GEOPHYSICAL RESEARCH LETTERS, VOL. 38, L01701, doi:10.1029/2010GL045571, 2011
Sensitivity of South American summer rainfall to tropical Pacific
Ocean SST anomalies
K. J. Hill,1 A. S. Taschetto,1 and M. H. England1
Received 21 September 2010; revised 24 October 2010; accepted 3 November 2010; published 5 January 2011.
[1] A suite of idealised ensemble experiments are used to
investigate the sensitivity of the Southern Hemisphere
atmospheric circulation to the location of SST anomalies
over the tropical Pacific Ocean. Of particular interest is the
response of South American rainfall during austral summer.
The experiments reveal an approximately opposite response
in rainfall over South America when the tropical SST
forcing is applied over the western and eastern Pacific
Ocean, respectively. The contrasting tropical rainfall
conditions are due primarily to the displaced Walker
circulation anomalies. Further south, the atmospheric
circulation over the South American subtropics is modulated
by teleconnection patterns that appear as a wave train. The
resulting circulation manifests as an anomalous cyclone over
central‐eastern South America which in turn leads to a
northward displacement of the westerly moisture transport
when the SST forcing is located further west. The opposite
pattern occurs when the SST forcing is applied to the
eastern equatorial Pacific. Citation: Hill, K. J., A. S. Taschetto,
and M. H. England (2011), Sensitivity of South American summer
rainfall to tropical Pacific Ocean SST anomalies, Geophys. Res. Lett.,
38, L01701, doi:10.1029/2010GL045571.
1. Introduction
[2] The tropical Pacific Ocean experiences a range of sea
surface temperature (SST) anomalies over a variety of space
and time scales. These anomalies can result from both small‐
scale stochastic processes such as tropical instability waves
and weather‐scale atmospheric forcing, through to large‐scale
climate modes of variability such as the El Niño South
Oscillation (ENSO). In addition, there are large inter‐event
variations in the pattern of SST anomalies associated with
ENSO phases and ENSO events. There is also the potential for
climate change to alter the way that ENSO manifests in SST
anomalies over the next century [e.g., Collins et al., 2010, and
references therein]. The goal of this study is to evaluate the
role of the longitudinal position of tropical Pacific SST
anomalies in driving rainfall variations over South America.
[3] ENSO plays a fundamental role in regulating South
American climate; for example during El Niño events,
enhanced subsidence east of the anomalous Walker circulation suppresses rainfall across northeast Brazil [Ropelewski
and Halpert, 1987]. This forms part of a dipole‐like precipitation anomaly pattern with increased rainfall further south
over subtropical South America, which is commonly seen
during El Niño events [e.g., Grimm, 2003]. At these times the
1
Climate Change Research Centre, University of New South Wales,
Sydney, New South Wales, Australia.
Copyright 2011 by the American Geophysical Union.
0094‐8276/11/2010GL045571
summer monsoon is weaker, leading to anomalously dry
conditions over the north. It has been suggested that El Niño
events also modulate the occurrence of extreme precipitation
associated with intense convection in the South Atlantic
Convergence Zone (SACZ) by displacing it northward
[Carvalho et al., 2002]. In addition, during El Niño, warm
moist air from the Amazon basin can be transported from the
tropical latitudes to the subtropics along an enhanced northwesterly low‐level jet east of the Andes [e.g., Marengo et al.,
2004]. The presence of this low‐level jet is conducive to
enhanced rainfall over the La Plata basin and a weakened
SACZ in southeastern Brazil [e.g., Herdies et al., 2002].
During La Niña events the rainfall conditions observed over
South America are almost opposite to El Niño [Grimm et al.,
2000]. For example, the summer monsoon is enhanced along
with the rainfall and convection in the SACZ, while further
south over the La Plata basin the rainfall is suppressed.
[4] The current understanding of the impact of different
flavors of El Niño on South American rainfall is somewhat
more limited. Silva and Ambrizzi [2006] showed observational evidence that the El Niño Modoki event of 2002/2003
displaced the position of the South American low‐level jet,
resulting in less moisture transport to the south compared to
the classical 1997/1998 event. Numerical ensemble experiments forced by the tropical Pacific SST anomalies of these
two types of El Niño events also show a northward displacement of the moisture fluxes, modifying the dipole
precipitation between the two event types [Hill et al., 2009].
Following on from Hill et al. [2009] this study examines the
role of the location of warm tropical Pacific SST anomalies
in driving an atmospheric circulation response and rainfall
variability over South America during austral summer.
2. Data Analysis and Atmospheric Model
[5] The model employed in this study is the National Centre
for Atmospheric Research (NCAR) Community Atmosphere
Model (CAM3), configured at T42 horizontal resolution
(approximately 2.8° latitude‐longitude) with 26 hybrid sigma‐
pressure vertical levels. For a detailed description of the model
see Collins et al. [2006]. Four experiments consisting of a
50 member‐ensemble were performed for 12 months, with an
idealised +1°C warming anomaly imposed over different
regions across the tropical Pacific (see Figure 1a). The SST
anomalies were linearly damped to zero over a 10° latitude/
longitude band; elsewhere the SST forcing is taken to be the
climatological monthly mean. The control experiment is
simply a 50 year integration forced by the 12‐month climatology of global SST.
[6] The anomaly regions of Figure 1a were not selected to
reflect the approximate position of anomalies associated
with El Niño, La Niña and El Niño Modoki events. For
L01701
1 of 6
L01701
HILL ET AL.: PACIFIC SST FORCING OF SOUTH AMERICAN SUMMER RAIN
L01701
Figure 1. (a) Regions defining the SST anomalies that were superimposed on the annual cycle to generate the WPAC,
CW PAC , CE PAC and E PAC idealised Pacific warming experiments. (b) Ensemble mean precipitation anomalies
(mm day−1) across the tropical Pacific averaged between 5°N–5°S for each experiment set. Precipitation anomalies
(mm day−1) and anomalous vertically integrated moisture flux and divergent moisture flux between 1000‐500hPa over
South America for the (c, g) WPAC, (d, h) CWPAC, (e, i) CEPAC and (f, j) EPAC ensemble sets. Units in Figures 1g–1j
are kg s−1 for contour shading and kg m−1 s−1 for the vectors. Maximum vector length (in kg m−1 s−1) is shown below Figure
1j. The black vectors and colour shaded regions are statistically significant at the 95% confidence level according to a two‐
tailed student t‐test.
instance, one could argue that a better idealised representation of a La Niña event would be a cooling in the eastern
Pacific coupled with a warming in the west. Here, the
locations merely span the zonal extent of the Pacific Ocean
and provide a broad means of comparing the resultant
atmospheric circulation and rainfall impacts associated with
warm SST anomalies in each region. The imposition of a
+1°C warming anomaly is also highly idealised: for example, typical anomalies in the eastern Pacific can be of much
larger magnitude than those in the west. Nonetheless, given
the goal of this study is to better understand the impact of
SST anomalies as a function of geographic location along the
tropical Pacific, it is appropriate that a uniform SST anomaly
be applied. Where possible, comparison will be made with
the climate regime and atmospheric circulation observed
during El Niño, La Niña, and El Niño Modoki events, but this
is not the central goal of the study. Furthermore the focus of
this paper is on austral summer, i.e., December, January and
February (DJF), when South America experiences its monsoon season, and when warming events in the Pacific tend to
have their greatest impact.
3. Results
[7] Figure 1b compares the simulated rainfall anomalies
in the four ensemble experiments, averaged over the tropical
2 of 6
L01701
HILL ET AL.: PACIFIC SST FORCING OF SOUTH AMERICAN SUMMER RAIN
Pacific (5°N–5°S), while the precipitation anomalies over
South America are shown in Figures 1c–1f. The simulated
moisture advection is illustrated through the vertically
integrated moisture flux and convergence anomalies in
Figures 1g–1j. The direct impact on the atmosphere from the
tropical SST anomalies is examined via the Walker circulation, which is represented by the 200hPa velocity potential
anomalies across the globe (Figure 2, left) and the vertical
velocity anomalies across the tropics (Figure 2, right). The
Southern Hemisphere low‐level circulation anomalies are
highlighted in Figure 3 (left), illustrating the 850hPa winds
and sea level pressure (SLP) anomalies. The upper level
circulation response is indicated by the 200hPa asymmetric
streamfunction anomalies (Figure 3, right). As we are
interested in both the direct forcing in the tropics (i.e., the
Walker circulation anomalies) and the teleconnections into
the subtropics (i.e., wave train anomalies) the following
analysis will be stratified accordingly.
3.1. Tropical Response
[8] In the two western Pacific experiments (CWPAC and
WPAC) there is reduced South American rainfall north of the
equator and enhanced rainfall south to approximately 20°S
(Figures 1c and 1d). These rainfall anomalies result from an
enhanced summer‐time circulation over the continent, i.e.,
from the southward progression of the summer monsoon
into the Southern Hemisphere. This leaves the far north dry,
while south of the Equator convection and precipitation
increase, which is typical of La Niña conditions [Grimm,
2003]. The reduced rainfall north of the equator is associated with an anomalous anticyclonic circulation that leads
to easterly flux and divergence of moisture over the continent, while to the south increased moisture in the monsoon
region is advected to central and eastern South America by
anomalous westerly moisture fluxes (Figures 1g and 1h).
These westerly moisture flux anomalies result in increased
available moisture over the Amazon basin and in the region
where the SACZ forms.
[9] The above rainfall anomalies are the result of changes
in the Walker circulation, which in the western Pacific
experiments (Figures 2a–2d) exhibit anomalous subsidence
over the north of the continent, acting to reduce convection
and rainfall during DJF. There is also anomalous subsidence
over the Indian Ocean (Figures 2a–2d) and anomalous uplift
over the Indonesian region in these experiments, particularly
in the WPAC case (Figures 2b and 2d). As the SST forcing is
displaced eastward in the CWPAC experiment, the uplift
anomaly also shifts to the east, having a maximum magnitude around the dateline (Figure 2d). Polewards of 5°S, over
South America, there is anomalous uplift in both the WPAC
and CWPAC experiments (Figures 2a and 2c) which coincides with the increased convection and precipitation of the
summer monsoon in that region (Figures 1c and 1d and
Figures 1g and 1h).
[10] A common feature of the eastern Pacific experiments
(EPAC and CEPAC) is the significantly reduced rainfall across
northern South America, which extends southward into the
region where the SACZ forms (Figures 1e and 1f). Associated with this reduced rainfall in both cases is an anomalous divergence of moisture driven by an anomalous
easterly moisture flux (Figures 1i and 1j), which indicates a
weakening of the climatological moisture advection into the
SACZ. The widespread anomalous divergence of moisture
L01701
also indicates an overall weakening of the summer monsoon, which leads to a decrease in moisture availability.
There is also increased rainfall over northwest South
America in the eastern Pacific experiments. In the CEPAC
experiment this positive rainfall anomaly stretches from the
northwest coast to 60°W, while in the EPAC experiment it is
confined to the northwest coast.
[11] The strongly enhanced rainfall over the northwest
coast of South America in the EPAC case is driven by the
SST anomaly located directly adjacent to the coast, enhancing local uplift and strengthening the subsidence over the
northern proportion of the continent. This reduces rainfall in
that region and is similar to the classical El Niño response
over South America. The winds converge over the northwest
coast in this case (Figures 1i and 1j and Figures 3e and 3g)
and in turn diverge over northeastern South America, where
reduced rainfall occurs. In contrast, the SST anomaly and
resultant uplift in the CEPAC case is located further offshore,
drawing with it the strongest subsidence and divergence
of moisture. The weaker subsidence over South America
allows more of the widespread climatological monsoonal
uplift and precipitation to occur over the northwest, despite
the offshore location of the SST anomalies, which can be
seen in the plots of vertical velocity (Figure 2g). Further east
of the uplift over northeastern South America, the subsidence
appears to strengthen, resulting in a rainfall deficit. The
velocity potential anomalies in CEPAC exhibit a double
Walker circulation structure (Figures 2e and 2f), which is a
feature of El Niño Modoki events [Ashok et al., 2007].
3.2. Subtropical Response
[12] In the subtropics the rainfall anomalies are also
opposite in sign between the western and eastern Pacific
experiments. When SST warming is applied in the western
(eastern) equatorial Pacific Ocean, rainfall is significantly
reduced (enhanced) around 30°S, 50°W. These opposing
subtropical circulation anomalies seem to be the result of a
Pacific South America (PSA) [Mo, 2000, and references
therein] teleconnection (Figure 3, right) emanating from the
region of warm SST anomalies in the tropical Pacific
[Grimm, 2003; Silva and Ambrizzi, 2006; Hill et al., 2009].
The teleconnection structure manifests as a mid‐latitude
wave train that originates in the upper troposphere by
divergent outflow from strong tropical deep convection.
This stationary Rossby wave shows an equivalent barotropic
pattern seen by the asymmetric streamfunction in the upper
troposphere and the low‐level circulation anomalies in the
southern subtropics (Figure 3). The teleconnection appears
to modulate the South American mid‐latitude circulation in
the western (eastern) Pacific experiments via an anticyclonic
(cyclonic) wind anomaly over the region (Figure 3, left).
This anomalous circulation in the subtropics drives anomalous southeasterly (northwesterly) moisture flux acting
to weaken (strengthen) the northwesterly low‐level jet.
Consequently, this circulation impacts further north, leading to an enhanced (suppressed) SACZ and higher (lower)
precipitation due to the anomalous westerly (easterly) moisture advection.
4. Summary and Conclusions
[13] This study investigates the sensitivity of the circulation over South America to the location of a uniform +1°C
3 of 6
L01701
HILL ET AL.: PACIFIC SST FORCING OF SOUTH AMERICAN SUMMER RAIN
L01701
Figure 2. (left) Anomalous velocity potential at 200hPa and (right) anomalous vertical velocity averaged between 5°N–5°S
for the (a, b) WPAC, (c, d) CWPAC, (e, f) CEPAC and (g, h) EPAC experiments sets. Units in Figure 2 (left) are 106 m2 s−1
and 10−2 m s−1 in Figure 2 (right). Vertical velocity is upwards for negative values (blue shaded regions). The dashed line
indicates statistical significance at the 95% level according to a two‐tailed student t‐test.
SST anomaly imposed along the tropical Pacific Ocean. In
response to the progressive displacement of the SST anomalies across the experiments, the large‐scale circulation is also
displaced throughout the atmosphere. The shifted circulation
anomalies are highlighted by the regions of uplift in the vertical velocity (Figure 2) and by the quadrapole pattern straddling the equator in the 200hPa asymmetric streamfunction
anomalies (Figure 3, right), as in the Gill‐Matsuno response to
diabatic heating imposed on the equator.
[14] Due to the westward location of the SST anomalies in
WPAC and CWPAC, these two ensemble sets show similarities to La Niña impacts in terms of the average tropical
Pacific response (Figure 1b). Yet both central Pacific
experiments (CWPAC and CEPAC) exhibit a more Modoki‐
like atmospheric circulation across the equatorial Pacific,
with a clear double Walker circulation, particularly in the
CEPAC case (Figures 2e and 2f). However, the South
American rainfall response in the CEPAC and CWPAC
experiments is almost opposite in sign, revealing a very
different teleconnection to South American rainfall when
SST anomalies are applied either side of the dateline. The
EPAC case exhibits the most El Niño‐like circulation and
rainfall response over South America, even though the SST
anomaly of +1°C is relatively modest and only extends as
far west as 120°W, compared to the dateline in observed
intense El Niño events. Finally we note that the difference
between the atmospheric response of the WPAC and CWPAC
experiments is relatively small, which is due primarily to the
sensitivity to the warm underlying SSTs where these two
idealised anomalies are applied.
[15] As emphasised previously, the applied SST anomalies of +1°C are not fully representative of the anomalies
4 of 6
L01701
HILL ET AL.: PACIFIC SST FORCING OF SOUTH AMERICAN SUMMER RAIN
L01701
Figure 3. (left) Anomalous SLP (mb) and 850hPa wind anomalies (m s−1) and (right) 200hPa asymmetric streamfunction anomalies (m2 s−1) for the (a, b) WPAC, (c, d) CWPAC, (e, f) CEPAC and (g, h) EPAC ensemble sets. The dashed line
in Figure 3 (right) indicates statistical significance at the 95% level according to a two‐tailed student t‐test. The maximum
anomalous wind vector scale is in m s−1 below Figure 3g.
that can occur over the tropical Pacific Ocean during El Niño
events. The eastern Pacific can experience much larger variations in SST than the central and western Pacific; for
example the 1997/1998 El Niño saw SST anomalies of up to
2.4°C in the eastern Pacific during DJF. In contrast, the 2002/
2003 El Niño Modoki showed average maximum SST
anomalies of just 1.2°C over the central Pacific during austral
summer. This does not necessarily mean that the impact of
Modoki events is, a priori, weaker than traditional ENSO
episodes; they can have significant impacts around the globe
[e.g., Ashok et al., 2007; Weng et al., 2007; Wang and
Hendon, 2007; Taschetto et al., 2009].
[16] Overall the simulated tropical rainfall anomalies over
South America reveal a predominantly opposite spatial distribution between the eastern and western Pacific experiments. This is particularly the case for rainfall over eastern
South America in the SACZ region. Here, in the WPAC and
CWPAC experiments, there is a normal‐to‐enhanced austral
summer circulation, including increased monsoon convection and precipitation plus westerly moisture advection,
favouring higher precipitation. In contrast, in the eastern
Pacific experiments (CEPAC and EPAC), the monsoon is
suppressed with weakened moisture advection into the
SACZ region.
[17] Further south in the subtropics the rainfall response
also shows a reversal in sign between the WPAC/CWPAC and
the CEPAC/EPAC experiments. This anomaly reversal occurs
not only due to the modified anomalies in the tropics but it is
also enhanced by the modified trajectory of the PSA‐like
wave train that emanates from the region of maximum SST
anomaly in each experiment. This has important implications for interannual rainfall variability over South America.
5 of 6
L01701
HILL ET AL.: PACIFIC SST FORCING OF SOUTH AMERICAN SUMMER RAIN
In addition, there may also be implications, for future climate if global warming results in a non‐uniform temperature
transformation of the tropical Pacific.
[18] Acknowledgments. This study was supported by the Australian
Research Council. The model simulations were conducted on the Australian
National Computational Infrastructure (NCI) supercomputing facility.
References
Ashok, K., S. K. Behera, S. A. Rao, H. Weng, and T. Yamagata (2007),
El Niño Modoki and its possible teleconnection, J. Geophys. Res.,
112, C11007, doi:10.1029/2006JC003798.
Carvalho, L. M., C. Jones, and B. Liebmann (2002), Extreme precipitation
events in southeastern South America and large‐scale convective patterns
in South Atlantic Convergence Zone, J. Clim., 15, 2377–2394.
Collins, M., et al. (2010), The impact of global warming on the tropical
Pacific Ocean and El Niño, Nat. Geosci., 3, 391–397.
Collins, W. D., et al. (2006), The Community Climate System Model
version 3 (CCSM3), J. Clim., 19, 2122–2143.
Grimm, A. M. (2003), The El Niño impact on the summer monsoon in
Brazil: Regional processes versus remote influences, J. Clim., 16, 263–280.
Grimm, A. M., V. R. Barros, and M. E. Doyle (2000), Climate variability
in southern South America associated with El Niño and La Niña events,
J. Clim., 13, 35–58.
Herdies, D. L., A. da Silva, M. A. F. Silva Dias, and R. Nieto Ferreira
(2002), Moisture budget of the bimodal pattern of the summer circulation
over South America, J. Geophys. Res., 107(D20), 8075, doi:10.1029/
2001JD000997.
L01701
Hill, K. J., A. S. Taschetto, and M. H. England (2009), South American
rainfall impacts associated with inter‐El Niño variations, Geophys.
Res. Lett., 36, L19702, doi:10.1029/2009GL040164.
Marengo, J. A., W. R. Soares, C. Saulo, and M. Nicolini (2004), Climatology
of the low‐level jet east of the Andes as derived from the NCEP‐
NCAR reanalyses: Characteristics and temporal variability, J. Clim., 17,
2261–2280.
Mo, K. (2000), Relationships between low‐frequency variability in the
Southern Hemisphere and sea surface temperature anomalies, J. Clim.,
13, 3599–3610.
Ropelewski, C. F., and M. S. Halpert (1987), Global and regional scale
precipitation patterns associated with the El Niño Southern Oscillation,
Mon. Weather. Rev., 115, 1606–1626.
Silva, G. M., and T. Ambrizzi (2006), Inter‐El Niño variability and its
impact on the South American low‐level jet east of the Andes during
austral summer: Two case studies, Adv. Geosci., 6, 283–286.
Taschetto, A. S., C. C. Ummenhofer, A. Sen Gupta, and M. H. England
(2009), Effect of anomalous warming in the central Pacific on the
Australian monsoon, Geophys. Res. Lett., 36, L12704, doi:10.1029/
2009GL038416.
Wang, G., and H. H. Hendon (2007), Sensitivity of Australian rainfall to
inter‐El Niño variations, J. Clim., 20, 4211–4226.
Weng, H. Y., K. Ashok, S. K. Behera, S. A. Rao, and T. Yamagata (2007),
Impacts of recent El Niño Modoki on dry/wet conditions in the Pacific
rim during bo rea l sum mer, C l i m . Dy n ., 2 9(2–3) , 11 3–129,
doi:10.1007/S00382-007-0234-0.
M. H. England, K. J. Hill, and A. S. Taschetto, Climate Change Research
Centre, University of New South Wales, Sydney, NSW 2052, Australia.
([email protected])
6 of 6