The combined effect of climate oscillations in producing extremes: the 2020 drought in southern Brazil

Abstract

The 2020 drought in southern Brazil, which culminated in late summer and early autumn (February-March-April), displayed one of the most deficient rainfall totals in such trimester. This period of the year has already been dominated by negative rainfall deviations since the end of the 1990s. This recent drought represents, therefore, a significant worsening in an already unfavorable situation of water availability. Such long-term behavior is due to the combination of opposite phases of two interdecadal oscillations in the sea surface temperature: the positive phase of the Atlantic Multidecadal Oscillation and the negative phase of the Pacific Interdecadal Oscillation. This combination produces variation in the atmospheric basic state that favors less rainfall in southern Brazil at this time of the year and more frequent occurrence of droughts. For an extreme event to occur, it is usually necessary that, in addition to interdecadal oscillations, an interannual oscillation event occurs that also favors drought, such as the events of Central El Niño in 2020 and La Niña in 2009 and 2012, years of droughts in southern Brazil during the same phase combination of the two interdecadal oscillations. Anthropic climate changes can intensify the frequency and intensity of these extreme events.

Full text

Revista Brasileira de Recursos Hídricos
Brazilian Journal of Water Resources
Versão On-line ISSN 2318-0331
RBRH, Porto Alegre, v. 25, e48, 2020
Scientic/Technical Article
https://doi.org/10.1590/2318-0331.252020200116
1/12
This is an Open Access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution,
and reproduction in any medium, provided the original work is properly cited.
The combined effect of climate oscillations in producing extremes: the 2020 drought
in southern Brazil
O efeito combinado de oscilações climáticas na produção de extremos: a seca de 2020 no Sul do Brasil
Alice Marlene Grimm1 , Arlan Scortegagna Almeida2, Cesar Augustus Assis Beneti2 & Eduardo Alvim Leite2
1Universidade Federal do Paraná, Curitiba, PR, Brasil
2Sistema de Tecnologia e Monitoramento Ambiental do Paraná, Curitiba, PR, Brasil
E-mails: grimm@sica.ufpr.br (AMG), arlan.scortegagna@simepar.br (ASA), cesar.beneti@simepar.br (CAAB), eduardo.alvim@simepar.br (EAL)
Received: July 17, 2020 - Revised: September 29, 2020 - Accepted: October 02, 2020
ABSTRACT
The 2020 drought in southern Brazil, which culminated in late summer and early autumn (February-March-April), displayed one of
the most decient rainfall totals in such trimester. This period of the year has already been dominated by negative rainfall deviations
since the end of the 1990s. This recent drought represents, therefore, a signicant worsening in an already unfavorable situation of
water availability. Such long-term behavior is due to the combination of opposite phases of two interdecadal oscillations in the sea
surface temperature: the positive phase of the Atlantic Multidecadal Oscillation and the negative phase of the Pacic Interdecadal
Oscillation. This combination produces variation in the atmospheric basic state that favors less rainfall in southern Brazil at this
time of the year and more frequent occurrence of droughts. For an extreme event to occur, it is usually necessary that, in addition
to interdecadal oscillations, an interannual oscillation event occurs that also favors drought, such as the events of Central El Niño in
2020 and La Niña in 2009 and 2012, years of droughts in southern Brazil during the same phase combination of the two interdecadal
oscillations. Anthropic climate changes can intensify the frequency and intensity of these extreme events.
Keywords: Extreme drought; Combination of climate oscillations.
RESUMO
A seca de 2020 no Sul do Brasil, que culminou no nal do verão e início do outono (fevereiro-março-abril), apresentou um dos mais
decientes totais pluviométricos em tal trimestre. Tal período do ano já vinha apresentando predominância de desvios pluviométricos
negativos desde o nal dos anos 1990. Esta recente seca representa, portanto, piora signicativa num quadro já desfavorável de
disponibilidade hídrica. Tal comportamento de longo prazo deve-se à combinação de fases opostas de duas oscilações climáticas
interdecadais na temperatura da superfície do mar: a fase positiva da Oscilação Multidecadal do Atlântico e a fase negativa da Oscilação
Interdecadal do Pacíco. Tal combinação produz variação no estado básico da atmosfera que favorece estiagem no Sul do Brasil nessa
época do ano e a ocorrência mais frequente de secas. Para que ocorra um evento extremo, geralmente é necessário que, em adição a
oscilações interdecadais, ocorra um evento de oscilação interanual que também favoreça a seca, como os eventos de El Niño Central em
2020 e La Niña em 2009 e 2012, anos de secas no Sul do Brasil durante a mesma combinação de fases das duas oscilações interdecadais.
Mudanças climáticas antrópicas podem intensicar a frequência e intensidade destes eventos extremos.
Palavras-chave: Seca extrema; Combinação de oscilações climáticas.
a

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The combined effect of climate oscillations in producing extremes: the 2020 drought in southern Brazil
INTRODUCTION
The 2020 drought and its impacts
The trimester February-March-April (FMA) 2020 was a
very dry period in southern Brazil, also preceded by some dry
months in the previous year, especially in late austral winter and
early spring. It was one of the worst ever droughts in the region,
which affected several sectors, such as agriculture, hydropower
generation and water supply to the population.
At the institutional level, the drought in southern Brazil was
recognized in March 2020. Earlier that month, the National Water
Agency (ANA), which is the Brazilian federal regulator, summoned
water resources agencies and stakeholders from the three southern
states (Paraná, Santa Catarina and Rio Grande do Sul) to a crisis
room where regular meetings were held. The National Center
for Monitoring and Alerts of Natural Disasters (Cemaden) was
assigned to survey the drought from the meteorological perspective.
In March 2020, Cemaden veried a widespread drought in the
southern states with intensity ranging from weak to extreme,
according to its Integrated Drought Index (IIS), which is calculated
based on data from the previous six months (Centro Nacional de
Monitoramento e Alertas de Desastres Naturais, 2020a). From the
hydrological perspective, the problem was rst observed through
monitoring made by the hydropower sector, as monthly natural
inows to the Subsystem South were below 70% of long-term
averages since July 2019, despite a mild recovery in November
of that year (Operador Nacional do Sistema Elétrico, 2020a).
Electricity supply in Brazil depends on the National Interconnected
System (SIN), which comprises four subsystems, with Subsystem
South encompassing the three above-mentioned southern states.
The impacts on the hydropower sector were evident in the
drainage basins of Iguaçu and Uruguay rivers, which account for
about 80% of the energy storage capacity of Subsystem South.
In May 2020, inows reached historical minima in the reservoirs
of hydroelectric power plants Governador Bento Munhoz (since
1930) and Barra Grande (since 1964), these two containing the
largest reservoirs in Iguaçu and Uruguay rivers respectively, which
reveals the severity of the hydrological drought. In both cases, the
analyzed data corresponded to 7-days averages of streamows
recorded at gauges located immediately upstream of the reservoirs,
in locations not inuenced by reservoir operations, at stations
codes 65310000 and 70200000 of the National Water Agency
information system (Agência Nacional de Águas, 2020). As a
result of low inows and reservoir depletion, the rst half of
May saw an alarming energy storage of only 14% in Subsystem
South capacity (Operador Nacional do Sistema Elétrico, 2020b).
Despite critical hydrometeorological conditions, the impacts
on the hydropower sector were mitigated by the very existence of
the SIN. Due to the fact that hydrometeorological conditions were
favorable in other parts of the country, energy could be transferred
from the remaining subsystems while generation was reduced, or
even interrupted, and allowed water to be stored in Subsystem
South reservoirs. Moreover, due to SARS-CoV-2 pandemic, energy
demand has decreased after March 2020. This allowed, for example,
Governador Bento Munhoz Hydroelectric Plant to recover its
storage above 50% in early July of the same year (Companhia
Paranaense de Energia Elétrica, 2020). Conversely, water supply
reservoirs suffered a dramatically reduction in its storage, possibly
enhanced by the pandemic and increase in domestic use. Following
a quarter with precipitation anomalies ranging from -30% to
-70%, in May 2020 the government of Paraná decreed a state
of emergency for 180 days due to drought. On early July 2020,
rotating water supply interruptions, with duration of a few days,
were still being faced by 3.5 million inhabitants of the Metropolitan
Region of Curitiba, capital of Paraná state.
Regarding drought impacts on agricultural sector, signicant
losses in soybean, corn and common bean crops occurred in the
South Region, although heterogeneously among the three states.
By the end of May 2020, the state most affected had been Rio
Grande do Sul (Centro Nacional de Monitoramento e Alertas de
Desastres Naturais, 2020b, 2020c). Its average yield in soybean,
corn and common bean fell, respectively, 33%, 19% and 12% in
the quarter March-April-May of 2020, when compared to the same
period of the previous year (Instituto Brasileiro de Geografia e
Estatística, 2020). Rice production, which is the state’s second
most important crop after soybean, was not affected. However,
the intense consumptive demand for irrigation of rice paddies
has caused conicts regarding the fulllment of multiple uses of
water, as was reported by the Secretariat for the Environment and
Infrastructure of Rio Grande do Sul (Sema-RS).
In view of all the impacts caused by this drought, it is
important to disclose its possible origins, so that water managers
can be aware of the causes and mechanisms that can produce severe
droughts and extreme rainfall events, and realize that extremes
may happen beyond those limits already observed within the
usually short period of available data. Therefore, some climate
oscillations will be presented in the following.
The importance of climate oscillations
Natural climate oscillations are important ingredients
for both extreme drought and precipitation events (Grimm &
Tedeschi, 2009; Tedeschi et al., 2015, 2016; Grimm et al., 2016).
Certain phase combinations of climate oscillations from different
origins and time scales (interdecadal, interannual and intraseasonal)
can produce intense and persistent droughts or cause extreme
precipitation by intensifying and anchoring synoptic patterns in
certain regions.
Precipitation over South America, and Brazil in particular,
is signicantly impacted by the most important interannual climate
oscillation, El Niño-Southern Oscillation (ENSO) (Grimm,
2003, 2004, 2011; Tedeschi et al., 2015, 2016; Cai et al., 2020).
In addition, it also displays interdecadal variability produced by
the main global and regional climate oscillations in this time scale,
especially the Atlantic Multidecadal Oscillation (AMO) and the
Interdecadal Pacic Oscillation (IPO) (Grimm & Saboia, 2015;
Grimm et al., 2016). Although a very signicant contribution is
also provided by intraseasonal time scales, such as the Madden
Julian Oscillation (Grimm, 2019), in the present case study of the
severe and persistent drought over southern Brazil that culminated
at the austral late summer and early autumn of 2020, the focus
will be on the longer time scales.

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ENSO is a coupled ocean – atmosphere climate oscillation.
Its opposite phases, the events El Niño and La Niña, denote
sea surface temperature (SST) conditions that are, respectively,
above and below average in the central/eastern tropical Pacic,
in addition to anomalies of atmospheric circulation coupled with
them. This is the main climate oscillation on interannual time scales,
with global climate impacts, including signicant effects on South
America. They can be produced directly, as on the west coast of
the continent, which experiences the local effects of the perturbed
SST, as well as indirectly, through atmospheric teleconnections
from the Pacic, which disturb the atmospheric circulation over
the continent, changing precipitation and temperature (Cai et al.,
2020, and references therein).
El Niño (La Niña) events, despite having in common
persistent above (below) normal SST in the equatorial Central-
Eastern Pacic, exhibit differences between them related to the
distribution of these equatorial SST anomalies (Kao & Yu, 2009;
Tedeschi et al., 2015, 2016; Cai et al., 2020). Such anomalies may
extend from the Central Pacic to the east, till the coast of South
America, or they may be more concentrated in the Central Pacic.
Therefore, El Niño (La Niña) events are classied into East El
Niño (EEN) and Central El Niño (CEN) (East La Niña and
Central La Niña), according to the location of the strongest SST
anomalies in the east or central equatorial Pacic Ocean.
If SST anomalies have different distributions, their effects
can also be different. The impact of ENSO on precipitation in
southern Brazil is produced by tropic-extratropic atmospheric
teleconnections, due to the propagation of Rossby waves produced
in the tropics/subtropics (Grimm & Silva Dias, 1995; Cai et al.,
2020). In El Niño, for example, the anomalous warming of the
equatorial ocean surface increases evaporation and heats the
air above, increasing atmospheric instability and favoring the
formation of deeper clouds in the equatorial Central-East Pacic,
where convection is usually weak, due to the lower climatological
SST. The formation of these deeper clouds and, therefore,
greater transformation of water vapor into liquid water, produces
anomalous release of latent heat in the atmosphere, constituting
a source of energy that produces expansion of the atmospheric
column and divergence at high levels (next to the tropopause).
The air that diverges at high levels towards different latitudes
undergoes variation of the Coriolis force, which produces the
so-called Rossby waves, composed of centers of low and high
pressure, or centers of cyclonic and anti-cyclonic circulation.
The propagation of these waves does not occur equally in all
directions. There are preferential routes, depending on where
the forcing of the Rossby wave is, and some of them pass over
southern South America, perturbing the atmospheric circulation
and producing droughts or periods of longer and more intense
rain (e.g., Grimm & Ambrizzi, 2009). These perturbations can be
different, depending on the position of the main forcing in the
tropical Pacic Ocean, which can vary according to the position
of the main anomalies of SST (Grimm & Silva Dias, 1995).
The AMO describes the SST variations in the North Atlantic
Ocean with multidecadal variability in an approximate 65–70 years
cycle. The overall physical mechanism that drives the variability
in AMO is not yet well understood, but modeling studies indicate
that the SST variation in the Atlantic is associated with variations
in the Atlantic Meridional Overturning Circulation, formed by the
transport of the top layer warm and salty oceanic water from the
equatorial to North Atlantic region followed by the ow of deep
cold water southward (Knight et al., 2005; Parker et al., 2007).
This meridional circulation is enhanced (weakened) during the
AMO positive (negative) phase.
The IPO is a wide Pacic basin decadal to multidecadal
oscillation that displays some geographical similarity to ENSO
except that the meridional scale of tropical anomalies is broader
(Power et al., 1999; Parker et al., 2007). There are, however,
differences with respect to ENSO: the tropical SST anomalies
associated with the IPO extend further west and are relatively
weak in the far eastern tropical Pacic. Besides, unlike ENSO,
the opposite SST anomalies in the midlatitude North Pacic
associated with the IPO are comparable to those near the equator,
constituting an important component of this oscillation.
There are several hypotheses on the origins of IPO, including
stochastic atmospheric forcing, SST advection associated with the
North Pacic gyre oscillation or upper-ocean circulation, SST
anomalies reinforced by unstable midlatitude ocean–atmosphere
interaction, and mechanisms based on wind-driven upper-ocean
circulation (Yang et al., 2020, and references therein). Modeling
experiments based on this last hypothesis, recently suggested the
AMO to be the source of the winds that can induce the IPO
pattern, so that a positive AMO leads negative IPO by 4–8 years
(Yang et al., 2020).
Objectives
This article intends to show that different natural climate
oscillations contributed to the occurrence of the severe drought
in the late summer and early autumn of 2020 in southern Brazil,
and therefore emphasize the importance of studying the impacts
of climate oscillations in different seasons and different phase
combinations. The results indicate that more intense extremes,
beyond those already observed, can happen due to these
combinations. Such information is important for planning the
use and storage of water resources in the short, medium and
long term, depending on the periods of such oscillations, and
preparing for water emergencies.
MATERIAL AND METHODS
Material
Precipitation data used are monthly and quarterly totals,
made available by the National Meteorological Institute (INMET,
Brazil) and by the Weather Forecast and Climate Studies Center
(CPTEC/INPE, Brazil).
Opposite phases of the analyzed climate oscillations are
characterized by oceanic and atmospheric variables. Oceanic data
are from the National Ocean and Atmospheric Administration
Extended Reconstructed SST V4 (NOAA ERSST-V4, Huang et al.,
2015) and HadISST1.1 (Rayner et al., 2003) sets. Atmospheric
data were obtained from the NCEP/NCAR Reanalysis data set
(Kalnay et al., 1996). All these data are provided by the NOAA/

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The combined effect of climate oscillations in producing extremes: the 2020 drought in southern Brazil
OAR/ESRL PSL, Boulder, Colorado, USA, from their website
(Physical Sciences Laboratory, 2020a). The indexes representing
the Atlantic Multidecadal Oscillation and Interdecadal Pacic
Oscillation were obtained from the same institution (Physical
Sciences Laboratory, 2020b).
Methods
The 2020 drought in the period FMA and associated SST
pattern is described through precipitation and SST anomalies,
which are deviations from the 1981-2010 climatology.
The state of the atmosphere and oceans during the two
types of El Niño (EEN and CEN) is characterized through anomaly
composites (anomaly means) over events of these phenomena.
The events are dened as in Tedeschi et al. (2015, 2016), according
to the position of the largest SST anomalies in the equatorial belt
of the Eastern Pacic (140°W – 90°W, 5°N – 5°S) or the Central
Pacic (160°E – 150°W, 5°N – 5°S), and are the same determined
in those studies. These regions correspond, respectively, to most
of that for Niño3 index (with less 10° at the west side, to separate
the regions and thus better distinguish the two types of ENSO),
and to that for Niño4 index. An El Niño (La Niña) event is
characterized if the ve-month running means of monthly SST
anomalies in each region are equal or greater than 0.5 K (equal or
less than −0.5 K) for at least six consecutive months (including
October–November–December of the beginning year (0) of the
event and January of the following year (+1)). It is called Central
ENSO (Central El Niño or Central La Niña) if the anomalies
satisfy the conditions in the Central Pacic (Niño 4) region, and
East ENSO (East El Niño or East La Niña) if the conditions are
satised in the Eastern Pacic (approximately Niño3) region. Some
episodes satisfy the conditions in both regions. In these cases,
the annual anomaly (August (0) to July (+)) is calculated in each
region (Central and East), and the episode is dened according
to the region with the highest value.
Since the Atlantic Multidecadal Oscillation (AMO) and
Interdecadal Pacic Oscillation (IPO) are the rst global scale
modes of SST interdecadal variability (e.g., Parker et al., 2007),
frequently these modes are represented by indexes based on
SST averaged over the regions in which these modes display the
largest SST variability. The AMO index is basically an index of the
SST anomalies area weighted averaged over the North Atlantic,
0 to 70°N (Enfield et al., 2001). The IPO index is based on the
difference between the SST anomalies averaged over the central
equatorial Pacic (10°S–10°N, 170°E–90°W) and those average in
the Northwest and Southwest Pacic (25°N–45°N, 140°E–145°W
and 50°S–15°S, 150°E–160°W, respectively), using HadISST1.1 data
(Henley et al., 2015). The data used span the period 1950-2020.
The SST patterns associated with the interdecadal oscillations
AMO and IPO are characterized through the correlation between
the indexes of these modes and the global SST in grid points
from the HadISST1.1 set (Rayner et al., 2003). The relationship
of these oscillations with precipitation at the global level is
characterized by the correlation between their indexes and the
outgoing long-wave radiation (OLR), a variable that satisfactorily
represents precipitation in tropical/subtropical regions. The negative
anomalies of this radiation are generally related to the weaker
thermal radiation emitted from the cold tops of deeper clouds
associated with stronger precipitation, while positive anomalies
are generally related to the stronger thermal radiation coming
from the surface, in cloudless regions and, therefore, without
rain. Therefore, negative correlation with OLR means positive
correlation with precipitation and vice versa.
RESULTS AND DISCUSSION
State of the climate in southern Brazil in February-
March-April 2020
In FMA 2020, there was a persistent and intense drought
in southern Brazil (Figure 1, upper left panel). It covered almost
the entire region. Although there were also some dry months in
2019 (in late winter and early spring), the greatest persistence and
extent of drought occurred in FMA 2020, as it was strong and
present in all these months (Figure 1, lower panels).
During this period, SST anomalies occurred in regions
connected with the climate oscillations CEN, AMO and IPO
(Figure 1, upper right panel).
The period from October 2019 to April 2020 was characterized
by the NOAA Climate Prediction Center (NOAA/CPC) as an El
Niño event, according to the general criterion that the Oceanic
Niño Index (ONI) (average three-months SST anomaly over the
Niño 3.4 region (5° N - 5° S, 120° - 170° W)), is above 0.5 K
(National Weather Service, 2020).
Analyzing the average SST anomalies during the event over
the regions in the equatorial Pacic, it is possible to conclude that
this event was a weak CEN, since the SST anomalies in the central
Pacic (Niño 4 region) were much stronger than in eastern Pacic
(Niño 3 region). Precipitation anomalies in southern Brazil were
even weakly positive on average in the spring 2019 (October-
November-December), but in late summer and early autumn
2020, anomalies became consistently negative across the region
according to the expected behavior for CEN (Tedeschi et al., 2016).
Besides the warming in the central equatorial Pacic,
associated with the CEN, there are also positive SST anomalies
in the extratropics of the North and South Pacic, associated
with the negative phase of the IPO, and in the North Atlantic,
associated with the positive phase of the AMO. There is a tendency
towards cooling in the eastern Pacic, which is also characteristic
of the negative phase of the IPO, although the values are small
in this region.
The characteristics and effects of the climate oscillations that
most contributed to the drought are described in the next section.
Effects of climate oscillations on precipitation in
southern Brazil
The drought that culminated during FMA resulted from
the superposition of effects, in southern Brazil, of different
climate oscillations that involve changes in the SST at different
time scales: the interannual oscillation ENSO (through a CEN
episode), and the two most important global modes of SST
interdecadal variability: the AMO and the IPO.

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Central El Niño
Figure 2 shows the difference between the two types of El
Niño for FMA, through anomaly composites of SST and OLR
(indicating precipitation) for these events, dened here according to
Tedeschi et al. (2015, 2016). The upper panels of Figure 2 show that
EEN has SST anomalies in general stronger than CEN, extending
from the Central Pacic to the west coast of South America.
In CEN the positive equatorial SST anomalies are concentrated
in the Central Pacic, with even negative anomalies in the far east
of the Pacic. Such differences produce different distributions of
anomalous convection over the Pacic, represented in the lower
panels of Figure 2 by OLR anomalies. Negative anomalies (in
shades of blue, purple and lilac) represent enhanced convection,
with more deep clouds and rain, while positive OLR anomalies (in
shades of green, yellow and red) indicate more subsidence, less
clouds and rain. For EEN, the equatorial anomalous convection
in the Pacic extends from the central Pacic to South America,
with anomalous subsidence in the subtropics and the equatorial
western Pacic. For CEN, the enhanced equatorial convection is
only in the Central Pacic and the subsidence in the subtropics
also does not extend to South America. On the contrary, in the
eastern Pacic the pattern of convection anomalies is reversed, with
greater subsidence in the equator and convection in the subtropics.
In the lower panels of Figure 2 it is possible to distinguish
the different effects of the two types of El Niño during FMA in
southern Brazil, through OLR anomalies over the region (inside
the red ellipse). While in the EEN the OLR anomalies are negative
(and therefore those of precipitation are positive), in CEN they
are positive (and those of precipitation are negative). These results
are consistent with those of Tedeschi et al. (2016) for autumn
(MAM), using observed rainfall data for South America.
The different impacts in southern Brazil are related to
the different distribution of convection anomalies in the Pacic,
as they produce different teleconnections that result in distinct
atmospheric disturbances over the extratropics of South America
(Figure 3). During EEN there is a cyclone (low pressure) to the
southwest of the continent and an anticyclone (high pressure) to
the southeast; in CEN the opposite occurs, with an anticyclone
to the southwest and a cyclone to the southeast. The difference
stems from the different wave propagation from the eastern
tropical Pacic, as there are different convection anomalies there,
Figure 1. Precipitation anomalies on FMA 2020 (top left panel, from INMET, in mm/trimester) and on February, March and April
2020 (bottom panels, from CPTEC/INPE, in mm/month). The top right panel shows the SST anomalies observed in FMA. The
ellipses of different colors indicate the main regions in which different climate oscillations contribute to SST anomalies: CEN (blue),
IPO (lilac), AMO (red) (see text) (from the NOAA/ESRL Physical Sciences Laboratory).

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The combined effect of climate oscillations in producing extremes: the 2020 drought in southern Brazil
Figure 2. Characteristics of East El Niño (EEN) and Central El Niño (CEN) for February-March-April, with regard to: (top panels)
sea surface temperature (SST) anomalies (K) and (bottom panels) outgoing long-wave radiation (OLR) anomalies (W/m2), whose
negative (positive) values indicate positive (negative) precipitation anomalies (from the NOAA/ESRL Physical Sciences Laboratory).
The red ellipse highlights southern Brazil. For EEN the equatorial anomalous warm SST and enhanced convection extend from Central
Pacic to South America, while for CEN they are concentrated in Central Pacic. As a consequence, in EEN (CEN) precipitation
anomalies are positive (negative) over southern Brazil.
Figure 3. Geopotential anomalies at high level (200 hPa) for February-March-April of (left) EEN and (right) CEN (from the NOAA/
ESRL Physical Sciences Laboratory). Positive (negative) anomalies indicate regions of higher (lower) pressure, with anticyclonic (cyclonic)
circulation. The lilac lines indicate Rossby wave propagation relevant to southern South America, showing a west-east pair cyclone/
anticyclone (anticyclone/cyclone) straddling extratropical South America for EEN (CEN). Since in the extratropics the Rossby waves
have equivalent barotropic structure, this pair also exists at low-level. Therefore, red lines indicate circulation and moisture transport
anomalies contributing to precipitation anomalies in southern Brazil.

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which modify the resulting Rossby wave that reaches southern
South America. This region in the eastern Pacic is important
for the propagation of teleconnections to the extratropics of
South America, since it is one of the few equatorial regions with
westerly winds at high levels, which facilitate the propagation of
Rossby waves to higher latitudes. Anomalies over southern South
America are, in reality, more inuenced by convection anomalies
in the eastern equatorial Pacic than in the central Pacic (Grimm
& Silva Dias, 1995; Grimm & Ambrizzi, 2009).
In the extratropics, circulation anomalies caused by Rossby
waves are equivalent barotropic, that is, they have the same sign
of anomalies at low and high levels (Grimm & Silva Dias, 1995;
Ting, 1996). In the case of EEN, the upper-level pattern with a
cyclone to the southwest and anticyclone to the southeast of South
America stimulates ascending motion over the subtropics of the
continent, east of the Andes, via advection of cyclonic vorticity,
which favors ascending motion to the east of the cyclonic anomaly
(Holton, 2004; Grimm, 2003; Tedeschi et al., 2016). At low-level,
the cyclone/anticyclone pair enhances the transport of moisture
from the north to southern Brazil (red lines), producing moisture
convergence and favoring precipitation in this region (Grimm,
2003; Tedeschi et al., 2016). In the case of CEN, the pattern with
an anticyclone to the southwest and cyclone to the southeast of
South America favors the opposite: subsidence over southern Brazil
and moisture transport from this region to the north, resulting in
reduced precipitation.
It is interesting to note that the greatest differences between
the impacts of EEN and CEN occur in the fall/winter of the
year following the start of the events, since it is in this period
that the greatest differences between the SST anomalies in the
eastern equatorial Pacic occur for the two types of EN, while the
differences are smaller in the spring of the event (Tedeschi et al.,
2015, 2016).
Atlantic Multidecadal Oscillation (AMO)
The quasi-interhemispheric structure of opposite SST
anomalies in the Atlantic Basin displaces the Intertropical
Convergence Zone (ITCZ) northward, and creates anomalies of
meridional circulation between the two hemispheres. There are
important AMO impacts on the precipitation over the tropical/
subtropical Atlantic basin, which follow from the meridional shifts
of the Atlantic ITCZ. Over South America, the strongest anomalies
occur over Northeast Brazil, whose rainy season depends on the
position of the ITCZ, but they also extend over southern Brazil.
Figure 4 shows, through correlation analysis with the
AMO index, the SST anomalies associated with this oscillation
in the FMA trimester (left panel), as well as the convection (or
precipitation) anomalies represented by OLR anomalies (right
panel). The positive correlation with OLR (which is negatively
correlated with precipitation), means that in the AMO positive phase
(AMO(+)), observed in FMA 2020, there is reduced precipitation
in FMA over southern Brazil (red ellipse).
This mode inuences the rst modes of interdecadal
monsoon precipitation variability over South America, as can
be seen in Grimm & Saboia (2015) and Grimm et al. (2016), in
which the rst two summer modes display negative anomalies over
southern Brazil corresponding to positive AMO phase.
Interdecadal Pacic Oscillation (IPO)
Since there are similarities between EEN and IPO positive
phase anomalous SST patterns (cf. Figures 2 and 5 (left)) there
are also similar inuences on the precipitation, here represented
by OLR. Remembering that OLR and precipitation anomalies are
negatively correlated, it is possible to see that the FMA precipitation
Figure 4. Correlation of the AMO index with (left) SST and (right) OLR, during February-March-April, indicating the sign of the
anomalies associated with the positive phase of the AMO (from the NOAA/ESRL Physical Sciences Laboratory). The ellipse on
the left panel indicates the region with highest components of the oscillation (North Atlantic), while on the right panel it highlights
southern Brazil, where positive correlation with OLR indicates negative correlation with precipitation. Correlations equal to 0.20 are
signicant to the 0.05 level. Therefore, the positive correlation with OLR above 0.2 in southern Brazil means that the positive phase
of AMO produces reduced precipitation in this region.

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The combined effect of climate oscillations in producing extremes: the 2020 drought in southern Brazil
anomaly patterns associated with the IPO positive phase and EEN
are similar over South America, with positive (negative) values over
the subtropical (northern/central-west) parts of the continent.
Therefore, the IPO negative phase (IPO(-)), observed in FMA
2020, would produce reduced precipitation over southern Brazil.
Combined effect of climate oscillations on precipitation
variability and extremes
The combined effects of CEN, AMO (+) and IPO (-)
in FMA are stronger, more comprehensive and consistent in
southern Brazil (Figure 1) because it is in this region that the
effects of these oscillations in these phases coincide in producing
reduced precipitation. In other regions the inuences are mixed.
For example, in most of the western and northern parts of Brazil,
AMO (+) and IPO (-) produce opposite effects on FMA rainfall
(Figures 4 and 5), while CEN favors reduced precipitation (Figure 2).
The result is mixed signs, with some prevalence of negative
anomalies (Figure 1). In the eastern part, there is little inuence
of IPO, although IPO(-) enhances rainfall in the southern part
of southeast Brazil and eastern part of North Brazil (Figure 5),
contrary to AMO (+) (Figure 4), and although CEN produces
few anomalies in the eastern part (Figure 2), they are positive.
In this part, positive anomalies predominate, although most are
weak (Figure 1). ENSO has more impact on the variance of
seasonal precipitation than interdecadal oscillations. The latter can
modulate the former but this does not preclude the occurrence of
ENSO events with opposite effects to those of the interdecadal
oscillations, as shows the analysis in the following.
Both modes, AMO and IPO, inuence the rst two
modes of monsoon precipitation interdecadal variability over
South America, but with different combination of phases, as
can be seen in Grimm & Saboia (2015) and Grimm et al. (2016).
While in the rst mode both oscillations are in the same phase, in
the second one they are in opposite phases, which is even more
effective in producing anomalies over southern Brazil in austral
autumn. Although the dynamical mechanisms of the AMO and
IPO impacts on the precipitation over southern Brazil is not
within the scope of this study, it is interesting to mention that
the inuence of AMO(+) and IPO(-) on the second interdecadal
mode of monsoon precipitation produces a low level divergence
center over central South America (and upper-level convergence
center) which weakens the monsoon circulation and reduces
the precipitation, especially in the subtropics (see Figures 5f-j
in Grimm et al. (2016) and corresponding analysis). The time
evolution of this precipitation mode also displays phase changes
in the late 1970s and late 1990s (Figure 3b in Grimm et al. (2016)),
as shown for AMO and IPO in Figure 6.
It is interesting to compare the evolution of the AMO and
IPO indexes (Figure 6, upper panel) and the FMA precipitation
interdecadal variability in two separate areas of southern Brazil,
which show that the behavior is consistent over this region (Figure 6,
bottom panels). The evolution of AMO and IPO in the period
1950-2020 is smoothed by a 4-year moving average, to emphasize
the slowest variations, but their phases remained respectively
positive and negative in FMA 2020, as the most recent averages
in Figure 6. It is possible to see that in decades with predominant
AMO(-) and IPO(+) (late 1970s till late 1990s) the precipitation
in the region was enhanced, but when these oscillations changed
phase to AMO(+) and IPO(-) in the late 1990s, precipitation
was reduced in the following decades. This combined effect of
the two oscillations is consistent with the superposition of the
effects previously described produced by each oscillation on the
precipitation in southern Brazil.
Figure 5. Correlation of the IPO index with (left) SST and (right) OLR, during February-March-April, indicating the sign of the
anomalies associated with the positive phase of the IPO (from the NOAA/ESRL Physical Sciences Laboratory). The rectangles on
the left panel show the areas used in the denition of the IPO index. The ellipse on the right panel highlights southern Brazil, where
negative correlation with OLR indicates positive correlation with precipitation. Correlations equal to 0.20 are signicant to the 0.05
level. Therefore, the negative correlation with OLR below -0.2 in southern Brazil means that the positive phase of IPO produces
increased precipitation in this region.

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The inuence of these slow climate oscillations is due to
changes in the basic state of the atmosphere produced by different
SST boundary conditions. This does not mean that extremes (of
drought or precipitation) cannot occur in the opposite direction
to that favored by the combination of phases of interdecadal
oscillations, but these will generally be less frequent and less intense.
Therefore, in periods when these climate oscillations favor above
(below) normal rainfall, large accumulations (severe droughts) are
more frequent. This relationship between climate oscillations and
extreme precipitation events occurs for interdecadal (Grimm et al.,
2016), interannual (Grimm & Tedeschi, 2009; Tedeschi et al.,
2015, 2016) and intraseasonal (Grimm, 2019) time scales. This is
why it is so important to know it for different seasons and phase
combinations, as it establishes the extreme limits for which water
resources managers must plan.
In addition to the combination of interdecadal oscillations
that produce an atmospheric basic state favorable to drought,
the occurrence of a really severe drought usually also counts
on the combined occurrence of a favorable climatic event
on interannual scale, such as CEN in 2020 or La Niña in
2009 and 2012. A similar condition is valid for producing large
accumulations of precipitation, when a phase combination of
interdecadal oscillations favorable to enhanced precipitation is
completed with the occurrence of a favorable climatic event
on interannual or intraseasonal time scale, such as EEN in
1992 and 1998. However, an interdecadal context favorable to
decient (excess rainfall) does not preclude the occurrence of
an interannual oscillation episode leading to excess (decient)
rainfall, as EEN in 2016 (La Niña in 1989), although the resulting
rainfall anomalies may be reduced.
Figure 6. (Upper panel) Indexes representing AMO (blue line) and IPO (red line) in the period 1950-2020, smoothed with a 4-years
running mean (from the NOAA/ESRL Physical Sciences Laboratory). The periods late 1970s - late 1990s and late 1990s – 2020, in
which the two oscillations had opposite phases, are limited by green ellipses. (Bottom panels) FMA rainfall anomaly series averaged
over two areas marked on the map (from CPTEC/INPE). In the decades with predominant IPO positive phase and AMO negative
phase rainfall anomalies were predominantly positive (blue bars), while in those with predominant IPO negative phase and AMO
positive phase they were predominantly negative (red bars).

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The combined effect of climate oscillations in producing extremes: the 2020 drought in southern Brazil
Possible role of anthropogenic climate change
Although a particular extreme drought cannot be attributed
to anthropogenic causes, they may have inuence on changing the
intensity and frequency of these extreme events (e.g., Skansi et al.,
2013). For example, extensive deforestation in the Amazon affects
evapotranspiration from the Amazon basin and alters the cascading
moisture recycling, which involves re-evaporation cycles along
the way of the moisture transport from the tropical Atlantic
and the entire Amazon basin towards the Paraná-La Plata basin.
According to Zemp et al. (2014), this moisture recycling contributes
about 17–18% to the precipitation over the La Plata basin, and
increases the fraction of precipitation over the La Plata basin
that originates from the Amazon basin from 18–23 to 24–29%
during the wet season. Therefore, deforestation can change the
amount of moisture that comes from the Amazon to the Paraná/
Plata Basin, and droughts produced by natural oscillations can be
intensied in southern Brazil.
CONCLUSIONS
Within the interdecadal context of an atmospheric basic
state more favorable or unfavorable to precipitation in a certain
region, the occurrence of interannual or intraseasonal time scale
climate events (such as El Niño, La Niña, or different phases of
the Madden-Julian Oscillation) that produce effects in the same
direction as the interdecadal oscillations, may lead to extreme
events of rainfall or drought. Climate oscillations and some of their
combinations alter the frequency (and sometimes the intensity)
of synoptic events that most inuence the weather in the region,
such as cold fronts in southern Brazil, producing different amounts
of precipitation. It is important that water resource managers are
aware of the possible effects of different combinations of climate
oscillations on extremes. These effects change according to the
season and to the phases of the oscillations, requiring detailed
study of possible interactions.
In the present case study not all of the natural global
or regional climate oscillations that explain smaller amounts of
low-frequency climate variability than AMO, IPO and ENSO
were in a phase favorable to drought in southern Brazil, which
means that other combinations may produce even more extreme
events. Preparing for water emergencies requires that managers
have an idea of how extreme such emergencies can be, and this
depends on the study of the effects of possible combinations of
climate oscillations.
ACKNOWLEDGEMENTS
The rst author thanks funding from CNPq and IAI grant
CRN3035, which is supported by US NSF Grant GEO-1128040.
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Authors contributions
Alice Marlene Grimm: Conceived the study and contributed the
climatic analysis on the causes of the drought.
Arlan Scortegagna Almeida: Contributed the information on the
impacts of the drought and to the discussion.
Cesar Augustus Assis Beneti: Contributed the information on the
impacts of the drought and to the discussion.
Eduardo Alvim Leite: Contributed the information on the impacts
of the drought and to the discussion.