Revista Brasileira de Meteorologia; 1989; Vol. 4(2), 351 -363
EXTREMES IN THE SOUTHERN OSCILLATION ANO THEIR RELATIONSHIP TO PRECIPITATION
ANOMALIES WITH EMPHASIS ON THE SOUTH AMERICAN REGION
VERNON E. KOUSKY & CHESTER F. ROPELEWSKI
Climate Analysis Center
National Meteorological Center
National Weather Service
Washington, D.C. 20233 USA
-
ABSTRACT
Within the tropical belt, the major swings in the Southern Oscillation alter the zonal
distribution of convection. During the mature phase of ENSO (the warm phase of the
SO), anomalous surface heating in the central Pacific results i n an eastward
displacement of the most intense convective activity from the maritime continent
(Indonesia, the Philippines and northern Australia) to the equatorial central Pacific.
Upper tropospheric anticyclonic circulation cells are found straddling the equator i n
the central Pacific in the vicinity of this convective activity and anomalous heating. This
heating reduces the strength of the east-west Walker circulation in this sector and
enhances the strength of the regional north-south Hadley circulation. Consequently,
the subtropical jet streams are stronger than normal in the Pacific in both
hemispheres, especially during the respective winter seasons.
Positive SST anomalies along the west coast of South America, associated with El
Niiio, are undoubtedly important in determining the intensity and position of the
ascending and descending branches of the east-west Walker circulation cell in the
Atlantic sector. During warm episodes the ascending branch of this cell is shified
westward from tropical South America to the region of the anomalously warm waters
in the eastern equatorial Pacific. This shifi results in a corresponding westward shifi of
the descending branch from the central equatorial Atlantic to coastal regions of
northern South America where negative precipitation anomalies are observed. Rainfall
is heavier than normal in coastal regions of northern and central Peru and Ecuador.
During a warm episode, the enhanced subtropical jet stream in the upper troposphere
over the Pacific extends eastward into the western Atlantic, especially during the
Southern Hemisphere late fall, winter and early spring months. In the vicinity of this
jet, surface fronts and cyclones are more vigorous than normal and precipitation
amounts are above normal. This is most evident in the region from southern Brazil
southward to east-central Argentina.
During the cold phase of the S0, many of the anomalies are opposite to those
experienced during ENSO. The rnonsoons (Indian, Australian, South American and
African) are enhanced, while convection is suppressed throughout the equatorial
Pacific. The subtropical jet streams are weaker than normal in both hemispheres,
which probably contributes fo weaker than normal surface cyclones and less than
normal precipitation at subtropical latitudes (e.g., United States Gulf Coast, and
east-central Argentina, Uruguay and southern Brazil).
The South American regions defined in this and previous studies are based on
relatively sparse data. More comprehensive data sets will undoubtedly lead to
refinements in the extent and configuration of these regions. Nonetheless, our present
understanding of the evolution of the Southern Oscillation provides a tool for long
range climate prediction.
352
Vernon E. Kousky & Chester F. Ropelewski
the anomalies are superposed.
Research during the last twenty-five years has
revealed the importante of tropical oceanlatmosphere
variability on global atmospheric circulation and
precipitation patterns. A seesaw in sea level preççure
(SLP) between the central Pacific and Indonesia, known
as the Southern Oscillation (Walker & Bliss, 19321, and an
abnormal warming of sea surface temperatures along the
west coast of South America, called El Niiio (see, e.g., the
works of Wyrtki, 1975; Hickey, 1975; Wooster & Guillen,
1974; Weare, 1982). have served as focal points for much
of this work. Beginning with the studies by Berlage (1966)
and Bjerknes (1966,1969,1972) it became evident that the
low index phase of the Southern Oscillation (SLP higher
than normal over Indonesia and lower than normal in the
central Pacific) and El Nino are closely related, and are
also accompanied by variations in equatorial zonal wind
and sea surface temperature (SST) in the central Pacific.
Several studies have explored i n detail the
teleconnections associated with the Southern Oscillation
(SO) both through data analysis (e.g., Bjerknes, 1969;
Trenberth, 1976; Julian & Chervin, 1978; Horel & Wallace,
1981; van Loon & Madden, 1981; van Loon & Rogers,
1981; van Loon, 1984; van Loon & Shea, 1987; Kiladis &
van Loon, 1988; Ropelewski & Halpert, 1987, 1989) and
through simulations using general circulation models
(e.g., Blackmon et al., 1983; Geisler et al., 1985). Although
the emphasis has been on anomaly patterns during the
warm (low index) phase of the S0, recent work has
shown that the cold (high Sol--above normal SLP in the
central Pacific and below normal SLP in the region of
Indonesia) phase is also characterized by significant
anomalies, which are essentially opposite to those of the
warm phase (Ropelewski & Halpert, 1989).
The warm (low S01) phase of the SO tends to
persist for periods of 12 to 18 months (Rasmusson &
Carpenter, 1982). with patterns of anomalous circulation
(Arkin, 19821 and precipitation (Ropelewski & Halpert,
1987) evolving in a similar fashion during each episode.
The cold (high Sol) phase has been founci to have similar
anomaly patterns but of the opposite sign (Ropelewski &
Halpert, 19891. Most warm (cola) episodes are
characterizeci by a similar evolution of circulation and
precipitation anomaly patterns. Thus, once an episode
(either warm or cold) has begun it may be possible to
make skillful long range predictions (severa1 months in
advance) for those regions generally affected by the S,O.
In this paper we will discuss the evolution of the SO
and associated oceanographic and atmospheric anomaly
fields, with emphasis on how these are interrelated and
how they affect the pattern of precipitation anomalies,
especially in the South American sector. However, before
discussing the anomalous atmosphere and ocean we will
briefly describe the mean climatological state upon which
CLIMATOLOGICAL ATMOSPHERK: CIRCULATION ANO
SEA SURFACE TEMPERATURE PATTERNS
The climatological atmospheric circulation features
within the tropics are primarily determined by the surface
temperature and tropospheric heating patterns. Sensible
heating (heating of the atmosphere by contact with a
warm underlying surface) provides positive buoyancy t o
the air and, if sufficiently strong, leads to condensation of
water vapor and the formation of deep cumulus clouds.
This contributes further to atmospheric heating through
the release of latent heat of condensation. It is necessary,
therefore, to consider the effects of surface heating and
latent heating within deep cumulus convection in order to
understand the observed atmospheric circulation
features.
Most of the land mass i n the Southern Hemisphere,
excluding Antarctica, is found at tropical and subtropical
latitudes. These regions are intensely heated by the sun
during the summer season. The atmospheric boundary
layer is also heated strongly at that time of year due to
contact with the Earth's surface (sensible heating).
Satellite imagery has revealed the diurnal nature of
convection in these regions; growing during the daytime
to a maximum in the late afternoon or early evening, and
then weakening during the nighttime to a minimum early
in the morning. The strong diurnal convection over
tropical continental regions in summer contributes to
tropospheric heating at lower and middle levels through
the release of latent heat of condensation. Thus, there is
considerable thermal contrast between the relatively
warm land areas and the cooler nearby oceanic regions
(e.g., Gutman & Schwerdtfeger, 1965). Since upper
tropospheric flows tends to be parallel to the mean
isotherms at extratropical latitudes, there is considerable
meridional (north-south) flow in the middle and upper
troposphere (3-12 km above the Earth's surface) near the
east and west coasts of continents during the summer
season. The summertime flow over South American
typifies this situation. Generally, upper tropospheric
anticyclonic circulation is observed over the continent in
the vicinity of Bolivia (due to summer heat sources) and
upper tropospheric cyclonic flow is found over the
nearby, relatively cool oceanic regions (Fig. Ia).
During the southern winter season, continental
regions in the Southern Hemisphere are c001 compared
with the summer season and the thermal contrast
between continents and adjacent oceans is relatively
small. Thus, the thermal gradient is mainly in the
Extremes in the southern oscillation and their relationship to precipitation anomalies
meridional (north-south) direction, which gives rise to
generally zonal (west-to-east) flow at mid-latitudes in the
middle and upper troposphere (see Fig. 1b).
Besides lhe heating over continental areas, certain
oceanic regions alço provide a great deal of heat to the
atmosphere. The oceans are warmest in the tropics and
Figure 1. Climatological (1978-1983) 200 mb stream function for a) December-February and b) June-August. The
non-divergent component of the flow is directed along the contours with speed proportional to the gradient.
The flow is clockwise about centers labeled H and counterclockwise about centers labeled L. Thus, high (low)
stream function corresponds to high (low) geopotential height in the Northern Hemisphere and to low (high)
geopotential height in the Southern Hemisphere. Arrows have been added to indicate the direction of the
flow.
354
Vernon E. Kousky & Chester F. Ropelewski
get progressively cooler with increasing latitude. Within
the tropics and subtropics, the eastern regions of the
oceans are cooler than the western regions due to
equatonnrard and westward moving cold currents along
continental margins and the upwelling of cold water from
below. This gives rise to an east-west sea surface
temperature gradient in the vicinity of the equator (Fig.
2). The regions of warmest water (western portions of the
tropical oceans) are significant sources of heat for the
atmosphere. There is a strong positive relationship
between regions experiencing deep atmospheric
convection. represented by low values of outgoing
longwave radiation in the tropics (Fig. 3), and heat
sources (continental and maritime) within the tropics (Fig.
2 1.
Changes in the atmospheric heating pattern, dueto
anomalously cold or warm sea surface conditions
primarily in the tropics, can have an important impact on
features of the atmospheric circulation, which in turn may
result in excessive rainfall in some regions and drought in
others. Such changes accompany the extremes in the
Southern Oscillation.
Onset phase
As the reservoir of warm water builds, expands and
deepens in the western Pacific, the extent of tropical
convection alço expands. At the same time, the low-level
easterlies in the western equatorial Pacific weaken and
may actually shift t o westerly (blow from west-to-east).
In most warm episodes there is then a general eastward
shift of the warmest water t o the vicinity of the date line,
which most often occurs during the Southern
Hemisphere winter and spring seasons (July-November).
At that time, there is a seasonal trend towards weaker
easterlies along the equator and a shift of the warmest
water southward towards the equator. This tendency for
phasing between warm episodes and the seasonal cycle
has been discussed by Rasmusson et al. (1988). Once the
warmest water extends t o the vicinity of the date line,
strong and persistent convective activity occurs in this
region marking the beginning of the mature ENSO
phase.
Matute phase
EVOLUTION OF WARM EPISODES IN THE TROPICAL
PACIFIC
Bulld-up phase
Prior t o the onset of a warm (El NiiioISouthern
Oscillation--ENSO) episode there is often an increase in
the reservoir of warm water found i n the western tropical
Pacific (Wyrtki, 1975). PreçUmably, stronger than normal
equatorial easterlies acting over a period of severa1
months to years gradually increases the amount of warm
water stored in this region. The enhanced easterlies result
in faster east-to-west equatorial ocean currents and
stronger than normal upwelling of cold subsurface water
along the equator i n the central portion of the basin.
Enhanced tropical convection is found over the region of
warmest water (the extreme western Pacific) while
convection is suppressed i n the relatively cold water
region of the central equatorial Pacific. This implies the
existence of a direct, thermal, east-west anomalous
circulation cell with rising motion in the regiop of
enhanced convection and sinking motion i n the region of
suppressed convection. This east-west circulation cell has
been called the Walker circulation (Bjerknes, 1969).
Periods of an enhanced Walker circerlation coincide with
periods of a weaker than normal meridional (Hadley)
circulation and weaker than normal subtropical jet
streams i n both hemispheres in the Pacific sector.
The mature phase of an ENSO episode features a
pattern of atmospheric anomalies which is nearly global
in extent. In most episodes the mature phase occurs
during the southern summer season (DecemberFebruary). This is due, in part, t o the fact that the
equatorial SSTs in the central and eastern Pacific are near
their seasonal peak at that time, and positive SST
anomalies may be sufticient to raise the SSTs above
28OC. the temperature considered t o be the threshold for
the onset of deep convection i n the tropics (Gadgil et al.,
1984). The extensive area of above normal SST in the
equatorial Pacific contributes to enhanced tropospheric
heating throughout the tropical Pacific. As a result, the
east-west thermal contrast and associated Walker
circulation are relatively weak and the north-south
thermal contrast and Hadley circulation are relatively
strong. Thus, during the mature phase of ENSO the
subtropical jet stream (winter hemisphere) is stronger
than normal i n the Pacific sector (Arkin, 1982). This
enhanced jet is found on the poleward flank of an
anomalous upper tropospheric anticyclonic circulation
center found at low-latitudes i n the central Pacific. and
forms part of a wave train of anomalous circulation
centers emanating from the central Pacific towards
higher latitudes of the winter hemisphere (Horel &
Wallace, 1981; Karoly, 1986).
During the onset and mature phaases of a warm
episode, there is considerable readjustment in the
structure of the upper ocean. The oceanic thermocline,
which is the boundary between the warm well-mixed
surface layer and the deeper colder oceanic waters, is
Extrernes in the southern oscillation and their relationship to precipitation anornalies
b
I20L
140E
160E
180
l60W
140W
120W
IOOW
BOW
60W
40*
2OW
O
20E
40E
6OE
80E
120E
140E
160E
180
160W
140W
120W
IOOW
80W
60U
4ON
20W
O
20E
4OE
60E
80E
Figure 2. ~limatolo~ical
(1950-1979) sea surface ternperatures for a) Decernber-February and b) June-August. Contour
severa1 is 1OC.
Vernon E. Kousky 81Chester F. Ropelewski
Figure 3. Climatological (1974-1983) outgoing longwave radiation for a) December-February and b) June-August.
Contour interval is 20 W m". Values less than 220 W m - 2 within the tropics have been shaded to emphasize
regions where deep convection occurs.
Extremes in the southern oscillation and their relationship t o precipitation anomalies
normally relatively shallow i n the east and deep i n the
west. Dliring a warm episode the east-west slope of the
thermocline changes such that it is deeper than normal i n
the east and shallower than normal i n the west. (Ocean
currents are alço affected by this thermal redistribution).
Since the depth of the thermocline is an indicator of the
amount of warm water i n the upper ocean (Rebert et al.,
1985), i t is evident that during the mature phase of ENSO
the volume of warm water decreases in the equatorial
western Pacific and increases i n the central and eastern
equatorial Pacific.
The onset and mature phases are frequently
characterized by the appearance of anomalously warm
water along the equatorial west coast of South America
called El Nitio. Although El Nino is not always closely
associated with the major central Pacific warmings and
may occur independently of them (Deser & Wallace,
1987), i t is quite often observed during the southern
summer at the same time that the SSTs are anomalously
warm in the central equatorial Pacific.
On possible cause for the warming along the South
American coast stems from the relaxation of the
easterlies in the central and western Pacific as the
enhanced convection propagates from west t o east. The
weakening of the easterlies initiates oceanic Kelvin waves
which propagate eastward at about 3 m s-l. These waves
may alço be initiated during the low-level westerly phase
of intraseasonal (30-60 day) oscillations, which are
observed i n the tropical atmospheric circulation and
convection patterns (Madden & Juiian, 1972; Weickmann,
1983; Weickmann et al., 1985). It has been suggested that
these oscillations play an important role during the onset
of warm episodes (Lau, 1985). Kelvin waves take about 60
days to trave1 from the central equatorial Pacific t o the
South American coast. As they propagate eastward they
cause the equatorial oceanic thermocline to deepen. The
net effect at the eastern boundary, where coastal
upwelling of cold subsurface nutrient-rich waters is
important for the fishing economies of Ecuador and Peru,
is to depress the thermocline so that upwelling brings
relatively warm water t o the surface resulting in positive
SST anomalies.
The positive SST anomalies along the South
American coast during the 1982-83 and 1986-87 ENSO
episodes appear to have evolved i n a manner similar t o
that described above. However, the composite of severa1
ENSO episodes occurring during the period 1950-1980
(Rasmusson & Carpenter, 1982) features first the
appearance of abnormally warm water along the South
American Coast and then the anomalous warming of the
water i n the central equatorial Pacific. Such episodes do
not appear to rely solely on the occurrence of oceanic
Kelvin waves t o produce positive 'SST anomalies. It
seems that a variety of factors, some local (Leetmaa,
1983) and others remote (Kelvin waves), are involved i n
357
anomalous coastal warmings. Further research is
necessary i n order t o understand the complex dynamics
associated with these warmings.
üecay phase
This phase of ENSO features, i n most instances, a
rather rapid return to near normal conditions, which
begins when the equatorial easterlies i n the central Pacific
are near the peak in their seasonal cycle (January). I n
some instances. there is a complete reversal i n the sign of
the anomalies and the subsequent development of the
opposite phase of the Southern Oscillation or a cold
episode. During the decay phase there is a rapid decrease
in the areal extent and magnitude of positive SST
anomalies in the equatorial Pacific. Just as the normal
seasonal cycle tends t o lag the astronomical seasonal
cycle, so the atmosphere lags the ocean during the ENSO
cycle. Thus, during the decay phase the subtropical jet
streams usually continue stronger than normal.
COLD PHASE OF THE SOUTHERN OSCILLATION
The central equatorial Pacific experiences cold
episodes as well as the warm episodes discussed above.
During a cold episode the patterns of anomalous
atmospheric circulation, SST and precipitation are t o a
large extent reversed from those experienced during
warm episodes. The colder than normal equatorial waters
are associated with a colder than normal troposphere and
abnormally strong off-equatorial cyclonic circulation at
upper tropospheric levels i n both hemispheres.
Subsidence is enhanced i n the central Pacific and clouds
are suppressed. Low-level easterlies are stronger than
normal and westward ocean currents and equatorial
upwelling in the ocean are stronger than normal. The
abnormally cold conditions in the central Pacific are
associated with an en hanced east-west thermal gradient
and an abnormally strong Walker circulation. The colder
than normal troposphere i n the equatorial Pacific results
in a weaker than normal thermal gradient in the
meridional (north-south) direction i n both hemispheres.
Thus, the subtropical jet streams, especially i n the winter
hemisphere, are weaker than normal. The contrast
between the upper tropospheric circulation pattern
observed during December 1986- February 1987 (warm
episode) (Fig. 4) and that observed during December
1988-February 1989 4cold episode) (Fig. 5) illustrates the
features discussed above. Further details on the
ocean/atmosphere variations prior to, during and
following the 1986-87 ENSO episode can be found i n
Kousky & Leetmaa (1989).
Vernon E. Kousky & Chester F. Ropelewski
Figure 4. 200 mb stream function, a) mean and b) anomalous, for December 1986-February 1987. Arrows have been
added to indicate the direction of the flow.
Extremes in the southern oçcillation and their relationship to precipitation anomalies
Figure 5. 200 mb stream function, a) mean and b) anomalous, for December 1988-February 1989. Arrows have been
added to indicate.the direction of the flow.
360
Vernon E. Kousky & Chester F. Ropelewski
SOUTHERN OSCILLATION-RELATED PRECIPITATION
ANOMALIES
The pioneering studies of Walker (1923,1924,1928)
and Walker & Bliss (1930, 1932) first documented the
global precipitation patterns asçociated with the warm
phase of the S0. Recent analyses, based on more
complete data sets, (Ropelewski & Halpert, 1987) have
greatly improved our understanding of the large scale
ENSO-precipitation relationships for many regions of the
globe. Specifically, ENSO-precipitation relationships
have been identified for regions of Australia, North
America, Africa, the Indian sub-continent, and most
importantly for this discussion, Central and South
America. For South America, the ENSO related
precipitation patterns extend t o mid-latitudes and are
somewhat more complicated. In general, the SO is
asçociated with a modulation of the global monsoon
circulation and pattern. During the warm phase of the
cycle, monsoon rainfall tends t o be less than normal for
large regions of the globe.
Three separate ENSO-related precipitation regimes
can be identified for South America (Fig. 6). These are: a)
the western coastal region, where normally arid areas are
often afflicted with heavy rains and flooding during
ENSO episodes, b) the northeastern section, including
Venezuela, Guyana, Surinam, French Guiana and the
near equatorial regions of Brazil. which tend to receive
less than normal rainfall, and c) the southeastern region,
a region of enhanced ENSO-related precipitation, which
covers extreme southern Brazil, Uruguay, and parts of
northern Argentina. The ENSO related precipitation
anomalies tend to be most pronounced in these regions
during the mature phase of the ENSO in the southern
summer (Table 1).
The ENSO -related precipitation defficiencies in the
northeastern portions of the continent, documented by
severa1 other authors (Hastenrath & Heller, 1977; Kousky
et al., 1984; Aceituno, 1988). are among the most
consistent of such relationships. Sufficient data for
analysis of ENSO-related precipitation exists from about
the turn of the century. Of the 18 ENSO episodes which
occurred over this 88 year span, 17 episodes were
associated with dry conditions during the period from
July(0) (onset phase) through the following March(+)
(mature phase). The historical record also reveals that 9
of the 11 driest July through March periods were
associated with ENSO while none of the wettest seasons
occurred with the warm phase of the S0. This consistent
relationship is a reflection of shifts, during ENSO, in the
large scale atmospheric circulation and, in particular, is
related to the westward displacement of an east-west
circulation cell i n the Atlantic similar to the Walker cell in
the equatorial Pacific. As noted i n Kousky et'al. (19841,
such a shifi results in enhanced subsidence over
northeastern South America and consequently a
suppression of precipitation.
In the southeastern region (Fig. 6) ENSO is
associated with increased precipitation during the
November(0) to February(+) period. The historical record
showç that 15 of the past 17 ENSO episodes were
associated with enhanced precipitation i n this region and
that 7 of the 10 wettest Wovember through February
periods were associated with ENSO. Conversely none of
the 10 driest periods were associated with ENSO.
Composite analysis (Arkin, 1982) shows that ENSO is
related to stronger than normal 200 mb subtropical jet
stream winds over South America.
During the cold, or high index, phase of the SO
global precipitation patterns tend to show the opposite
anomalies of those associated with the ENSO phase. To a
first approximation, the cold phase of the SO is
asçociated with enhanced precipitation over the major
monsoon regions of the globe (Ropelewski & Halpert,
1989). This relationship is particularly striking i n the
summer monsoons of the Indian sub-continent and
Northern Australia.
The tendency for inverse precipitation relationships
associated with the extremes of the SO tends to hold for
South America also. In the northeastern region
Ropelewski & Halpert (1989) found that 13 out of 17 cold
SO episodes were associated with above normal
precipitation in the June through March period (Table 1).
Furthermore, their analysis shows that 2 of the 3 wettest
such periods since 1940 were associated with the cold SO
phase. I n northeastern South America only one of the
extreme dry years (1904) was aççociated with the cold
phase of the SO. These relationships are alço consistent
with the correlation analysis of Aceituno (1988). and the
cluster analysis of Wolter (1987).
In the southeastern sections of South America the
cold, high index, phase of the SO has been linked to drier
than normal conditions during the June through
December period (Ropelewski & Halpert, 1989).
Southeastern South America is one of the few regions of
the globe were the SO-related precipitation anomalies
tend to occur at different times of the iear for the
opposite extremes of the SO. An examination of the
historical records shows that 17 out of 20 cold episodes
were associated with drier than normal conditions i n
southeastern South America. No high index year was
associated with extremely wet conditions and,
furthermore, 2 of the 3 driest years were associated with
this, the cold, phase of the S0. The tendency for drier
than normal conditions during the cold extreme of the
SO was clearly in evidence with the 1988 drought in
Uruguay, and Northern Argentina, and extreme southern
Brazil.
Extremes in the soúthern oscillation and their relationship to precipitation anomalies
361
ENSO YET
HIGH INDEX DRY
-
HIW INDEX DRY
Jun (0)-Dec(0)
60S,
8OW
-.
I
6 0 ~
I
40W
Jsos
Figure 6. Areas in South America which @ave consistent precipitation anomalies associated with extremes in the
Southern Oscillation.
362
Vernon E. Kousky & Chester F. Ropelewski
Table 1. Historical surnmary of Southern Oscillation related precipitation anornalies. The "'O" represents the-episode
year and
the year following. (After Ropelewski & Halpert, 1987,1989).
"+"
NORTHEASTERN REGION
SEASON
WARM
EPISODES
Total
Wet
SEASON
Dry
COLD
EPISODES
Total
Wet
Dry
SOUTHEASTERN REGION
SEASON
Nov(0)-Feb(+)
WARM
EPISODES
Total
1
Wet
15
SEASON
Dry
2
Jun(0)-Dec(0)
COLD
EPISODES
Total
20
Wet
3
Dry
17
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