Written by Javier Tomasella e José A. Marengo
Thursday, 20 January 2011 23:00
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Due to its geographical location and its vast territorial expanse, the Amazon basin is strongly influenced by two different types of rain regimes: the northern portion is subjected to a southern displacement (north-south) of the Intertropical Convergence Zone (ITCZ); the southern portion that results from the organized convection associated with the South Atlantic Convergence Zone (SACZ) during the months of summer. These convergence zones (ITCZ and SACZ) represent a meeting of the masses of different characteristics that determine rainfall over vast areas, particularly during the months of summer and autumn. These convergence regions can be observed from satellite images as areas of great nebulosity.
The regime of rains that affect the Amazon Basin have a great annual amplitude: well-defined rainy periods and dry seasons, however these periods are not the same in the north and south portions of the basin: the maximum amount of rainfall in the southern portion of the basin takes place two months before (December-January-February) the maximum amount in the central portion (February-March-April), located next to the main course, and six months before the months of maximum rainfall in the extreme northern portion of the basin (June-July-August), located in the northern hemisphere over the State of Roraima. The minimum amount of rainfall occurs in June-July-August in the central portion and to the south of the basin (look for more details in Marengo 1992 and Tomasella et al. 2010). Consequently, from a hydrological point of view, the Amazon basin is subdivided in three subregions with contrasting hydrologic regimes: the northern tributaries, the southern tributaries and the main waterway which receives contributions from tributaries located in the north and south. This difference between rainy periods between the northern and southern waterways determines that the contribution from southern and northern tributaries takes place, in normal years, in different periods.
One of the most important characteristics of tropical regions is the rainfall variation on an interannual scale (from one year to another) and interdecadal, processes that are much studied in scientific literature (e.g. Ronchail et al. 2002, Marengo 2004). These studies show that the northern portion of a basin is greatly affected by anomalous temperature changes, whether heating or cooling the sea surface in the tropical Pacific, a phenomenon known respectively as El Niño and La Niña. Warm episodes in the tropical Pacific (El Niño) tend to cause rain deficits in the northern and central portion of the basin (for example, the draughts of 1926, 1983 and 1998); cold episodes are generally associated with excessive rains in the northern portion of the basin. However, there are extreme events in the Amazon that are not associated with El Niño or La Niña: the draught of 2005 (Marengo et al. 2008 a, b, Tomasella et al. 2010) showed that part of the southern basin is affected by reductions in rainfall in consequence of anomalous warming of the Northern tropical Atlantic. This was the main phenomenon responsible for the draughts of 1963 and 2005.
Any alteration in the magnitude and in the period in which the rainy season occurs, whether in the north or south of the basin, can motivate an increase or decrease in the hydrometric levels of the main waterway. For instance, in the case of the decline of 1997, related with an El Niño event (third largest decline in the period between 1907-2009), the levels of the river in the main waterway were affected by a sudden drop in the tributary levels in the North; while in the decline of 2005 (seventh largest in the period between 1903-1999), the decline was increased by the output deficits in the tributaries located to the south of the main waterway (Tomasella et al. 2010).
A preliminary hydro-meteorologic analysis of the draught of 2010: its causes and consequences
Figure 1 shows the evolution of anomalies (which are deviations with regard to the normal period of 1961-1990) of the sea surface temperature – SST in the tropical Atlantic and Pacific since November 2009 up to October 2010. The areas in red indicate an occurrence of above average temperatures; blue signifies a cooling below average.
It is clear that during the rainy seasons in the northern portion of the basin, in other words from February to April, the prevalent condition in the tropical Pacific Ocean corresponds to the warm phase specifically, a weak El Niño event. At the same time, the north tropical Atlantic has been warmer than normal (up to 2º C warmer than normal in March-May 2010).
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The occurrence of the El Niño phenomenon, added to the warming of the tropical North Atlantic in early 2010, brought in consequence a reduction in rainfall in the northern portion of the basin, affecting the output of Northern tributaries, especially that of Negro River. On the other hand, the persistence of warming in the tropical Atlantic is associated with the occurrence of strong draughts in the tributaries of the southern waterway (Marengo et al. 2008), affecting the output of major rivers such as Madeira. It is worth highlighting that the rain’s deficit has direct consequences on the hydrologic regime, not only with regard to the magnitude of the floods during the rainy season, over the yield in the dry period due to the smaller recharging of aquifers.
This preliminary analysis suggests, therefore, that the El Niño phenomenon, added to the warming of the tropical North Atlantic, influenced unfavorably the floods and recharging of the tributaries in the Northern Region during the rainy seasons from December 2009 to March 2010. During the autumn in the southern hemisphere, the tropical Atlantic continued warming to such a degree that in the tropical Pacific the El Niño phenomenon was replaced by a cold phase, the La Niña phenomenon. This weather situation during the summer and autumn favored the occurrence of severe draughts in the tributaries of the southern waterway of the Amazonas River up to October 2010.
Figure 2 shows the hydrometric levels in two stations located in Rivers Negro in Barcelos and Madeira in Humaitá, which are the main tributaries of the northern waterway, and in the south of the Amazon River. It is clear that the hydrometric levels of Negro River in the period from February to April 2010 were close to the greatest decline recorded (1980), recovering between April and September 2010, but once again remained in very low levels between September and November, a period which corresponds to the height of the decline in the main waterway.
In the case of the Madeira River in Humaitá, the hydrometric levels remained well below the historic levels during 2010, reaching a minimum value slightly above the recorded historical minimum values in 2005. This hydrologic evolution is similar to the one observed in 2005.
As previously mentioned, the difference between the rainy periods in the north and south of the main waterway determines the occurrence of the contributions of the north and south tributaries, in normal years, in different periods of time (Tomasella et al. 2010). The displacement in times of contribution by the main waterway by the tributaries has as a main effect the lessening of the height of flooding and ebbing of rivers. However, the simultaneous occurrence of such heights or draughts in the tributaries of the northern waterway and to the south of the Amazon River causes an increase in the flooding/draughts in the main course and shows up as normally associated with the hydrologic extremes of the Amazon River.
That being the case, this preliminary analysis of the hydrological data indicates that the ebbing of 2010 was caused by the superimposition of a severe draught in the tributaries to the south of the main waterway in the months of October and November, combined with very low hydrometric levels for this time of the year in the northern tributaries; to which is added an exceptional reduction of the Andean part of the basin. A combination of deficiencies in the output of both margins of the river was responsible for the sudden drop in the output of the main course.
Certainly, both draughts (2005 and 2009) and the biggest flood in the history of the Amazon Rainforest in 2009 showed that in less than five years two climate extremes, which are opposed and of major intensity, affected the Amazon. Compared to other extremes in the past, the impacts seem to be greater now than in other draughts such as in 1926 or in 1998, and that shows that the populations living on the banks of Amazonian rivers are more exposed and more vulnerable in the present than in the past, due to the urban growth and economic development. It is not possible to say for sure that draughts such as the one in 2005 or 2010, or even floods such as in 2009, are a direct consequence of global warming or of deforestation in the Amazon. Ongoing studies by INPE are analyzing these extremes and employing robust climate models, aiming at determining whether hydrological extremes such as those recently observed can become more frequent in the future as a result of global changes in the climate.
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Javier Tomasella and José Marengo are researchers of INPE’s CCST (Science Center of the Land System).
References:
Marengo, J.A. 1992. Interannual variability of surface climate in the Amazon basin. International Journal of Climatology 12: 853-863.
Marengo, J.A. 2004. Interdecadal variability and trends in rainfall in the Amazon basin. Theoretical And Applied Climatology 78: 79-96.
Marengo J.A., Nobre A., Nobre, C.A., Tomasella J.; Cardoso M,. Oyama M. 2008a. Hydro-climatic and ecological behaviour of the drought of Amazonia in 2005. Philosophical Transactions of the Royal Society of London. Biological Sciences 21: 1-6.
Marengo J.A.; Nobre C.A.; Tomasella J.; Oyama M;. Sampaio G. ; Camargo H.; Alves, L.M. 2008. The drought of Amazonia in 2005. Journal of Climate 21: 495-516.
Ronchail J.G.; Cochonneau M.; Molinier J.L.; Guyot A.G.; de Miranda C.; Guimarães V.; and de Oliveira E. 2002. Interannual rainfall variability in the Amazon basin and sea-surface temperatures in the equatorial Pacific and tropical Atlantic Oceans, International Journal of Climatology 22: 1663–1686.
Tomasella, J.; Borma, L. S.; Marengo, J. A. ; Rodriguez, D. A.; Cuartas, L. A.; Nobre, C.A.; Prado, M.C.R. . The droughts of 1996-97 and 2004-05 in Amazonia: Hydrological response in the river main-stem. Hydrological Processes (Print), 2010.
