MONTHLY MEAN ATMOSPHERIC CIRCULATION, LOW LEVEL
TRANSPORT AND PRECIPITATION OVER CENTRAL SOUTH AMERICA
DURING ENSO EVENTS
Guillermo J. Berri1 and Germán Bertossa2
ABSTRACT
We study the relationship between low level moisture transport and precipitation in the central
part of South America (CSA) (40°S- 10°S), during the ENSO events of the period 1960-1998. We
also analyze the differences on the atmospheric circulation by considering horizontal wind
anomalies at 850 hPa and 200 hPa, and the pressure vertical velocity at 500 hPa. The analysis is
carried out for December, January and March. El Niño presents positive precipitation anomalies
over southern CSA (especially in central-northern Argentina) associated with a water vapor influx
over this region and upward motion at 500 hPa. In December and March, there is an anticyclonic
anomaly at 200 hPa that is less intense during March. In January, however, the 200 hPa positive
westerly wind anomalies adopt a wavelike pattern that reinforce the subtropical jet. La Niña
presents in general an opposite pattern. Precipitation deficits and downward motion prevail over
southern CSA, while at 200 hPa there is a cyclonic anomaly during December and March, less
defined during the latter one. In December there is a corridor of mean moisture transport from NW
to SE in CSA, that is characterized by positive rainfall anomalies. In January, an anticyclonic
anomaly located at low levels over southeastern Brazil with extension to the Atlantic Ocean,
facilitates the convergence in southern Brazil, Paraguay, Uruguay and northeastern Argentina where
positive rainfall anomalies prevail.
1
Member of the National Research Council of Argentina . Department of Atmospheric and Oceanic Sciences,
University of Buenos Aires, Ciudad Universitaria, 1428 Buenos Aires, Argentina. Email: berri@at.fcen.uba.ar
2
Department of Atmospheric and Oceanic Sciences, University of Buenos Aires, Ciudad Universitaria, 1428 Buenos
Aires, Argentina. Email: bertossa@at.fcen.uba.ar
DATA AND METHODOLOGY
The region of study, Central South America –hereinafter CSA- is delimited between 40oS10oS. Monthly mean values of specific humidity, zonal and meridional winds components and
pressure vertical velocity are obtained from the NCEP Reanalysis for the period 1960-1998. The
wind components and specific humidity are taken at the following standard pressure levels: 1000
hPa, 925 hPa, 850 hPa, 700 hPa while pressure vertical velocity is only selected at 500 hPa for the
same period. We also include zonal and meridional winds at 200 hPa to analyze the circulation in
upper levels.
Since the contribution of the upper levels is negligible, we consider the vertically integrated
water vapor flux only up to 700 hPa level by the following way:
Q  g w 
1

p  700hPa
p 1000hPa
qvdp
(1)
where v is the horizontal velocity vector, q is the specific humidity, g is the acceleration of gravity
and ρw is the density of water. Anomalies in the moisture transport and precipitation are obtained as
the difference between the mean value for El Niño and La Niña years, with respect to the mean
value of the period 1960-1998. The monthly precipitation data are obtained from the UEA-CRU
database (New, Hulme and Jones, 1999). All of these data are available on a uniform 2.5o lat x 2.5o
long grid resolution.
Table 1 shows the El Niño and La Niña years considered in this study. This monthly analysis
is carried out in summer (December and January of the following year.) and early fall (March)
El Niño years:
1963/64 – 1965/66 – 1969/70 – 1972/73 – 1976/77 – 1977/78 – 1982/83 –
1986/87 – 1991/92 – 1992/93 – 1994/95 – 1997/98
La Niña years: 1964/65 – 1970/71 – 1973/74 – 1975/76 – 1988/89 – 1995/96 – 1998/99
Table 1. El Niño and La Niña selected years
RESULTS
Figures 1 to 4 are described by the following way: left hand (right hand) side corresponds to
El Niño (La Niña) period and from the top, each Figure is arranged in a sequence DecemberJanuary- March, respectively.
Figure 1 shows the moisture transport (vector lines) and precipitation anomalies (shaded)
during ENSO events in December, January and March. In December of El Niño (EN), we find an
anomalous low-level cyclonic moisture circulation over CSA with a well-defined water vapor influx
into northern Argentina where positive rainfall anomalies are observed. Apparently, a deepening of
the continental low increases and facilitates this northerly advection of moisture (Grimm et al.
2000). During December of La Niña (LN), the opposite situation prevails over northern Argentina,
with southwesterly moisture transport and precipitation deficit. From NW to SE in CSA, across
Bolivia, Paraguay, southern Brazil and up to the Atlantic Ocean, there is a corridor of mean
moisture transport (Figure not shown), that is characterized by positive rainfall anomalies, flanked
to the north and the south by large regions with strong precipitation deficit as it is found in the right
above side of Figure 1. This corridor could reinforce the South Atlantic Convergence Zone (SACZ)
over the Atlantic Ocean. A strong SACZ would contribute to subsidence to both sides, which would
explain the reduced precipitation appeared over central-eastern Brazil and northeastern Argentina.
Nogués-Paegle and Mo (1997) found that intensification of the SACZ is associated with rainfall
deficits over the subtropical plains. An anticyclonic anomalous circulation located in central-east
Brazil, where precipitation deficit prevails, in connection with the southwesterly moisture transport
over Argentina, help to increase water vapor convergence over southern Brazil where more positive
precipitation anomalies are found.
In January, during EN there is a sort of bifurcation of moisture transport in CSA coming from
the NW that directs moisture flux towards central-east Brazil, causing moisture convergence in this
region (Grimm 2003), and to the south over central-north of Argentina where positive rainfall
anomalies are found, although less intense than in December. This bifurcation of the circulation
produces precipitation deficit over southern Brazil. During LN, a clearly anticyclonic circulation is
located over southeastern Brazil with extension to the Atlantic Ocean. This circulation facilitates the
convergence in southern Brazil, Paraguay, Uruguay and northeastern Argentina, where positive
rainfall anomalies prevail. In January we find the opposite pattern with respect to the preceding
months, i.e. positive precipitation anomalies appear over that same region. Similar results were seen
in other studies, for example Pisciotano et al. (1994) over Uruguay and Grimm et al. (2000) in
Southern South America.
Figure 1 Monthly precipitation anomalies (shaded) and 1000/700 hPa horizontal water vapor flux anomalies (vector)
in December, January and March for La Niña (right) and El Niño (left) events during the period 1960-1998.
March of EN presents a similar pattern of December, with precipitation deficit in northeastern
CSA (over most part of central Brazil), precipitation excess over northeastern Argentina and
southern Brazil and water vapor influx into northern Argentina although less intense than in
December. The water vapor influx into northern Argentina is displaced eastward with respect to
December in coincidence with the same positive precipitation anomalies displacement.
Figure 2 shows horizontal wind anomalies in 850 hPa for December, January and March. The
comparison of these figures with the moisture transport described in the Figure 1, reveals
differences in vectors size (for example, in December and March of EN over central-south of CSA).
As moisture transport is calculated by the equation (1), these differences suggest the presence of
more water vapor available (explained by the increase of the term specific humidity), connected in
general with positive precipitation anomalies (over Argentina in December and March of EN, for
example).
Figure 2 Monthly horizontal wind anomalies at 850 hPa in December, January and March for La Niña (right) and El
Niño (left) events during the period 1960-1998.
Figure 3 shows vertical motion anomalies in 500 hPa during the same period of the figures
explained in Figure 1. In general, regions with positive (negative) anomalies in precipitation
correspond to negative (positive) anomalies of vertical motion or upward (downward) motion. In
December of LN for example, the corridor with positive precipitation anomalies is related to
anomalous upward motion, flanked to the north and the south with anomalous downward motion
where precipitation deficit is found. In March of LN, there is a large region with excess (deficit)
precipitation over central- south of Brazil (eastern Argentina) associated with upward (downward)
motion, respectively, over those regions. The opposite situation prevails in March of EN.
Figure 3 Same as Figure 2 for monthly pressure vertical velocity anomalies at 500 hPa
Figure 4 shows 200 hPa wind anomalies for December, January and March. In general, a
clearly defined opposite pattern is found between EN and LN events. For example, in the
southeastern part of CSA we observe a cyclonic anomaly in December of LN, in connection with
downward motion in 500 hPa (see Figure 3) and precipitation deficit (see Figure 1). Instead in
December of EN there is an opposite pattern, with an anticyclonic anomaly, associated with upward
motion in 500 hPa and precipitation excess over the same region. Thus, during LN (EN), the
cyclonic (anticyclonic) anomaly in the upper levels is associated with downward (upward) motion
in the middle troposphere and precipitation deficit (excess). During LN (EN), besides the cyclonic
(anticyclonic) anomaly over the continent in 200 hPa, there are two anticyclonic (cyclonic)
anomalies over the midlatitudes of both the Atlantic and Pacific oceans. These two patterns have a
barotropic structure (they are also found in 850 hPa although less intense). In January of EN, the
200 hPa positive westerly wind anomalies adopt a wavelike pattern that reinforce the subtropical
jet. In LN, an easterly anomaly contributes to a weakening of the subtropical jet and there are also
cyclonic and anticyclonic anomalies to the south and southeastern of CSA, respectively. The former
is also found in 850 hPa (see Figure 2) but it is less intense. March presents a pattern that is similar
to December.
Figure 4 Same as Figure 2 at 200 hPa
ACKNOWLEDGEMENTS
This research was partially supported by Research Grant PICT99-6707 from Agencia
Nacional de Promoción Científica y Tecnológica (ANPCyT) of Argentina, Research Grant X126
from University of Buenos Aires and Research Grant CRN-055 from the Interamerican Institute for
Global Change Research (IAI).
REFERENCES
Grimm A., V. Barros and M. Doyle, 2000: Climate variability in Southern South America
associated with El Niño and La Niña events. J. of Climate, 13, 35-58.
______, 2003: The El Niño Impact on the Summer Monsoon in Brazil: Regional Processes versus
Remote Influences. Journal of Climate, 16, 263–280.
New, M., M. Hulme, and P. Jones, 1999: Representing twentieth-century space-time climate
variability. Part I: Development of a 1961-90 mean monthly terrestrial climatology. J. Climate, 12,
829-856.
Nogués-Paegle J, Mo K.C. 1997: Alternating wet and dry conditions over South America during
summer. Mon. Wea. Rev., 125, 279-291.
Pisciottano G, Diaz A, Cazes G, Mechoso C. 1994: El-Niño Southern Oscillation impact on rainfall
in Uruguay. J. Climate, 7, 1286-1302.