C century21. The trend in SST over the Indian
Ocean exhibits only minimal seasonal variation (figure
not shown), perhaps unsurprisingly given the high per-
sistence of SST on monthly time scales. This uniformity
between seasons in the trend in both spatial pattern and
magnitude also suggests that it is forced by mechanisms
operating over significant time and space scales, most
likely as a result of radiation changes associated with
anthropogenic climate change (Du and Xie 2008).
However, reconstructions of heat flux into the Indian
Corresponding author address: Andrea S. Taschetto, Climate
Change Research Centre, University of New South Wales,
Kensington, NSW 2052, Australia.
E-mail: [email protected]
1660 J O U R N A L O F C L I M A T E VOLUME 23
DOI: 10.1175/2009JCLI3086.1
� 2010 American Meteorological Society
Ocean over the late twentieth century disagree on both
the sign and magnitude of the net heat flux (Yu et al.
2007), with most datasets actually exhibiting a negative
flux south of 158S, suggesting that internal oceanic cir-
culation processes may have played a significant role in
a component of the observed surface warming.
Although the subject of Indian Ocean variability and
teleconnections with regional climate has been studied
extensively, the role of this emerging SST trend has re-
ceived far less attention. The sustained SST trend in the
Indian Ocean will probably modulate the historic tele-
connections in this region—for example, via the IOD—to
rainfall. The observed warming in Indian Ocean SST
adjacent to Africa has been suggested to have forced a
net shift of convection from land toward the ocean, re-
ducing rainfall in the Sahel region (Giannini et al. 2003).
Using an atmospheric general circulation model (AGCM)
ensemble forced with observed global SST over the pe-
riod 1930–2000, the authors found a low-frequency trend
in precipitation over the Sahel similar to observations,
with a marked drying from the 1960s to the 1980s. Their
study suggests that low-frequency trends in precipitation
can be accounted for by corresponding trends in Indian
Ocean SST.
It has also been argued that the warming of the trop-
ical Indian Ocean might have contributed to unusually
dry and warm conditions over North America and west-
ern Europe during the 1998–2002 period (Hoerling and
Kumar 2003). A subsequent study considered tropical
Indian Ocean SSTs as a separate case and found that a
large proportion of the observed Northern Hemisphere
circulation changes over the late twentieth century could
be forced simply by the linear trend in Indian Ocean
SSTs (Hoerling et al. 2004). The authors also found that
in 80% of model realizations forced only with warm
Indian Ocean SST, the North Atlantic Oscillation index
exhibited a positive trend, leading to a decrease in pre-
cipitation over the Mediterranean region and an increase
over northern continental Europe. The tropical SST
trend in the Indian Ocean was found to project onto the
Northern Hemisphere extratropical climate through at-
mospheric eddy feedbacks and modifications to the win-
tertime storm track. It is as yet unknown whether such a
mechanism exists in the Southern Hemisphere, and in-
deed what the impacts of an Indian Ocean warming on
the Southern Hemisphere extratropical climate might be.
It is likely that the surface warming of the Indian Ocean
also has a signal in regional precipitation, including over
Australia. The rainfall decline in Australia has been
particularly pronounced over the northeast, where de-
creases in mean annual rainfall have been as large as
200 mm decade21 since 1971 (Taschetto and England
2009). As Australian climate is affected by Pacific SST
variability, one may speculate that the decrease along
the east coast could be a result of changes in the El Nino
pattern over the past few decades. However, given the
absence of substantial trends in ENSO indices over the
twentieth century (Nicholls 2008), it is unclear whether
changes in El Nino activity could have forced the ob-
served Australian east coast drying trend. The causes of
the post-1950 drying trend over northeastern Australia
remain unknown (Nicholls 2006).
Trends in regional rainfall over Australia and Indian
Ocean rim nations warrant further investigation, espe-
cially considering their continuation into the early part of
the twenty-first century. Given the extensive linkages be-
tween regional climate and the Indian Ocean on seasonal
to interannual time scales, the role of lower-frequency
variability and trends in the Indian Ocean needs to be
considered. In this study, we investigate the atmospheric
response to a warming Indian Ocean using an ensemble
set of experiments with an AGCM forced by a linear
trend component of observed SST during the late twen-
tieth century. The configuration and forcing of the model
experiments and an evaluation of the relative contribu-
tions from the Indian and Pacific oceans are outlined in
section 2. Section 3 describes the forced changes in the
FIG. 1. (a) Linear trend in annual SST (8C century21) in the In-
dian Ocean over the period 1970–2005. Dashed lines indicate the
linear ramp at the southern boundary of the model forcing region.
(b) Time series of annual Indian Ocean basin-averaged SST (8C)
between 208S–208N and 408–1108E. Anomalies are relative to the
1970–2005 climatology. The HadISST1 dataset (Rayner et al. 2003)
was used to compute these trends.
1 APRIL 2010 L U F F M A N E T A L . 1661
global atmospheric circulation, and the resultant re-
gional trends are analyzed in section 4, with a focus on
Australia. Finally, section 5 discusses and concludes the
main findings of this study.
2. Atmospheric model and experimental design
a. Atmospheric model
The atmospheric model used in this study is the Com-
munity Atmosphere Model, version 3 (CAM3), devel-
oped by the National Center for Atmospheric Research
(NCAR). The CAM3 uses Eulerian spectral dynamics
with a T42 horizontal resolution (approximately 2.88 in
both latitude and longitude) and 26 levels in a hybrid
sigma pressure system in the vertical. The AGCM is
forced by prescribed global monthly SST values, as well
as sea ice fraction, linearly interpolated at each time step
to form the bottom boundary conditions for the atmo-
sphere. A more thorough description of CAM3 is given
by Collins et al. (2006), and an analysis of the model’s
hydrological cycle is given by Hack et al. (2006).
b. Experimental forcing and configuration
The premise of our experimental design is to analyze
the modifications to regional climate forced by the ob-
served warming of the Indian Ocean. In the main per-
turbation experiment we are interested in particular in the
climate modifications arising from the lowest-frequency
aspects of the observed SST changes—that is, the linear
trend component. By conducting experiments forced
solely by the observed changes in Indian Ocean SST, we
can elucidate the response of regional climate in iso-
lation from other potential forcing agents. Therefore,
the main experiment ensemble set applies SST pertur-
bations to the monthly varying climatological mean only
over the Indian Ocean region, defined as the ocean basin
north of 258S, bordered to the west by the African con-
tinent, to the east by the Indonesian archipelago, and to
the north by the Asian continent (Fig. 1a). Everywhere
else the boundary forcing is the climatological monthly
mean SST and sea ice fraction data from the Hadley
Centre (HadISST1; Rayner et al. 2003).
In this main Indian Ocean warming experiment, re-
ferred to as ITR, a linear trend over the period 1970–2005
is fitted to the SST time series at each model grid point,
with a mean 1970–2005 SST change of 10.938C century21
over the basin. The linear trend in SST at each grid
point is added to the monthly climatological values
over the period 1970–2005, so that the only variability
in SST is the seasonal cycle superimposed upon the
observed linear trend. At 158S, a ramp of width 108 in
latitude is used to border the forced region to minimize
edge effects (see Fig. 1a), and across this ramp the im-
posed trend is damped linearly to a value of zero at the
outside boundary.
To filter out the internal variability generated by the
atmosphere, an ensemble of experiments was carried out
with identical SST forcing as described previously, but
each with slightly varied atmospheric initial conditions,
taken at consecutive two-day intervals. The number of
ensemble members was five, yielding a total of 175 yr of
integration time. Although there is no upper bound on the
number of integrations desired, the computational load of
this ensemble size is already substantial (Taschetto and
England 2008). We employ a bootstrap statistical analysis
to quantify the significance of modeled trends (Efron
1982). This technique can be particularly effective in
dealing with climate data from non-Gaussian fields, be-
cause other hypothesis tests often make invalid assump-
tions about the normality of the sampling distribution
(Kiktev et al. 2003). Our approach determines whether
the mean of the bootstrap distribution of ensemble
member linear trends at each model grid point is signifi-
cantly different from zero at the 90% confidence level
using a bootstrap t test. In sections 3 and 4, we find a
number of major features in the atmospheric circulation
common to the output of all five ensemble members,
suggesting that this modest ensemble size is sufficient for
our purposes.
c. Assessing the relative contributions from Indianand Pacific ocean warming
Additional experiment sets were performed in order
to assess the relative contributions from surface warm-
ing of the Indian and Pacific oceans during the late
twentieth century (see Table 1). These additional sim-
ulations allow us to examine the linearity or otherwise of
the regional climate response to Indian Ocean surface
warming and also to support the results of our primary
experiment set forced only by the warming trend in
the Indian Ocean. The AGCM experiments consisted
of seven-member multidecadal ensemble sets using
CAM3, integrated from 1950 to 2006 and forced by
monthly varying SST from HadISST1 in the different
domains. These experiments differ from the ITR by in-
cluding not only the observed trend in SST but also the
interannual variability. For consistency, only the 1971–
2005 period from these additional experiments is ana-
lyzed here.
The geometry of the forcing regions used in the addi-
tional experiment set is as follows: The first set was forced
by the trending SST field only in the Indian Ocean north of
258S, which also includes year–year variability (ITR1VR).
Outside this region, the monthly varying climatological
1662 J O U R N A L O F C L I M A T E VOLUME 23
SST is applied (i.e., no trend and no year–year variabil-
ity). Similarly, the second set was forced by the trending
and interannually varying SST only in the Pacific Ocean,
from 308S to 308N, with monthly mean SST climatology
elsewhere (PTR1VR). A third experiment set was forced
by the trending and interannually varying SST in both the
Indian and Pacific oceans (IPTR1VR). Beyond the domain
of varying SST in each experiment set, anomalies were
damped to zero over a 108 latitude and longitude band to
avoid unrealistic atmospheric circulation resulting from
edge effects from the boundary forcing.
Figure 2 shows the mean linear trend in regional
precipitation from each of these three ensemble sets. Of
course, we do not expect a priori that any of the model
ensembles should look like the observed trends, as the
experiments include just one forcing agent (i.e., SST) out
of many possible drivers (greenhouse, ozone, aerosols,
etc). Nevertheless, a general comparison with National
Centers for Environmental Prediction (NCEP)–NCAR
reanalysis (not shown) indicates that the large-scale rainfall
pattern is captured by the IPTR1VR experiment (Fig. 2c).
For instance, a general increase of precipitation is seen
over the tropical Indian Ocean in the reanalysis product,
consistent with the SST warming over the region. In
addition, NCEP–NCAR reanalysis shows a rainfall reduc-
tion over India, the Arabian Sea, and the Bay of Bengal
(not shown), which is also captured by the IPTR1VR
simulation (Fig. 2c). Despite this large agreement be-
tween the simulated trends and the precipitation pattern
from the NCEP–NCAR reanalysis over certain regions,
the same cannot be said for the Climate Prediction Center
Merged Analysis of Precipitation (CMAP) dataset—
for example, CMAP and NCEP–NCAR reanalysis ex-
hibit opposite trends over the tropical Indian Ocean
(not shown). Previous studies have reported a discrep-
ancy between the long-term rainfall trends over the trop-
ical Indian Ocean between observations and reanalysis
products (e.g., Arakawa and Kitoh 2004; Kumar et al.
2004; Quartly et al. 2007). It is possible that the re-
analysis product is biased toward a larger positive rain-
fall trend, because their models use SST as boundary
conditions in the data assimilation. While the rainfall
trends in the reanalysis are consistent with the SST
warming, precipitation trends from CMAP agree with
the long-term changes in surface salinity over the Indian
Ocean (Durack and Wijffels 2009, manuscript submitted
to J. Climate).
Despite discrepancies in Indian Ocean rainfall trends
between observations, both CMAP and the reanalysis
data show the east–west pattern over Australia, consis-
tent with the Australian Bureau of Meteorology dataset.
None of the experiments, however, simulate an east–
west pattern of rainfall trend over Australia, unlike the
observations. Instead, the Indian Ocean SST warming
generates a negative trend in the north and a positive
trend in the southern regions of Australia (Fig. 2a),
whereas broadly the opposite pattern is exhibited for the
experiment forced by trending Pacific SST (Fig. 2b). The
experiment IPTR1VR shows a decreasing trend across
the continent, which is particularly strong in the north
and east. This suggests that while the drying trend in the
east may be partially explained by changes in the trop-
ical Indian and Pacific SST, the rise of rainfall in the
northwest must be related to other factors.
In general, there is a marked linearity in atmospheric
response to the forcing in the experiment with SST trends
in the Indian and Pacific oceans combined, relative to the
response in the individually forced sets. Nonlinear re-
sponses in rainfall trend are seen over the South China
Sea and Malaysia. However, the overall linearity is quite
clear and this increases the confidence in the in-
terpretation of results from the ITR experiment forced
only by the linear trend in Indian Ocean SST. Focusing on
the region over the central Indian Ocean and northern
Australia, the experiment set forced only by Pacific
Ocean SST exhibits a precipitation trend of opposite sign
to the Indian Ocean and combined sets. This suggests that
SST trends over the Indian Ocean may be a potential
factor in forcing trends over these regions. In contrast, the
Australian region south of approximately 188S shows
a rainfall decline apparent primarily in the PTR1VR ex-
periment. The modeled decrease over northern and
northeastern Australia in the combined forced IPTR1VR
experiment, and in the ITR1VR set, is in agreement with
the results of the ITR experiment as outlined in section 4.
This aspect of the climate response to Indian Ocean
TABLE 1. Outline of the experiments analyzed in this study. Unless otherwise indicated in the text, the results are described for the ITR
experiment from 1971 to 2005.
Experiment Region Forcing Ensemble size
Control Global oceans Monthly SST climatology 5
ITR Tropical Indian Ocean SST trend 5
ITR1VR Tropical Indian Ocean SST trend and variability 7
PTR1VR Tropical Pacific Ocean SST trend and variability 7
IPTR1VR Tropical Indian and Pacific oceans SST trend and variability 7
1 APRIL 2010 L U F F M A N E T A L . 1663
warming is in agreement with observed trends over far
northeastern Australia, as outlined in section 4c.
3. Global response to Indian Ocean warming
Here, we analyze the circulation changes imposed by
the linear trend component of Indian Ocean SST and
their projection onto global climate from the ITR exper-
iment. A global response to the warming Indian Ocean
can be seen in zonally averaged zonal winds (Fig. 3)
throughout the depth of the troposphere, suggesting a
large-scale reorganization of the wind field at multiple
levels in response to the imposed surface heating. The
trends in the Northern Hemisphere extratropics re-
semble the zonal wind anomaly structure of the positive
phase of the northern annular mode (NAM) shown by
Thompson et al. (2003), with an increase in westerlies
north of about 408N and a weakening to the south. To
investigate this modification further, we conduct an em-
pirical orthogonal function (EOF) analysis of modeled
sea level pressure (SLP) poleward of 208 latitude. The
leading spatial pattern and associated principal compo-
nent time series of SLP under the condition of a warm-
ing Indian Ocean are displayed in Fig. 4.
In the Northern Hemisphere, the first EOF mode
(Fig. 4b) resembles the SLP signature of the NAM, with
positive eigenvectors extending from North America to
western Europe and also over the North Pacific. The
principal component time series for the leading EOF
mode (Fig. 4d) exhibits a notable positive trend, which is
found to be significant at the 90% confidence level
using the bootstrap t test. This result is consistent with
the findings of Hoerling et al. (2004)—suggesting that
a positive trend in the NAM, and the associated re-
gional climate change in the Northern Hemisphere ex-
tratropics, can be forced significantly by a warming Indian
Ocean.
In the Southern Hemisphere, the strongest response in
zonal winds (Fig. 3) is an enhancement of the subtropical
jet, which is driven by divergent outflow associated with
increased convection over the tropical Indian Ocean. The
positive trend in zonal winds associated with this en-
hancement is strongest near 200 hPa, but also extends to
the surface from 208 to 408S, corresponding to a weaken-
ing of the subtropical easterly trade winds in this band.
Over the Southern Hemisphere extratropics, the pattern
of zonally averaged trends is weaker than that exhibited
in the Northern Hemisphere and has a different zonal and
vertical structure. The spatial pattern of the leading EOF
in the Southern Hemisphere SLP (Fig. 4a) has a roughly
annular form similar to the observed southern annular
mode (SAM), described by Thompson and Wallace (2000),
although negative eigenvectors extend equatorward at
the Indian Ocean longitudes, weakening the ‘‘annular’’
shape. The principal component time series (Fig. 4c) for
the leading EOF shows a slight negative trend and is not
significant at the 90% confidence level. Given these re-
sults, we conclude that it is unlikely that the observed
positive trend in the SAM (Marshall 2003) has been
forced in any significant way by a warming of the Indian
Ocean. Other factors such as changes in stratospheric
FIG. 2. Ensemble mean linear trends in annual precipitation
(mm century21) during 1971–2005 from the atmospheric model
forced by observed trending and monthly and interannually vary-
ing SST in the (a) Indian, (b) Pacific, and (c) Indian and Pacific
oceans combined. Areas within the dashed lines are significant at
the 90% level based on a Student’s t test.
1664 J O U R N A L O F C L I M A T E VOLUME 23
ozone (Gillett and Thompson 2003) and greenhouse
gases (Cai et al. 2003) have clearly played a greater role.
It is worth noting that negative eigenvectors in the
Southern Hemisphere extend equatorward at Indian
Ocean longitudes in Fig. 4a. This suggests a regional
weakening of the SAM (i.e., toward its negative phase),
indicating that the Indian Ocean warming may have a
compensating effect upon extratropical climate over these
longitudes in the Southern Hemisphere. We examine the
mechanisms associated with this zonally asymmetric mod-
ification to the SAM in a discussion of regional dynamics
in section 4.
4. Regional atmospheric response
a. Annual mean
The direct response to the warming of Indian Ocean
SST is an overall decrease in sea level pressure over the
ocean basin (Fig. 5b) and enhanced deep convection in
the atmosphere, which is clearly visible in Fig. 5a as an
increase in upward motion over the tropical Indian
Ocean. This feature in our experiment is consistent with
the results of Giannini et al. (2003), who found that the
observed SST warming caused preferential and in-
creased convection over the Indian Ocean. We find this
trend toward enhanced uplift over the ocean to be a ro-
bust response; it is present in each of the five ensemble
members of the ITR experiment as well as the IPTR1VR
experiment forced by the actual observed SST over both
the Indian and Pacific oceans.
The convection-driven uplift over the tropical Indian
Ocean leads to upper-tropospheric divergence, visible in
the divergent component of horizontal winds at 220 hPa
in Fig. 5a. At lower levels a marked convergence in near-
surface winds can be seen over much of the tropical
Indian Ocean (Fig. 5b), causing a reorganization of the
local wind field in the region. Farther east is a corre-
sponding suppression of tropical convection over the
Bay of Bengal, northern Australia, and the Coral Sea,
shown as a broad area of subsiding air, associated upper-
level convergence and positive trends in sea level pres-
sure. The zonal and vertical structure of the vertical
velocity anomalies is examined in Fig. 6, revealing that
the strong east–west sign reversal seen in the 500-hPa
velocities is consistent throughout the depth of the
troposphere. Again, this feature is also present in the
IPTR1VR ensemble forced by SST in the Indian and
Pacific oceans combined.
Another vertical circulation response to the imposed
SST warming is enhanced uplift over the southern Indian
Ocean, outside the region of forced warming. Surface
winds (Fig. 5b) reveal an anomalous cyclonic circulation
centered near 408S, 658E and enhanced westerlies from
FIG. 3. Ensemble mean linear trend in global annual zonally averaged zonal winds [(m s21)
century21] in the atmosphere in response to Indian Ocean warming. The contour interval is
0.5 (m s21) century21. The zero contour is shown in heavy black line, and positive (eastward)
contours are dashed.
1 APRIL 2010 L U F F M A N E T A L . 1665
308 to 408S, indicating a tendency toward stronger baro-
clinicity and intensification of the storm track over the
southern Indian Ocean, evidenced by a reduction in geo-
potential height (not shown). This increased cyclogenesis
over the southern Indian Ocean is also apparent in the
spatial structure of the SLP response (Fig. 4a), where the
annular structure deviates equatorward at 408–808E.
Although we do not conduct a detailed analysis of the
model storm track trends, the enhancement of mid-
latitude depressions over the southern Indian Ocean in
the model is likely a result of the increased meridional
temperature gradient arising from the warmed tropical
SST. It is also likely a result of the increase in moisture
content of prefrontal air masses from 708 to 908E, where
lifting results in condensation and the release of latent
heat, warming the overlying air and resulting in further
intensification of midlatitude systems.
To analyze the nature of changes to advection of
moisture in the lower and middle troposphere arising
from the warming of the Indian Ocean, Fig. 7 shows the
trend in specific humidity and winds, integrated from
800 to 500 hPa. In the control run forced by climato-
logical monthly SST, the levels over 800–500 hPa ac-
count for 56% of global-averaged moisture transport,
calculated using the bulk formula Q � kUk. Figure 7a
shows that the increased uplift over the tropical Indian
Ocean and subsidence to the north of Australia result in
an east–west dipole in specific humidity trends over these
areas. This dipole is consistent with the vertical circulation
described previously and is a robust feature seen in both
the ITR and IPTR1VR simulations. Over the tropical In-
dian Ocean, warmer SSTs increase evaporation rates and
lead to higher specific humidity, while the deep convec-
tion drives a strong convergence of the tropical wind field.
FIG. 4. (top) Spatial loading pattern of the first EOF of (a) Southern and (b) Northern Hemisphere SLP (Pa)
calculated poleward of 208 latitude for the ensemble mean of the Indian Ocean warming experiments, and associated
(bottom) normalized annual means of the principal component time series of the (c) Southern and (d) Northern
Hemisphere.
1666 J O U R N A L O F C L I M A T E VOLUME 23
FIG. 5. Ensemble mean trends in atmospheric circulation over 1971–2005 in response to
Indian Ocean warming: (a) annual vertical velocity (shaded) at 500 hPa [(Pa s21) century21] and
divergent component of horizontal annual wind (arrows) at 220 hPa [(m s21) century21]; (b) sea
level pressure (hPa century21) and near-surface annual winds [(m s21) century21]. Negative
(positive) values in the vertical velocity represent upward (downward) motion. Trend values
determined to be significantly different from zero at the 90% confidence level using a bootstrap
t test in (a) and Student’s t test in (b) are bounded by dashed lines, trend vectors significantly
different from zero at the 90% confidence level are shown in black.
1 APRIL 2010 L U F F M A N E T A L . 1667
Farther east, an anomalous anticyclonic circulation cen-
tered near 108S, 1308E coincides with an area of de-
creasing moisture that takes a maximum value over the
Coral Sea and northeastern Australia. This region of
atmospheric drying coincides with the region of anom-
alous subsidence seen in Fig. 5a.
Linear trends in modeled annual rainfall during
1971–2005 are shown in Fig. 8. The regional response
is dominated by an increase over the tropical Indian
Ocean and a decrease over India, the Bay of Bengal, the
South China Sea extending to the Indonesian archipel-
ago and the Coral Sea. This pattern is highly correlated
with changes to vertical velocity (Fig. 5a) and horizontal
moisture transport (Fig. 7a), indicating that the precip-
itation change in the model is accounted for by the mod-
ifications to the regional atmospheric circulation described
earlier. The increase over the tropical Indian Ocean and
decrease over surrounding tropical land areas is consis-
tent with the results of Giannini et al. (2003), whereby a
reduced land–sea temperature gradient forces preferen-
tial convection over the ocean and weakens the monsoon
mechanism.
It is of interest to compare the spatial pattern of the
regional rainfall response in the ITR experiment set with
the response in the IPTR1VR experiment with combined
forcing from both the Indian and Pacific oceans (Fig. 2c).
The pattern of increase over the tropical Indian Ocean,
and decrease over India, the Bay of Bengal, parts of the
Indonesian archipelago, the Coral Sea, and northeastern
Australia is largely retained when the SST forcing is
extended east across the Pacific Ocean. This result in-
dicates that the atmospheric response in the ITR exper-
iment is not particular to the isolated warming imposed
over the Indian Ocean. In contrast, the PTR1VR exper-
iment set (Fig. 2b), forced solely by Pacific Ocean SST
trends plus interannual variability, exhibits a rainfall
increase over northeastern Australia and the Coral Sea,
which is opposite to both the combined forced IPTR1VR
set and the observed trends. To the extent that a linear
superposition between individual and combined atmo-
spheric forcings from the Indian and Pacific oceans ex-
ists, our results suggest that the Indian Ocean is leading
the atmospheric response over land areas of southern
Asia, Indonesia, and tropical Australia.
b. Seasonal evolution
Although the magnitude and spatial pattern of the
applied SST trend forcing is annually constant, a sea-
sonal component to the atmospheric response is possi-
ble, because the trend forcing is projected onto natural
unforced seasonal variability in the atmospheric circu-
lation. In the case of horizontal moisture transport, we
FIG. 6. Ensemble mean trends in atmospheric annual vertical velocity [(Pa s21) century21] in
response to Indian Ocean warming from 1000 to 100 hPa over the Southern Hemisphere
tropics (08–238S), across longitudes spanning the Indian Ocean (608–1158E) and Australia
(1158–1508E). The contour interval is 0.05 (Pa s21) century21. The zero contour is shown in
a heavy black line, and positive (downward) contours are dashed.
1668 J O U R N A L O F C L I M A T E VOLUME 23
expect that the effect of the trend in Indian Ocean SST
will be nonuniform, given that the flow regimes vary
with season across the region.
Specific humidity and wind trends (Fig. 7b) for the
austral summer [December–January–February (DJF)]
reveal a decrease in atmospheric moisture content over
eastern parts of the Indonesian archipelago, the South
China Sea, and northeastern Australia, which is associ-
ated with the region of subsidence also seen in the annual
trends over this region. In addition, a reduction of mois-
ture is seen over the southeastern Indian Ocean in DJF.
Trends in moisture transport during the austral au-
tumn [March–April–May (MAM)] exhibit a marked
dipole in moisture (Fig. 7c) qualitatively resembling the
annual plot (Fig. 7a) but of greater magnitude. The in-
creasing moisture and wind field convergence associated
with enhanced convective activity reaches a maximum
over the Indian Ocean during MAM (Fig. 7c), with the
corresponding area of subsidence over northeastern
Australia also at a maximum.
With the increasing moisture content of the lower
troposphere over the tropical Indian Ocean at a maxi-
mum in austral autumn, the annual trend toward in-
creased inflow of moisture to extratropical latitudes also
reaches a peak during these months. The increasing trend
of this moisture advection pathway can be seen clearly in
Fig. 7c extending from near 108S, 908E to western and
southern Australia.
During austral winter [June–July–August (JJA)], pat-
terns of atmospheric moisture in the Southern Hemi-
sphere tropics east of 1008E are weak as the ITCZ is at its
northernmost position, corresponding to the dry season
in this region. An area of increasing moisture over the
Indian Ocean south of the equator is advected toward the
southeast (Fig. 7d) as a result of enhanced midlatitude
baroclinic eddies over the southern Indian Ocean, lead-
ing to a trend of increasing moisture transport into the
Southern Hemisphere extratropics.
During austral spring [September–October–November
(SON)], the ITCZ would ordinarily be held north over
the strongly heated south Asian landmass; however, the
warming trend in SST enhances deep convection over the
FIG. 7. Ensemble mean of the (a) annual and (b)–(e) seasonal
trends in the specific humidity [(g kg21) century21] and horizontal
wind [(m s21) century21] in response to Indian Ocean warming,
integrated over vertical levels in the 800–500-hPa band. Trend
values determined to be significantly different from zero at the
90% confidence level using a bootstrap t test are bounded by
dashed lines, trend vectors significantly different from zero at the
90% confidence level are shown in black.
1 APRIL 2010 L U F F M A N E T A L . 1669
tropical Indian Ocean (Fig. 5a) by weakening the land–
sea temperature gradient and drawing the ITCZ south-
ward of its mean climatological position. The increased
specific humidity over the southern equatorial Indian
Ocean resulting from this enhanced uplift is evident in
Fig. 7e, as is the drying trend over continental South Asia.
With the ITCZ enhanced over the tropical Indian Ocean,
there is an increase in eastward advection of moisture
south of Java, Indonesia, and over subtropical Australia.
c. Australian rainfall response to Indian Oceanwarming
Observed annual trends (Lavery et al. 1997) over
1971–2005 (Fig. 9f) show an east–west pattern associ-
ated with a substantial increase in the northwest and a
drying over the northeast. This observed east–west pat-
tern is not captured in the simulation ensemble means.
Instead, the modeled annual rainfall trends (Fig. 9a) show
a significant decrease over the north and northeast and an
increase along the east coast and over southwest Western
Australia. Although the observed rainfall decrease over
far northeastern Australia corresponds to the modeled
responses in the ITR experiment (Fig. 2a), the strongest
magnitude of the drying trend is obtained in IPTR1VR,
suggesting a combined response to both tropical Indian
and Pacific oceans (Fig. 2c).
The situation is different south of approximately 188S,
where the Indian Ocean warming tends to increase rain-
fall (see Fig. 9a for ITR and Fig. 2a for ITR1VR), whereas
the Pacific Ocean warming tends to decrease rainfall
(Fig. 2b). This suggests that the Pacific Ocean may play
a role in the decline of rainfall over the southeastern re-
gions of Australia. It is important to note, however,
that the magnitude of the modeled rainfall trend across
Australia is generally much weaker than the observations.
The cause of the modeled annual rainfall decrease
over northern and northeastern Australia is an induced
subsidence trend in the Southern Hemisphere tropics
from 1258 to 1608E (Figs. 5 and 6), causing an anoma-
lous divergent anticyclonic circulation over northern
Australia (Fig. 5b). The subsidence-induced drying is
exacerbated by the anomalous anticyclonic circulation,
causing an offshore trend in horizontal winds, both at
the surface (Fig. 5b) and integrated over 800–500 hPa
(Fig. 7a). This offshore trend opposes the northerly wind
FIG. 8. Ensemble mean linear trends in (a) annual and (b)–(e)
seasonal precipitation (mm century21) during 1971–2005 from
CAM3 forced by increasing Indian Ocean SST. Trend values de-
termined to be significantly different from zero at the 90% confi-
dence level using a bootstrap t test are bounded by dashed lines.
1670 J O U R N A L O F C L I M A T E VOLUME 23
pattern of Watterson (2001), which was found in a model
simulation to be correlated with monthly rainfall over
northeastern Australia (coefficients greater than 0.80
during both January and July). Over northern Australia,
the decreasing trend in specific humidity (Fig. 7a)
combines with an easterly trend in near-surface winds
north of 158S and east of 1258E (Fig. 5b) to weaken the
northwesterly monsoon surge in this region.
The increase in onshore moisture transport from the
southeastern Indian Ocean to the northwestern and west-
ern portions of the continent (Figs. 5b and 7a) combined
with enhanced uplift within the prevailing west coast
trough (Fig. 5a) may account for the slight increase in
modeled precipitation over Western Australia (Fig. 9a),
although the signal is not significant at the 90% level. It
has been shown by Smith (2004) that observed rainfall
trends over the twentieth century showed their most
significant increase over a similar area; however, the
observed 1971–2005 trends are mostly in the range 100–
300 mm century21, whereas our ensemble mean has a
maximum increase of only 110 mm century21. A rainfall
increase over the northwestern regions of Australia closer
in magnitude to the observed was shown by Rotstayn
et al. (2007) to appear in experiments, including Asian
anthropogenic aerosols in a coupled GCM. It is thus
possible that the additional inclusion of increasing an-
thropogenic aerosols would produce a modeled rainfall
increase over the northwestern regions of Australia
closer in magnitude to observations.
The modeled rainfall response in DJF (Fig. 9b) over
Australia is not large, exhibiting a slight decrease of ap-
proximately 150 mm century21 over the north and north-
east, which is not significant at the 90% level. Trends in
specific humidity and wind (Fig. 7b) suggest that the de-
crease over northeastern Australia is accounted for by an
atmospheric drying and offshore trend in winds.
The modeled decrease in precipitation over the north
and northeast is most extensive during MAM, with the
trend significant at the 90% confidence level across a
large area. During MAM the ITCZ is in a transient state,
centered near the equator where the observed SST in-
crease since 1970 (Fig. 1) has been greatest. With the
mean ITCZ location coinciding with the region of greatest
oceanic warming, the available thermal energy for con-
vective instability is maximized and the regional west–east
pattern of uplift and subsidence is greatest.
Farther south, a region of increased MAM rainfall ex-
tends from Western Australia to the southern and south-
eastern (Fig. 9c) regions of the continent. This band of
increased rainfall is associated with an enhanced moisture
advection from the tropical eastern Indian Ocean to the
southern regions of Australia (Fig. 7c).
During JJA, modeled rainfall trends over Australia are
weak and not significant in the ITR experiment (Fig. 9d).
During SON, there is a band of increased rainfall ex-
tending from the northwestern regions of the continent to
the eastern inland (Fig. 9e), as a result of the anomalous
FIG. 9. Linear trends in (a),(f) annual, (b),(g) DJF, (c),(h) MAM,
(d),(i) JJA, and (e),( j) SON mean precipitation over the period
1971–2005 (mm century21) for (left) the ITR experiment and (right)
observations estimated from a monthly gridded rainfall dataset
interpolated from high-quality station data (Lavery et al. 1997).
Areas within the thin black contours are significant at the 90% level
based on a Student’s t test.
1 APRIL 2010 L U F F M A N E T A L . 1671
moisture advection pathway (Fig. 7e). The observed rain-
fall trend is, however, not significant during this season.
5. Discussion and conclusions
We have sought to examine the climate response to
observed surface warming of the tropical Indian Ocean
through an ensemble of AGCM experiments, forced
only by the linear trend component of Indian Ocean
SST, with monthly varying climatological values else-
where across the globe. To test statistical significance we
conducted a five-member ensemble of the 35-yr runs
with identical forcing. Our forcing methodology allowed
us to examine, to a linear approximation, the particular
atmospheric signal of a warming Indian Ocean in iso-
lation from other potential forcing agents. An additional
set of experiments with the trend-forced SST region
extending east across the Pacific Ocean exhibited a cli-
mate response in agreement with our major findings.
Over the Northern Hemisphere extratropics, the warm-
ing tropical Indian Ocean induces a significant positive
trend in the NAM, which is in agreement with the findings
of Hoerling et al. (2004). No significant trend is forced in
the SAM; however, the Southern Hemisphere storm track
is enhanced over Indian Ocean longitudes as a result of an
anomalous increase in the meridional temperature gra-
dient and release of latent heat in baroclinic eddies.
It is worth noting that the changes simulated in the ITR
experiment are consistent with the responses of the hy-
drological cycle to global warming reported by previous
studies. For instance, Held and Soden (2006) show that in
a warmer-world scenario, the horizontal moisture trans-
port and the pattern of evaporation minus precipitation
tend to be enhanced. The intensification of the hydro-
logical cycle is also seen in our experiments over the In-
dian Ocean, with an enhancement of precipitation over
the tropical latitudes and a reduction over the sub-
tropics, especially in the Northern Hemisphere.
The regional response of the tropical atmosphere to
the linear trend component of Indian Ocean SST occurs
via significant modifications to the vertical circulation
in the tropics and a reorganization of moisture advec-
tion pathways. Surface warming over the tropical Indian
Ocean generates a decrease in sea level pressure and
enhances atmospheric convection, causing a convergence
of the wind field in the lower levels and a weakening of
the monsoon over adjacent land areas of southern Asia.
Convective outflow in the upper levels leads to a broad
area of anomalous subsidence over the South China Sea,
extending south over the western Pacific Ocean to the
Coral Sea. These major features remain robust when the
imposed warming is extended east into the Pacific Ocean.
A schematic diagram showing the regional climate re-
sponse to Indian Ocean warming is given in Fig. 10.
In general the Indian Ocean warming was found to be
efficient in forcing dry conditions over northern and
northeastern Australia. It also captures a broad region of
increased rainfall over the Indian Ocean itself, consistent
with warmer SST driving increased local convective cloud
formation and precipitation. The simulated decrease
over northern and northeastern Australia is a result of an
anomalous zone of subsidence caused by the divergent
outflow from increased convective activity over the trop-
ical Indian Ocean, coupled with an offshore trend in the
lower-tropospheric wind field. The confluence of these
two factors leads to a drying signal over northeastern
Australia detectable in the annual precipitation and during
MAM. Experiments forced with Pacific Ocean warming
FIG. 10. Schematic diagram representing the anomalous regional atmospheric response to
surface heating over the Indian Ocean. Horizontal vectors shown are indicative of the near-
surface layer (black) and the 800–500-hPa layer (red/blue). The vertical bold blue (red) arrows
indicate convection (subsidence), and the green arrow shows anomalous circulation at high
levels of the atmosphere.
1672 J O U R N A L O F C L I M A T E VOLUME 23
show a significant precipitation decrease in the south and
east but do not generate a rainfall decline over the far
northeastern region where the observed annual trend is
strongest. On the other hand, the IPTR1VR experiment
found to better capture the annual drying trend over the
eastern and northeastern regions Australia.
The modeled increase in annual and MAM rainfall
over Western Australia, caused by a weakening of the
prevailing offshore winds and increased atmospheric
moisture content, is of the same sign as the observed
changes but of smaller magnitude. In addition, the ob-
served east–west pattern of rainfall trends over Australia
is not clearly obtained in our experiments, indicating the
importance of other factors in these changes, such as
greenhouse, ozone, or aerosol forcing; for example, it
could be speculated that the increase of anthropogenic
aerosols over Asia has contributed to the intensification
of rainfall over northwestern Australia. Indeed, Rotstayn
et al. (2007) suggest that rainfall over this area has been
enhanced by a combination of increasing Asian anthro-
pogenic aerosols and warming Indian Ocean sea temper-
atures. However, Shi et al. (2008a) argue that this increased
rainfall in models may be a response to an unrealistic re-
lationship between the modeled eastern Indian Ocean SST
and northwest Australian rainfall. These hypotheses must
be tested with further experiments.
One limitation of our results linking Indian Ocean
warming to Australian rainfall trends is that the model
trends across northern Australia reach a maximum in the
austral autumn, whereas the observed trends peak in the
summer months. This may be a consequence of the linear
assumption implicit in using a single forcing mechanism
in the ITR experiment. Indeed, the ITR1VR and IPTR1VR
experiment sets, which include both the SST variability
and trends over the Indian Ocean and the Indian and
Pacific oceans, respectively, exhibited their strongest
modeled decrease over north and northeastern Australia
during DJF (figure not shown). The negative trend in
annual rainfall over the east coast of Australia is better
simulated in the IPTR1VR experiment, suggesting that
changes in both the Indian and Pacific oceans may ac-
count for a component of the observed drought.
Previous studies have suggested an influence of the
SAM on Australian rainfall trends (e.g., Meneghini et al.
2006 and Sen Gupta and England 2006). It is likely that
recent trends in the SAM may account for some fraction
of rainfall changes, particularly over the southern regions
of Australia. In addition, the warming of the Tasman Sea
may play an additional role in modulating Australian
rainfall, via alterations in the SAM as suggested by Shi
et al. (2008b). Here, we focused on atmospheric changes
arising from the recent warming over the tropical Indian
Ocean. Further investigation must be carried out to assess
the relative impact of the southern extratropics compared
to the tropical forcing studied here.
Over the Pacific Ocean, anthropogenic forcing by
greenhouse gases may have weakened the Walker circu-
lation, reducing the zonal gradient of SLP between the
western and central tropical Pacific Ocean (Vecchi et al.
2006). However, Vecchi et al. (2006) show that the region
of relative SLP increase reaches west into the Indian
Ocean, with a maximum adjacent to northwestern
Australia where precipitation has increased significantly in
recent decades. A broad increase of SLP across northern
Australia from the Pacific Ocean to the Indian Ocean
would fail to account for the observed east–west gradient
in rainfall trends over Australia observed in recent
decades.
The major modifications to the atmospheric circula-
tion imposed by the observed trend in Indian Ocean SST
suggest that this is an important climatic perturbation
through which changes in the global heat budget are pro-
jected onto regional climate. Given the resulting changes
in tropical atmospheric circulation and the projected
continuation of the Indian Ocean SST trend, studies into
the future evolution of this teleconnection should be
conducted.
Acknowledgments. This research was supported by the
Australian Research Council (ARC). Alex Sen Gupta
provided some assistance with Fig. 10.
REFERENCES
Arakawa, O., and A. Kitoh, 2004: Comparison of local precipitation–
SST relationship between the observation and a reanalysis data-
set. Geophys. Res. Lett., 31, L12206, doi:10.1029/2004GL020283.
Ashok, K., Z. Guan, and T. Yamagata, 2001: Impact of the Indian
Ocean dipole on the relationship between the Indian monsoon
rainfall and ENSO. Geophys. Res. Lett., 28, 4499–4502.
——, ——, and ——, 2003: Influence of the Indian Ocean dipole on
the Australian winter rainfall. Geophys. Res. Lett., 30, 1821,
doi:10.1029/2003GL017926.
Cai, W., P. H. Whetton, and D. J. Karoly, 2003: The response of the
Antarctic Oscillation to increasing and stabilized atmospheric
CO2. J. Climate, 16, 1525–1538.
——, T. Cowan, and A. Sullivan, 2009: Recent unprecedented
skewness towards positive Indian Ocean dipole occurrences
and its impact on Australian rainfall. Geophys. Res. Lett., 36,
L11705, doi:10.1029/2009GL037604.
Collins, W. D., and Coauthors, 2006: The formulation and atmo-
spheric simulation of the Community Atmosphere Model
version 3 (CAM3). J. Climate, 19, 2144–2161.
Drosdowsky, W., and L. E. Chambers, 2001: Near-global sea sur-
face temperature anomalies as predictors of Australian sea-
sonal rainfall. J. Climate, 14, 1677–1687.
Du, Y., and S. Xie, 2008: Role of atmospheric adjustments in the
tropical Indian Ocean warming during the 20th century in
climate models. Geophys. Res. Lett., 35, L08712, doi:10.1029/
2008GL033631.
1 APRIL 2010 L U F F M A N E T A L . 1673
Efron, B., 1982: The Jackknife, the Bootstrap, and Other Resampling
Plans. Society for Industrial and Applied Mathematics, 92 pp.
England, M. H., C. C. Ummenhofer, and A. Santoso, 2006: Inter-
annual rainfall extremes over southwest Western Australia
linked to Indian Ocean climate variability. J. Climate, 19, 1948–
1969.
Giannini, A., R. Saravanan, and P. Chang, 2003: Oceanic forcing of
Sahel rainfall on interannual to interdecadal time scales. Sci-
ence, 302, 1027–1030.
Gillett, N. P., and D. W. J. Thompson, 2003: Simulation of recent
Southern Hemisphere climate change. Science, 302, 273–275.
Hack, J. J., J. M. Caron, S. G. Yeager, K. W. Oleson, M. M. Holland,
J. E. Truesdale, and P. J. Rasch, 2006: Simulation of the global
hydrological cycle in the CCSM Community Atmosphere Model
Version 3 (CAM3): Mean features. J. Climate, 19, 2199–2221.
Held, I. M., and B. J. Soden, 2006: Robust responses of the hy-
drological cycle to global warming. J. Climate, 19, 5686–5699.
Hoerling, M. P., and A. Kumar, 2003: The perfect ocean for
drought. Science, 299, 691–694.
——, J. W. Hurrell, T. Xu, G. T. Bates, and A. S. Phillips, 2004:
Twentieth century North Atlantic climate change. Part II:
Understanding the effect of Indian Ocean warming. Climate
Dyn., 23, 391–405.
Kiktev, D., D. M. H. Sexton, L. Alexander, and C. K. Folland, 2003:
Comparison of modeled and observed trends in indices of
daily climate extremes. J. Climate, 16, 3560–3571.
Kumar, A., F. Yang, L. Goddard, and S. Schubert, 2004: Differing
trends in the tropical surface temperatures and precipitation
over land and oceans. J. Climate, 17, 653–664.
Lavery, B., G. Joung, and N. Nicholls, 1997: An extended high-
quality historical rainfall dataset for Australia. Aust. Meteor.
Mag., 46, 27–38.
Marshall, G. J., 2003: Trends in the southern annular mode from
observations and reanalyses. J. Climate, 16, 4134–4143.
Meneghini, B., I. Simmonds, and I. N. Smith, 2006: Association
between Australian rainfall and the southern annular mode.
Int. J. Climatol., 27, 109–121.
Nicholls, N., 1989: Sea surface temperature and Australian winter
rainfall. J. Climate, 2, 965–973.
——, 2006: Detecting and attributing Australian climate change: A
review. Aust. Meteor. Mag., 55, 199–211.
——, 2008: Recent trends in the seasonal and temporal behaviour
of the El Nino–Southern Oscillation. Geophys. Res. Lett., 35,
L19703, doi:10.1029/2008GL034499.
Quartly, G. D., E. A. Kyte, M. A. Srokosz, and M. N. Tsimplis, 2007:
An intercomparison of global oceanic precipitation climatol-
ogies. J. Geophys. Res., 112, D10121, doi:10.1029/2006JD007810.
Rayner, N. A., D. E. Parker, E. B. Horton, C. K. Folland,
L. V. Alexander, D. P. Rowell, E. C. Kent, and A. Kaplan,
2003: Global analyses of sea surface temperature, sea ice, and
night marine air temperature since the late nineteenth cen-
tury. J. Geophys. Res., 108, 4407, doi:10.1029/2002JD002670.
Rotstayn, L. D., and Coauthors, 2007: Have Australian rainfall
and cloudiness increased due to the remote effects of Asian
anthropogenic aerosols? J. Geophys. Res., 112, D09202,
doi:10.1029/2006JD007712.
Saji, N. H., and T. Yamagata, 2003: Possible impacts of Indian
Ocean dipole mode events on global climate. Climate Res., 25,151–169.
——, B. N. Goswami, P. N. Vinayachandran, and T. Yamagata,
1999: A dipole mode in the tropical Indian Ocean. Nature, 401,
360–363.
Sen Gupta, A., and M. H. England, 2006: Coupled ocean–
atmosphere–ice response to variations in the southern an-
nular mode. J. Climate, 19, 4457–4486.
Shi, G., W. Cai, T. Cowan, J. Ribbe, L. Rotstayn, and M. Dix,
2008a: Variability and trend of northwest Australia rainfall:
Observations and coupled climate modeling. J. Climate, 21,
2938–2959.
——, J. Ribbe, W. Cai, and T. Cowan, 2008b: An interpretation
of Australian rainfall projections. Geophys. Res. Lett., 35,
L02702, doi:10.1029/2007GL032436.
Smith, I., 2004: An assessment of recent trends in Australian
rainfall. Aust. Meteor. Mag., 53, 163–173.
Taschetto, A. S., and M. H. England, 2008: Estimating ensemble size
requirements of AGCM simulations. Meteor. Atmos. Phys.,
100, 23–36.
——, and ——, 2009: An analysis of late twentieth century trends in
Australian rainfall. Int. J. Climatol., 29, 791–807.
Thompson, D. W. J., and J. M. Wallace, 2000: Annular modes in the
extratropical circulation. Part I: Month-to-month variability.
J. Climate, 13, 1000–1016.
——, S. Lee, and M. P. Baldwin, 2003: Atmospheric processes
governing the Northern Hemisphere annular mode/North
Atlantic Oscillation. The North Atlantic Oscillation: Climatic
Significance and Environmental Impact, Geophys. Monogr.,
Vol. 134, Amer. Geophys. Union, 81–112.
Ummenhofer, C. C., A. Sen Gupta, M. J. Pook, and M. H. England,
2008: Anomalous rainfall over southwest Western Australia
forced by Indian Ocean sea surface temperatures. J. Climate,
21, 5113–5134.
——, ——, A. S. Taschetto, and M. H. England, 2009: Modulation
of Australian precipitation by meridional gradients in east
Indian Ocean sea surface temperature. J. Climate, 22, 5597–
5610.
Vecchi, G. A., B. J. Soden, A. T. Wittenberg, I. M. Held, A. Leetmaa,
and M. J. Harrison, 2006: Weakening of tropical Pacific atmo-
spheric circulation due to anthropogenic forcing. Nature, 441,
73–76.
Washington, R., and A. Preston, 2006: Extreme wet years over
southern Africa: Role of Indian Ocean sea surface tempera-
tures. J. Geophys. Res., 111, D15104, doi:10.1029/2005JD006724.
Watterson, I. G., 2001: Wind-induced rainfall and surface tem-
perature anomalies in the Australian region. J. Climate, 14,
1901–1922.
Yu, L., X. Jin, and R. Weller, 2007: Annual, seasonal, and in-
terannual variability of air–sea heat fluxes in the Indian
Ocean. J. Climate, 20, 3190–3209.
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