CLIMATIC VARIABILITY BETWEEN NORTH BRAZILIAN RIVERS AND
SEA SURFACE TEMPERATURE
(ID 201119367NKZVB)
Maria Elisa Siqueira Silva1
Carlos Batista da Silva2
Sérgio Melo Gadelha2
Abstract. This study has search variables which could influence climatic variability of
river outflow over north and central areas of Brazil, at Amazon and Paraguay basins.
Firstly, it was possible to note, through linear relations, that the two continental chosen
regions should suffer influence from distinct aspects of oceanic climatic variability. The
central region (represented by Aquidauana River) showed high correlations with South
Equatorial Pacific and South Tropical Atlantic areas, while the north region (represented
by Amazon River/Obidos) presented high correlations values in relation to both South
Equatorial Pacific and Tropical Atlantic. Aliseos from northeast strengthening was
suggested as a contributing factor for the increasing in hydrological cycle at Amazon
Basin case. The results through the linear model application suggest that outflow
variability can be well explained by SST averages at particular areas over the Pacific
Ocean, showing a R2=80.1%. SST averages at North Pacific, South Pacific and west
Equatorial Pacific (Niño4) are the most important regions that can be related to the
climatic variability of Aquidauana river outflow.
Keywords: climatic variability; river outflow; sea surface temperature
INTRODUCTION
Climatic variability of river outflow is attested in association to sea surface temperature
for rivers located at north and central portions of Brazil. Although these rivers are
placed in the same tropical area, they are located at different sub-areas (distant by more
than 2700 km), as stated in Figure 1, whose climate is partially defined by distinct
sources from the global oceans. Climate for both regions receives influence mainly from
Pacific and Atlantic variability. Results from many studies in climatology reveal the
direct and non-direct influence of oceanic surface variability over rivers outflow
throughout the world (Molion and Moraes, 1987; Hastenrath, 1990; Mechoso and
Iribarren, 1991; Robertson and Mechoso, 1998; Genta et al., 1998; Camilloni and
Barros, 2000; Gomes Filho, 2000; Trigo et al., 2002; Prowse and Beltaos, 2002;
Cardoso et al., 2004; Barros et al., 2004; Cardoso, 2005; Xu et al., 2007). The north
region of Brazil is characterized by great amount of hydro resource from Amazon Basin
1 Teacher at the Department of Geography - University of São Paulo (USP) - email: elisasiq@usp.br
2 Graduation student from the Physic Geography Program - USP
(133.000 m3/s, 73% of the hydro source from the Amazon River is placed in Brazil).
Amazon is the greatest basin in relation to those around the globe, both in extension and
in water volume, and presents the greatest hydro energy power in Brazil. The other
studied river stays at central part of Brazil, at Paraguay Basin, and comprehends the
Pantanal region that constitutes a particular ecosystem. This area is characterized by
great plains and flooded portions during almost the entire year. Although the area is
highly plain, there are many small hydroelectric power plants installed there. While the
Amazon Basin vegetation is dominated by rain forest, the Pantanal area is characterized
by savanna-like vegetation (Cerrado) and tall grass. The seasonality is fair marked in
Pantanal, with lower temperatures during the dry month (winter month – Jun to Sep)
and more rain during the summer month (Nov to Mar). On the other hand, Amazon
Basin presents small temperature amplitude during the year due to its position closer to
equatorial areas. Seasonality of rain presents distinct patterns accordingly to the region
inside the Amazon Basin due to its extensive area. Obidos, the fluvial station considered
here in the Amazon Basin, is characterized by a rainy period during Dec and May. In
relation to atmospheric systems that contribute to the climate in these two areas, frontal
colds and South Atlantic Convergence Zone (SACZ) are prominent systems that affects
Pantanal region. Intertropical Convergence Zone (ITCZ) and local convection, mainly
between December and May, are synoptic and local systems that contribute to Amazon
climate.
Many scientific studies had already attested the relation between sea surface
temperature variability and hydrological cycle, particularly over South America (SA).
Causes of precipitation climatic variability are linked to Pacific anomalies, as ENSO
and PDO phenomena, what is dependent on time scale. The first modes of Principal
Component Analysis (PCA) applied on SST data are related to areas with great
variability over the Atlantic and Pacific Oceans. These areas are many times related to
hydrological cycle over SA. The most important pattern that can be depicted at Pacific
Ocean is that characterized by areas with SST anomalies with opposite signals in
regions near the equator and those displayed over higher latitudes, in both hemisphere,
the so-called horseshoe pattern. Another important pattern observed over the oceans and
that can be related to SA climate variability is the meridional gradient of SST anomalies
over the Tropical Atlantic, showing may times opposite signals between north and south
areas. Positive SST anomalies over south tropical areas of Atlantic are many times
related to ITCZ displacement to southern positions. This study aims to depict oceanic
areas that are well related to the hydrological variability over Amazon and Paraguay
basins, becoming thus possible to suggest possible links between continental and
oceanic areas. Based on this result, it is also an objective to consider stochastic
modeling for river outflow simulation in annual scale.
Hydrographic Basins
Amazon Basin
Araguaia-Tocantins Basin
Paraíba River Basin
São Francisco River Basin
Paraná River Basin
Paraíba do Sul River Basin
Uruguai RiverBasin
Paraguai River Basin
Fig 1 Brazilian hydrographic basins. Amazon and Paraguay basins are shaded in green and
violet colors, respectively. Red dots show Obidos and Aquidauana outflow observations.
After: http://www.suapesquisa.com/geografia/bacias_hidrograficas.gif
DATA AND METHODS
Outflow monthly data from Obidos and Aquidauana rivers were collected from National
Agency for Hydro Resources in Brazil (ANA). Precipitation data were obtained from
CPTEC/INPE, from a compiled file with spatial resolution equal to ¼ degree;
precipitation data were originally from the national rain gauge. Aquidauana River lies at
Paraguay basin that is characterized much more by subtropical climate than tha Amazon
Basin. The period select for Aquidauana River was 1966-2000; for Obidos River, the
period was 1974-2004. SST monthly data were obtained from the Earth System
Research Laboratory – ESRL/NOAA, with 2 degree of spatial resolution (time series
originally from 1854 to 2008). Firstly, river outflows and precipitation (for areas
upstream to the fluvial observation) time series were compared themselves to assess the
possible influence of surface characteristics on the variability of outflow data. Secondly,
oceanic areas with significant linear correlation (above 95%) between SST and outflow
for both rivers were defined. SST averages over the selected oceanic areas were
considered as independent variables in the elaboration of stochastic models for river
outflow simulation. Besides SST means values, climatic indices were also considered as
potential predictors, North Atlantic Oscillation (NAO), South Antarctic Mode (SAM)
Pacific Decadal Oscillation (PDO), South Oscillation Index (SOI) and NIÑO1+2,
NIÑO3+4 e NIÑO4. Stochastic models were constructed as linear regression between
river outflow and oceanic variables (SST average and climatic indices) for annual and
monthly scales (only annual results are presented here). Linear tendency and seasonal
cycle were previously removed from the time series studied, that is, from outflow and
precipitation data for each continental area and from SST. All the maps showed here
present correlation computed between time series without lag (lag=0) and for January as
the beginning month for the two time series.
RESULTS
Linear correlation between Aquidauana outflow and SST, between 1966 and 2000,
shows a well known spatial patter that is related to ENSO or PDO oscillation,
depending on the time scale, as can be seen in Figure 2. Positive SST anomalies over
Tropical Pacific closer to SA west coast are related to increased outflow at Aquidauana.
Linear correlation above 0.4 can be noted over the south Tropical Pacific near the coast
of Chile. In this case, the horseshoe pattern is well marked with negative correlation
over North and South Pacific reaching up 40 degrees of latitude in both Hemisphere.
Between 40S and 60S there is another area positively well correlated with Aquidauana
outflow, showing values greater than 0.6 near Antarctic regions. Correlation over South
Atlantic show opposite signals between north and south areas what can be related to the
first mode of variability for PCA applied on SST data, as referenced by (Chaves, 2005).
In the case of Amazon/Obidos River, the SST pattern over Tropical Pacific is similar to
that related to Aquidauana outflow, except by the sign inversion. Negative (positive)
anomalies over Equatorial and Tropical Pacific are related to intensified (weakness)
river outflow (or precipitation) at Obidos. Major values (|r| > 0.5) are located over
southern Equatorial Pacific, close to the west SA coast, between 20S and Equator, as
shown in Figure 3. Therefore, both cases, Obidos and Aquidauana outflows, seams to
reflect the importance of Pacific variability. Another area which is well defined by high
linear correlation values, in case of Obidos, is the Tropical North Atlantic, over passing
absolute correlations to values greater than 0.5. The results also show a small area of
positive correlation at Equator south side. Negative SST anomalies over north Tropical
Atlantic and positive SST anomalies over south Tropical Atlantic can be related to the
ITCZ displacement to south, providing more precipitation and intensifying runoff over
the north of SA. Negative SST anomalies over north tropical Atlantic can also be related
to the strengthening of subtropical high pressure in this region, providing the increase of
water vapor transport by intensified aliseos from northeast. Back to Pacific Ocean, the
results obtained in this study show the canonical pattern that influence South America,
that is, positive SST anomalies over the Equatorial East Pacific contribute to the
weakening and strengthening of hydrologic cycle over north and central SA and, viceversa. The influence over Central America, represented here by Paraguay Basin, is
much less evident in the scientific results already shown, yet.
Although physic and dynamic explanations constitute the more confident method to
search oceanic aspects that can influence river outflow variability, stochastic modeling
is considered as a practical way for simulation and prediction solutions. Aquidauana
outflow simulation was addressed trough stochastic modeling based on linear multiple
regression. Climatic indices and averaged SST for areas well correlated were considered
in the model development. The better linear model built considered SST averages at
North Pacific (Area1), South Pacific (Area2) and west Equatorial Pacific, named Nino4
(Area3), with R2 equal to 80.1% for smoothed data by a 6-years-window running mean.
Data not filtered showed R2 equal to 60.5%. Observed and simulated data by the
developed linear model are shown in Figure 4. In this case, Pacific variability seems to
be more representative to the climatic variability over central SA. In contrast, no
Atlantic areas were chosen for Aquidauana river outflow.
Figure 2 Spatial correlation between SST and Aquidauana River outflow (Paraguay
Basin), for lag=0. All shaded areas are statistically significant at a level of 95%.
Figure 3
Spatial correlation between SST and Amazon River outflow (at Obidos)
(Amazon Basin), for lag=0. All shaded areas are statistically significant at a level of 95%.
Figure 4
Simulated and observed data for Aquidauana outflow anomaly (m 3/year)
between 1966 and 2000. The data are smoothed by running mean for each 6 years. The
period between 1985 and 1990 is characterized by missing data.
CONCLUSIONS
This study has provided to confirm, in general, the possible differences in climatic
variability influence over north and central areas of Brazil. Besides that, it was possible
to note that Pacific Oscillation seems to be important for both Amazon and Paraguay
outflows climatic variability. On the other hand, the influence from Atlantic Ocean on
outflows seems to be more dependent on the river localization, whether in the north or
in central Brazil. This study has search variables which could influence climatic
variability of outflow over north and central areas of Brazil, at Amazon and Paraguay
basins. Firstly, it was possible to note, through linear relations, that the two continental
regions chosen should suffer influence from distinct aspects of oceanic climatic
variability. The central portion (represented by Aquidauana River) showed high
correlations with South Equatorial Pacific and South Tropical Atlantic areas, while the
north portion (represented by Amazon River/Obidos) presented high correlations values
in relation to both South Equatorial Pacific and Tropical Atlantic. In the last case,
aliseos from northeast strengthening was suggested as a contributing factor for the
increasing in hydrological cycle. The results through the linear model application
suggest that outflow variability can be well explained (R2=80.1%) by SST averages at
particular areas over the Pacific Ocean. SST averages at North Pacific, South Pacific
and west Equatorial Pacific (Nino4) are the most important regions that can be related to
the climatic variability of Aquidauana outflow. Analysis of atmospheric flow in specific
periods should bring comprehension for the main mechanisms that can be important to
the climatic variability definition for the studied areas.
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