INTERNATIONAL JOURNAL OF CLIMATOLOGY
Int. J. Climatol. 23: 529–539 (2003)
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/joc.900
ANOMALIES OF THE SOUTH AMERICAN SUMMER MONSOON
ASSOCIATED WITH THE 1997–99 EL NIÑO–SOUTHERN OSCILLATION
K.-M. LAUa, * and JIAYU ZHOUb
Laboratory for Atmospheres, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
Goddard Earth Sciences and Technology Center, University of Maryland, Baltimore, MD 21250, USA
a
b
Received 21 January 2002
Revised 31 January 2003
Accepted 3 February 2003
ABSTRACT
We describe the rainfall and circulation anomalies of the South American summer monsoon (SASM) during
December–January–February (DJF) of 1997–98 (El Niño) and 1998–99 (La Niña). The most pronounced rainfall
signals in DJF 1997–98 include (a) excessive rainfall over northern Peru and Ecuador, (b) deficient rainfall over northern
and central Brazil, and (c) above-normal rainfall over southeastern subtropical South America. The rainfall anomalies in
(a) and (b) are associated with the excitation of an anomalous east–west overturning cell with rising motion and low-level
westerlies over the equatorial eastern Pacific, coupled to sinking motion and low-level easterlies over northern Brazil.
The easterlies turn sharply southeastward on encountering the steep topography of the Andes, enhancing the summertime
low-level jet (LLJ) along the eastern foothills of the Andes near 15–20 ° S, possibly contributing to the increased rainfall
in (c).
During DJF 1997–98, the sea-surface temperature-induced warming spreads and expands over the entire tropical
troposphere. The eastward expansion of a warm upper tropospheric geopotential and temperature ridge from the Niño-3
region, across subtropical South America to the southeast Atlantic, enhances warming over the Altiplano Plateau,
hydrostatically strengthening the Bolivia high. Similar to previous warming events, the South Pacific high is weakened,
and the South Atlantic high is strengthened. During DJF 1998–99, as cold water develops over the equatorial central
Pacific, the SASM anomalies in the tropics are weaker and less organized and appear to be in transition to the opposite
phase to those found in DJF 97–98. In the subtropics, notable features include a weakening of the LLJ, a rainfall pattern
associated with a poleward shift of the South Atlantic convergence zone, and development of the Pacific–South America
teleconnection pattern. Published in 2003 by John Wiley & Sons, Ltd.
KEY WORDS:
ENSO; South American summer monsoon; interannual variability; low-level jets
1. INTRODUCTION
The occurrences of severe droughts, at intervals of 2–7 years, with devastating impacts in the Amazon and
northeast Brazil have been known for a long time (Walker, 1928). Many previous studies (e.g. Ropelewski
and Halpert, 1987; Aceituno, 1988; Marengo, 1992; Rasmusson and Mo, 1993) have shown that the severe
droughts over northeastern Brazil, also known as the Nordeste droughts, are strongly influenced by sea surface
temperature (SST) anomalies in the tropical Pacific associated with the El Niño–southern oscillation (ENSO).
Others have shown that SST anomalies in the tropical Atlantic may be important (Hasternrath and Heller,
1977; Moura and Shukla, 1981; Mechoso et al., 1990; Curtis and Hastenrath, 1995; Nobre and Shukla, 1996;
Rao et al., 1996). Results from dynamical models have also suggested that the reduced latent heating from
deficient rainfall over the Amazon Basin during El Niño may reduce the intensity of the Bolivian high in
austral summer and hence signals the weakening of the South American summer monsoon (SASM) during
an El Niño (Silva Dias et al., 1983; Bell et al., 1999).
* Correspondence to: Dr K.-M. Lau, Laboratory for Atmospheres, Code 910, NASA/GSFC, Greenbelt, MD 20771, USA;
e-mail: [email protected]
This article is a US Government work and is in the public domain in the USA.
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K.-M. LAU AND J. ZHOU
On the other hand, the low-level jet (LLJ) at Santa Cruz, Bolivia, was found to have been strengthened
during a special pilot balloon observation period in the austral summer of 1997–98 (Douglas et al., 1999).
Others have found a strong relationship between the strength of the LLJ, moisture transport and rainfall
variability in the region of Uruguay and southern Brazil in austral summer (Berri and Inzunza, 1993; Berbery
and Collini, 2000). Recent studies have observed that during El Niño the increased summertime LLJ enhances
the climatological SASM low-level flow from northwestern Brazil to subtropical southeastern South America,
suggesting an enhancement of the SASM due to El Niño (Zhou and Lau, 1998, 2001).
The years 1997–99 witnessed the strongest ENSO event of the 20th century. The amplitude of the SST
anomaly in the equatorial eastern Pacific was about four times the standard deviation of interannual variability
for the previous two decades, with the SST warming centre of 1997–98 located at around 100 ° W, much
closer to the South American continent than in previous events (Bell et al., 1999). Given the strength of the
1997–99 warm event, it will be instructive to examine the anomalies of the SASM with respect to the SST
anomalies during that period. In this study, we will describe aspects of the aforementioned features regarding
SASM anomalies observed during the austral summers of 1997–98 (El Niño) and 1998–99 (La Niña). The
observed anomalies will be evaluated against the known ENSO response of the SASM from previous events,
to determine the extent to which the anomalies are related to ENSO forcings.
2. DATA DESCRIPTION
The key data set used in this study is the monthly mean National Centers for Environmental Prediction
(NCEP) reanalysis for the period 1979–2001 (Kalnay et al., 1996), with recent rerun to correct the problem
of processing satellite temperature data over land (http://wesley.wwb.noaa.gov/tovs− problem/). The spatial
resolution of the data is 2.5° latitude by 2.5° longitude with 17 pressure levels in the vertical. For SST, we
use the monthly NCEP 1° × 1° analysis, which incorporates in situ data from the global telecommunication
system, as well as satellite observations from the National Environment Satellite Data and Information Service
(NESDIS; Reynolds and Smith, 1994). For rainfall, we use the Climate Prediction Center Merged Analysis
Product (CMAP) data, which merge satellite-derived rainfall estimates over oceans and gauge observations
over land on a 2.5° × 2.5° latitude–longitude global grid (Xie and Arkin, 1997). In all the results presented,
the anomalies are defined with respect to the climatology for the period 1979–2001.
3. RESULTS
The following analysis will be focused on the SASM anomalies in December–January–February (DJF)
1997–98 and 1998–99. To evaluate the relevance of the 1997–99 anomalies to ENSO forcings, we briefly
discuss the basic features of the response of the SASM to ENSO based on past events. The general features
described in the next section are not new and have been reported by many previous studies (e.g. Mechoso
et al., 1990; Nogues-Paegle and Mo, 1997; Aceituno, 1998; Hastenrath, 2000).
3.1. The canonical ENSO response (CER)
For convenience and data consistency, we use the rainfall and circulation anomalies derived from the first
empirical orthogonal mode of low-pass filtered DJF SST and rainfall anomaly for the period of 1980–95 as
the basis of the CER (see Zhou and Lau, 2001). Since the CER patterns are based on statistics from multiple
events keyed to tropical eastern Pacific SST anomalies, they represent patterns that reflect the response to
ENSO SST anomalies.
The CER features deficient rainfall in north-northeast Brazil and the equatorial Atlantic, excessive rainfall
over the west coast of Ecuador and northern Peru, and the subtropical South America region of Paraguay,
Uruguay, southern Brazil and northern Argentina (Figure 1(a)). The northwest African high and the South
Atlantic high (SAH) are enhanced, while the subtropical high of the South Pacific is weakened (Figure 1(b)).
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Figure 1. The CER pattern showing (a) ENSO mode of DJF rainfall and (b) the regression of the ENSO mode against sea-level pressure
and 850 hPa wind. Major cyclonic (C) and anticyclonic circulation centres (A) are indicated. Unit of rainfall is in mm day−1 , sea level
pressure is in hPa and wind in m s−1 (adapted from Zhou and Lau (2001))
This large-scale surface pressure anomaly pattern is consistent with the eastward shift of the Walker
circulation during an El Niño, signalled by low-level westerly anomalies over the equatorial eastern Pacific
and anomalous easterlies over the equatorial Atlantic. The anomalous easterlies over northern Brazil turn
sharply southeastward as they are deflected by the Andes, leading to an enhanced LLJ. The LLJ reinforces
the northwesterly flow, which is also strengthened as part of the anticyclonic (geostrophic) flow around the
enhanced SAH, associated with an intensified South Atlantic convergence zone (SACZ).
3.2. Rainfall and circulation anomalies during 1997–99
The rainfall anomaly pattern during DJF of 1997–98 is shown in Figure 2(a). Compared with CER, the
rainfall deficit over the Atlantic intertropical convergence zone and northern Brazil is more extensive, including
much of the Amazon region. The region of excessive rainfall over southeastern South America is found near
30–35 ° S, which is further poleward by about 5° latitude compared with CER. Also noted is enhanced rainfall
along the west coast of equatorial central Africa (0–15 ° S), which is not apparent in the CER (not shown in
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Figure 2. DJF 1997–98 anomaly pattern for (a) rainfall, (b) 850 hPa wind and streamlines, and (c) 200 hPa velocity potential; (d),
(e) and (f) are the same as (a), (b) and (c) respectively, except for DJF 1998–99. Major anticyclonic and cyclonic centres are denoted
by letters A and C respectively. Unit of rainfall is mm day−1 , wind is m s−1 , and velocity potential is 106 m2 s−1
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Figure 1(a)). As discussed below, the Africa heat source may further modify the response of the SASM to
the 1997–99 ENSO.
During DJF 1997–98, the most prominent circulation features are two well-defined low-level anticyclones
on opposite sides of the equator (marked by the letter ‘A’ over northern and southern Brazil in Figure 2(b)),
with easterlies sandwiched between them. The anticyclones are coupled with a cyclone pair over the tropical
eastern Pacific, approximately symmetric about the equator, but with centres (indicated by letter ‘C’) separated
further in the meridional direction. The overall circulation anomaly in DJF 1997–98 indicates an increased
northwest Africa high and enhanced SAH. The cyclonic flow off the coast of Chile confirms a weakening
of the South Pacific high (SPH). These features are similar to CER, except the anticyclone centres are more
pronounced over the land regions of eastern South America. Additionally, a pair of cyclones straddling the
equator is found over the coast of West Africa. The locations of the paired anticyclones and cyclones straddling
the equator are consistent with the Rossby wave response to a heat source over the equatorial tropical Pacific,
a heat sink over northern Brazil, and a secondary heat source over the west coast of Africa (Gill, 1980).
They are also in agreement with the observed rainfall anomalies shown in Figure 2(a). The corresponding
anomalous equatorial rising and sinking motions are evident in the 200 hPa velocity potential anomalies
(Figure 2(c)). The aforementioned features suggest that, in DJF 1997–98, anomalous east–west overturning
circulations are more intense and compact in horizontal scale compared with CER.
In conjunction with the overturning cells, strong low-level easterlies are confined to the land regions
of northern Brazil, while low-level westerlies, instead of easterlies in CER, are found over the equatorial
Atlantic, stretching eastward to the west coast of Africa. As is evident in Figure 2(b), the low-level anomalous
easterlies over northern Brazil are deflected by the steep topography of the Andes and split into two branches.
The northern branch flows northward into the Caribbean Sea, and the southern branch becomes a part of
the LLJ that penetrates southward into subtropical South America. While the northern branch opposes, the
southern branch substantially enhances the climatological summer monsoon circulation over these regions.
The increased pressure gradient from west to east on the eastern slope of the Andes (not shown) coupled
to the strong northwesterly flow spun off by the anticyclone over southern Brazil reinforces and extends the
climatological LLJ southward. Compared with the CER, the LLJ in 1997–98 is much more enhanced and
penetrates deeper into the extratropics, pushing the maximum rainfall anomaly further poleward.
During DJF 1998–99, above-normal rainfall is found over northwestern South America (Figure 2(d)) in
association with the establishment of a pair of anomalous cyclones: one centred to the west of Colombia, and
another one over tropical western Brazil (marked by the letter ‘C’ in the two locations shown in Figure 2(e)).
Although deficient rainfall is still found over northeastern and eastern Brazil, the anomalous anticyclones
are substantially weakened from DJF 1997–98. Increased rainfall is found over the equatorial Atlantic,
where the anomalous westerlies have turned northeastward, merging with a cyclonic circulation anomaly
over northwestern Africa (Figure 2(e)). South of 25 ° S, a pronounced wave-like pattern is established, with
anticyclonic circulation over the South Pacific, eastern South Atlantic, and cyclonic circulation over the
western South Atlantic. Over subtropical southeastern South America, the rainfall anomaly pattern implies
a southward displacement of the SACZ (Figure 2(d)). The anomalous LLJ shows a weak reversal from
the previous year, with a southeasterly flow emanating from Uruguay (∼35 ° S) and merging with the
cyclonic circulation over Bolivia and western Brazil. This circulation pattern represents a weakening of
the climatological SASM LLJ. The partial reversal of the anomalous east–west overturning motion is evident
in the upper-level velocity potential anomalies (Figure 2(f)) with rising motion over northern Peru and
northwestern Brazil. Figure 2(f) also shows that the rising motion over West Africa has been further enhanced
in DJF 1998–99. This rising motion has no correspondence in CER and may be related to intrinsic fluctuation
of heat sources/sinks over West Africa. Overall, the 1998–99 anomalies are consistent with a warm event in
retreat, developing anomalies with opposite signs relative to those in DJF 1997–98.
3.3. Geopotential anomalies
During DJF 1997–98, widespread tropospheric warming induced by the SST anomalies in the central and
eastern tropical Pacific has led to increased geopotential height anomalies over much of the tropics north
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Figure 3. DJF 200 hPa geopotential height for (a) 1997–98 anomaly, (b) 1997–98 total field, (c) 1998–99 anomaly, and (d) 1998–99
total field. Letters H and L denote relative highs and lows, and BH denotes the Bolivia high. Units are in gpm, with 12 000 gpm
subtracted
of 30 ° S (Figure 3(a)). This is consistent with a strong zonally symmetric component of the temperature
response in the upper troposphere (see discussion in Section 3.4). In addition to the overall geopotential
increase, relative highs and lows are well established. A pair of highs on opposite sides of the equator
over the equatorial Pacific (indicated by letter ‘H’ in Figure 3(a)), and a similar pair of lows (indicated by
letter ‘L’) over the tropical Atlantic, is dynamically consistent with the Rossby wave response to heating
and cooling in the near-equatorial regions noted above. The southern high centre extends southeastward,
forming a ridge emanating from the tropical South Pacific across the Altiplano, southeastern Brazil, to
the southeastern South Atlantic. Similar patterns are noted in temperature anomalies in the mid-to-upper
troposphere (not shown). As a result of the southeast extension of the warm ridge, the Bolivia high is
increased in DJF 1997–98 (Figure 3(b)) compared with that in DJF 1998–99 (Figure 3(d)). In Figure 3(a),
the anomalous high over the southeastern South Atlantic approximately collocates with the lower tropospheric
anticyclone (see Figure 2(b)), suggesting an equivalent barotropic vertical structure, typical of extratropical
teleconnection patterns. Another pronounced feature in Figure 3(a) is an elongated low height anomaly in
the southwestern region of the domain, which contrasts with the increased height in the tropics. The tight
latitudinal gradient implies an enhanced geostrophic westerly jet stream at 200 hPa near 30 ° S spanning the
southeastern Pacific, subtropical South America and the southwestern Atlantic. The vertical profile of zonal
wind (not shown) indicates an equatorward shift of the climatological subtropical jet stream at 100–200 hPa
by about 5° latitude, coinciding approximately with the latitude of the maximum precipitation anomaly over
subtropical South America. The general warming of the upper troposphere and increased westerlies over the
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Altiplano during warm events have been noted in previous studies (Vuille, 1999; Garreaud and Aceituno,
2001).
During DJF 1998–99, the 200 hPa anomalous geopotential (Figure 3(c)) features an anomalous low over
the near-equatorial eastern Pacific in direct response to the establishment of cold water there. Over the tropical
Atlantic, two anomalous upper highs replace the lows in 1997–98. The weakening of the Bolivia high is
quite obvious. As shown in Figure 3(d), the maximum height over the Altiplano is 12 440 gpm (440 + 12 000)
during 1998–99 compared with the maximum of 12 505 gpm (505 + 12 000) during 1997–98 (Figure 3(b)).
In the extreme southern part of the domain, a pronounced high–low–high teleconnection pattern is found.
Comparing Figures 3(c) and 2(e), the upper and lower level patterns appear to be approximately barotropic.
However, some degree of baroclinicity is also evident in the westward shift of the upper-level low with
respect to the centre of the low-level cyclone over Uruguay. This mixed barotropic–baroclinic feature may
be related to the latitudinal shift of the SACZ associated with intraseasonal variability, as well as interannual
teleconnection signals (Kodama, 1992; Nogues-Paegle and Mo, 1997; Liebmann et al., 1999; Robertson and
Mechoso, 2000; Mo and Nogues-Paegle, 2001).
3.4. Temperature cross-sections
During DJF 1997–98, the SST-induced tropospheric warming is remarkably extensive, starting from about
1 K at the surface in the Niño-3 region (120–140 ° W) and increasing upward to a maximum of more than
3 K between 400 and 200 hPa (Figure 4(a)). In the upper troposphere the warming area expands laterally,
developing a pronounced zonally symmetric component across the entire domain, with a warming of 1–1.5 K
over the Altiplano (∼70 ° W). The zonal expansion of the temperature anomaly in the upper troposphere during
El Niño has been attributed to temperature advection (Lau et al., 1998) and appears to have contributed to
the enhancement of the Bolivia high in DJF 1997–98. Interestingly, over the oceanic region of 100–80 ° W,
Figure 4. Height–longitude cross-section showing large-scale temperature anomalies (K) along 15 ° S for (a) DJF 1997–98 and (b) DJF
1998–99
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off the coast of Peru, a shallower vertical structure is found below 700 hPa. In DJF 1998–99 (Figure 4(b)),
the tropospheric temperature is less extensive, with the largest negative anomaly found near 300–400 hPa
over the Niño-3 region. Here, the shallow temperature anomaly near 80–100 ° W appears to have intensified
around 850 hPa and weakened underneath. The physical interpretation of the shallow feature is unclear, but
it may be linked to diabatic heating anomalies associated with changes in stratus clouds in that region during
an El Niño (Norris and Leovy, 1994).
3.5. Meridional winds
The vertical cross-sections of the anomalous meridional wind v along 15 ° S from 80 to 55 ° W are shown
in Figure 5. During DJF 1997–98, most pronounced is the large negative v signal east of the Andes
encompassing regions over western Brazil (55–65 ° W) and extending vertically from the surface to above
400 hPa (Figure 5(a)). The maximum anomalous northerly wind reaches over 4 m s−1 , in the jet core region
between 700 and 800 hPa. In the upper troposphere, the meridional wind anomaly is positive, with a maximum
over the Altiplano. The deep vertical extent and the tight east–west gradient of the LLJ anomalies are likely
due to deflection of the low-level easterlies and the channelling effect by the Andes. The meridional structure
0
100 (a) 0.5
1
1.5
2
200
2
1.5
300
0.5
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0.5
-1
-1.0
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500
600
700
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80 W 75 W 70 W 65 W 60 W 55 W
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Figure 5. Height–longitude cross-section showing meridional wind anomaly along 15 ° S for (a) DJF 1997–98 and (b) DJF 1998–99.
Units: m s−1
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is consistent with an induced anomalous local meridional overturning with rising motion to the south and
sinking motion to the north. This overturning reinforces the climatological SASM local Hadley cell. As noted
previously, during DJF 1997–98, the increased LLJ, and its further penetration into the extratropics, may
be important in bringing abundant moisture and rainfall to subtropical South America (Berri and Inzunza,
1993; Berbery and Collini, 2000). In contrast, the LLJ is only weakly developed during DJF 1998–99. A
weak reversal of the climatological LLJ is noted with anomalous northward flow (positive v) confined to
below 800 hPa, and the eastern foothills of the Andes (Figure 5(b)). Overall, anomalous meridional flow is
less organized with smaller vertical scales, indicating the lesser impact of the 1998–99 La Niña.
4. CONCLUSIONS
In this section, we provide a summary discussion of the main results of this paper, with the aid of the schematic
shown in Figure 6. Similar to previous warm events, the most pronounced rainfall signals in DJF 1997–98
include (a) excessive rainfall over northern Peru and Ecuador, (b) deficient rainfall over north-northeastern
and central Brazil, and (c) above-normal rainfall over subtropical southeastern South America near 30 ° S.
These rainfall anomalies are associated with possible response of the SASM to ENSO through three possible
pathways.
1. Anomalous equatorial east–west overturning: in response to warmer SST in the tropical eastern Pacific
in DJF 1997–98, anomalous east–west overturning motions develops, with rising motion (excess rainfall)
and low-level westerlies over the equatorial eastern Pacific and broad subsidence (suppressed rainfall)
and anomalous easterlies over northern Brazil. The ascending and descending motions are associated
with a weakened SPH (SPH < 0) and a strengthened SAH (SAH > 0) respectively, dynamically
consistent with the Rossby wave response to tropical heating/cooling. During the 1997–98, the SSTinduced east–west overturning is found to be more intense and localized compared with the CER, with
more extensive subsidence over the land region of north-northeastern Brazil. Also found is a secondary
rising branch over the west coast of Africa in response to an anomalous heat source there (not shown in
Figure 6).
2. Direct tropospheric warming: during DJF 1997–98, the SST anomaly in the equatorial eastern Pacific
induces overall warming in the mid-to-upper troposphere over the tropical–subtropical South Pacific–South
America–South Atlantic sector. In the upper troposphere, in addition to the broad-scale warming, two
BH > 0
LLJ
SPH < 0
SAH > 0
Figure 6. Schematic illustrations of key elements of the three-dimensional response of the SASM to El Niño of 1997–98. See text for
definition of symbols
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anomalous high pressure or warm ridges straddling the equator are found. The southern branch enhances
warming over the Altiplano Plateau, increasing hydrostatically the height field over the Bolivia high
(BH > 0 in Figure 6). Dynamically, a reduced heating over Brazil will force a negative height anomaly
over the Altiplano (Silva Dias et al., 1983). However, in DJF 1997–98, the direct thermal forcing due to
SST overcompensates the effect due to reduced latent heating over northern and central Brazil, leading
to a net increase in the Bolivia high. The increased heating over the Altiplano Plateau leads, through
thermal wind balance, to an equatorward displacement and intensification of the upper-tropospheric jet
over subtropical South America, and wave development further downstream.
3. Pacific–South America teleconnection: the SASM anomalies may be affected by atmospheric teleconnection
induced by remote heating over the tropical eastern Pacific. A major departure of the response in DJF
1997–98 from the CER is the displacement of the SAH anomaly, which appears to split into two lowlevel anticyclones: one immediately to the south of the Amazon Basin, and the other over the southeastern
Atlantic (not shown in Figure 6). Although the former is dynamically consistent with a Rossby wave
response to induced sinking motion, it may be affected by the topography of the Andes. The anticyclone
over the southeastern Atlantic is likely to be affected by fluctuations of the Pacific–South America teleconnection linking the South Pacific and the South Atlantic by the upper-level jet stream in association
with the displacement of the SACZ.
All the aforementioned pathways may interact to produce an enhancement of the LLJ east of the Andes.
The low-level meridional wind associated with the anomalous low-level anticyclone (SAH) south of the
Amazon and the pressure gradient set up by the steep topography of the Andes reinforce the climatological
LLJ. The increased LLJ constitutes the lower branch of the anomalous local Hadley cell, with rising motion
over subtropical southeastern South America, equatorward return flow east of the Altiplano, and sinking
motion over northern Brazil. The LLJ may be important in regulating rainfall variability over subtropical
South America, via moisture transport from the Amazon. It is important to point out that though our results
suggest that Pacific SST may have been instrumental in initiating the anomalies, it is the subsequent chain
of mechanisms, including forcing by Atlantic SST and interaction with regional and local processes, that
ultimately leads to the observed anomalies of 1997–99. The regional-scale interactions merit further study.
ACKNOWLEDGEMENTS
This research is supported by the NASA Earth Science Enterprise, Global Modeling and Data Analysis
Program. The authors would like to acknowledge useful comments by two anonymous reviewers on an
earlier version of the paper.
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Published in 2003 by John Wiley & Sons, Ltd
Int. J. Climatol. 23: 529–539 (2003)