A quick look at the 2012 record flood in the Amazon Basin

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 40, , doi: /grl.50245, 2013 A quick look at the 2012 record flood in the Amazon Basin Prakki Satyamurty, 1 Claudia Priscila Wanzeler da Costa, 2 Antonio Ocimar Manzi, 3 and Luiz Antonio Candido 3 Received 13 December 2012; revised 5 February 2013; accepted 8 February 2013; published 11 April [1] The water level at the Manaus Port on the Negro River reached its record value of m on 29 May This is higher than the previous record in the year 2009 by 20 cm. The rise of the level from November 2011 till the record maximum is the highest in the past several decades. A cooler South Atlantic and a normal or slightly warmer North Atlantic were associated with a record flood in the Amazon Basin in The seasonal atmospheric moisture convergence and the precipitation over the Amazon Basin are well correlated. During the period October 2011 through May 2012 the moisture-flux convergence was 38% more intense than climatology. The rainfall equivalence of this excess moisture convergence is about 2.5 mm d 1 in the western Amazon Basin and 1.8 mm d 1 in the whole Amazon Basin. Citation: Satyamurty, P., C. P. W. da Costa, A. O. Manzi, and L. A. Candido (2013), A quick look at the 2012 record flood in the Amazon Basin, Geophys. Res. Lett., 40, , doi: /grl Introduction [2] The flood plains in the Amazon Basin are seasonally subjected to inundations and recessions every year. In some years the flood peaks are very high and in some others they are very low compared to normal years [Satyamurty et al., 2012]. Either situation causes economic and sociological problems to the region. November through April is considered rainy season in the Amazon Basin [Rao and Hada, 1990; Marengo, 2004]. Because the basin s slope is small, there is a long delay between the rainy season and the manifestation of flood in the central and eastern portions of the Amazon Basin. Usually, around the first week of June the river level at Manaus reaches its yearly maximum and around 15 October it falls to its yearly minimum. In the rainy season of the year there was less precipitation in the basin, especially in the southwestern sector, and the Amazon River ran low in 2005 [Marengo et al., 2008; Zeng et al., 2008]. In 2009 there was record flood as seen by the water level at Manaus Port [Marengo et al., 2011]. The following year, 2010, experienced widespread deficit of 1 CESTU, Programa de Pós-Graduação em Clima e Ambiente, Universidade do Estado do Amazonas, Manaus, Brazil. 2 Programa de Pós-Graduação em Clima e Ambiente, Universidade do Estado do Amazonas, Manaus, Brazil. 3 Instituto Nacional de Pesquisa Amazônicas, Manaus, Brazil. precipitation in the basin and the region experienced one of the worst drought conditions [Lewis et al., 2011]. This drought was considered more severe than the 2005 drought. Recently, 2012 experienced record flood level as registered at the Manaus Port on 29 May and maintained the level for 10 days. These quick and large variations in the past decade has drawn the attention of climatologists. [3] The effects of El Niño Southern Oscillation (ENSO) on the rainfall in Amazonia are understood in terms of the Walker circulation cell [Ropelewski and Halpert, 1987]. The effects of the Tropical Atlantic SST (sea surface temperature) anomalies on the rainfall over tropical South America during the March-April-May (MAM) rainy season for the Nordeste of Brazil were investigated by Pezzi and Cavalcanti [2001]. Although they did not discuss the effects on the Amazon Basin rainfall, their figures show that the Center for Weather Prediction and Climate Studies model predicted deficit rainfall for all conditions in the Atlantic in MAM. This feature could be a systematic error of the model. Yoon and Zeng [2009] studied the Atlantic influence on the Amazon rainfall. They concluded that cooling of the North Atlantic could induce wet conditions over the Amazon Basin, especially in the dry season. They added that the North Atlantic influence is stronger than that of the South Atlantic. [4] In the present study, we investigated the atmospheric moisture convergence, the Pacific ENSO and the Atlantic SST conditions to get clues for explaining the behavior of the Amazon climate in Methodology [5] In this paper, the climatological year for the Amazon Basin is considered to be October through September, so that the rainy season and the high flood situation are in the middle of the climatological year. A large rectangular area between 80 W and 40 W and 5 N and 25 S, designated the Target Area (TA1), is considered to contain the Amazon Basin. The vertically and laterally integrated water vapor fluxes at the four boundaries of the rectangular area, TA1, i.e., the transports into and out of the area, are obtained [see Satyamurty et al., 2012] on a daily basis. From the daily values, the monthly, seasonal, and annual averages are calculated. The sum of the four transports across the four boundaries, respecting the signs, gives us the net moisture convergence over the Basin, C W. [6] On seasonal time scales the hydrological balance over a sufficiently large area, A, is given by Corresponding author: P. Satyamurty, CESTU, Programa de Pós-Graduação em Clima e Ambiente, Universidade do Estado do Amazonas, Manaus, AM, CEP: , Brazil. (saty.prakki@gmail.com) American Geophysical Union. All Rights Reserved /13/grl C W ¼ ðp EÞA (1) where E and P are the area average evapotranspiration and precipitation rates, respectively. Considering E to be fairly constant over the years, the interannual variability in the precipitation is explained by the variability of C W. Thus, the C W 1396

2 Figure 1. Regression between seasonal precipitation and moisture convergence over the Amazon Basin (TA1) for the 26 rainy seasons from 1986 to Continuous line: Linear fit. Dashed line: Quadratic fit. R 2 is the variance of precipitation explained by moisture convergence. calculations should be able to explain basin-scale floods and droughts. [7] One plausible argument for considering E to be fairly constant from one year to the other is that transpiration by the forest decreases in wet years because the cloudy skies reduce photosynthesis and in dry years the evaporation is reduced because there is less water available. Moreover, as was shown in Satyamurty et al. [2012], the contribution of the moisture convergence (62%) to precipitation is much larger than that of evapotranspiration (38%) and therefore the C W anomaly is more important for basin scale floods or droughts than the variations in evapotranspiration. In equation (1) the storage term (dw/dt, where w is precipitable water) is neglected because the difference between the w values before and after the rainy season is of the order of 10 mm whereas the seasonal precipitation and evapotranspiration are of the orders of 1000 and 500 mm, respectively. [8] The vertically integrated moisture convergence field Convq = R r.(qv) dp/g is calculated using gridded reanalysis wind (V) and specific humidity (q) data at the gridpoints, whereas the basin mean net moisture convergence, C W, is calculated as a line integral of vertically integrated moisture flux along the boundaries of the TA1, following Satyamurty et al. [2012]. [9] In the same way, another Target Area, TA2, containing the Western Amazon Basin [see Satyamurty et al., 2012], is also considered and the moisture transports across its boundaries and the net convergence are calculated. The meteorological data set used is the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis at 2.5 resolution. [10] The daily river level data at the Manaus Port obtained from the Geological Survey of Brazil is used to obtain the yearly range of the water level for investigating its variations and its relationship with the SST conditions in the Pacific and the Atlantic obtained from NOAA. Figure 2. Monthly maximum Negro River water level at Manaus Port during two recent record flood years. May 2008 through July 2009 (continuous line) and May 2011 through July 2012 (dashed line). 3. Results 3.1. Relation Between Moisture Convergence and Precipitation [11] The seasonal mean and area mean net moisture convergence over the Amazon Basin (TA1) and the precipitation for the 26 rainy seasons from 1986 to 2012 are plotted Figure 3. Annual range of Negro River level at Manaus Port and the linear trend line. 1397

3 in the dispersion diagram shown in Figure 1. The diagram shows a good relation between the two variables. The quadratic regression shows that nearly 60% of the variance in the precipitation can be explained by the variability in the moisture convergence over the basin. Especially, when the moisture convergence was higher than kg s 1 the rainfall was in excess of 1650 mm. Thus, moisture convergence is a very important factor for determining an abnormal wet rainy season. SATYAMURTY ET AL.: AMAZON FLOOD IN River Level Variations [12] The rate of rise of the water level during the period November 2011 through May 2012 is the highest so far recorded. Figure 2 shows the monthly peak height of the water level of the Negro River at Manaus Port (3.1 S, 59.9 W) for the two periods May 2008 to July 2009 and May 2011 to July This graph provides a ready comparison of the two most recent record floods in the Amazon Basin. One important aspect of the graphs is that the fall of water level from July to October and the rise from November to March are steep in both years. However, the rise in was faster than in This indicates that the amount of rain in the Negro River Basin during the rainy season October 2011 through April 2012 was higher than in the previous flood year. [13] Another important observation is shown in Figure 3 where the annual range of the river level, that is the difference in levels between the peak in a given year and the lowest level in the previous year, is plotted. In the rise was only about 11 m whereas it is 13 m in In fact the range was the highest in the year , but the peak Negro River level was below 28 m (figure not shown). [14] What is interesting is a gradual positive trend of the annual range since From 1904 to the late 1960s there was no significant long-range trend. Since 1970 the rate of increase in the range has been about 4 m in about 40 years. That is, the trend is approximately 10 cm year 1. In no other period during the recorded history of the river level measurements at Manaus such a trend was observed. Superposed on the linear trend are interannual oscillations in the range. Because the river level range is a function of rainfall amount in the basin in the rainy season, we can safely infer that the rainfall in the Negro River Basin suffered large oscillations and the interannual rainfall variability has gradually increased. This observation provides a strong basis for the argument that the climate extremes have been growing in intensity since 1970 in the Amazon Basin. [15] The Amazon River and its tributaries to the south also must have contributed to the Negro River level at Manaus. The argument is, if the Amazon River were running low, the flood waters from the Negro River would not accumulate to high levels. Or, if the Amazon River level is also simultaneously high, the Negro River runoff would be slower and the level rises What Influenced the 2012 Amazon Flood? [16] To investigate the causes for excess rainfall in the Amazon Basin during 2011 and 2012, we have examined the sea-surface temperature situation in the equatorial Pacific and the tropical Atlantic oceans and the moisture convergence over the Amazon Basin. Figure 4 shows the progression of the SST anomalies in the Niño 1 + 2, Niño 3.4, Figure 4. Evolution of mean monthly values of SST anomalies ( C) in the Pacific in Niño (10 S 0 /90 W 80 W) and Niño 3.4 (5 S 5 N/170 W 120 W) regions (top), SST anomalies in North Atlantic ( 5 N 20 N/60 W 30 W) and South Atlantic (20 S 0 /30 W 10 E) (middle) and moisture convergence (10 7 kg s 1 ) in TA1 (15 S 0 /75 W 45 W) and TA2 (15 S 5 N/75 W 60 W) (bottom). tropical North Atlantic and tropical South Atlantic oceans along with the monthly net moisture convergence anomalies. [17] The negative SST anomalies of the order of 1 C in the equatorial Pacific (Figure 4, top) constitute La Niña conditions since the middle of Cool anomalies persisted in the Niño 3.4 region till May This prolonged, though weak, La Niña situation may have favored excess rainfall in the Amazon basin, especially in the northern parts. The Niño region also shows negative anomalies till January That is, most of the peak rainy season of was under the influence of La Niña, cold phase of ENSO. [18] From September 2011 through February 2012 the surface temperature in the tropical North Atlantic (Figure 4, 1398

4 middle) was slightly warmer than climatology and the South Atlantic was cooler than normal. This constitutes interbasin thermal gradient from the South Atlantic to North Atlantic [Pezzi and Cavalcanti, 2001] which, according to Yoon and Zeng [2009], does not favor excess rainfall in the Amazon Basin. They found, based on 27 years data, that cooling of the North Atlantic could induce wet conditions over the Amazon Basin, especially in the dry season, by shifting the Intertropical Convergence Zone southward. Moreover, the authors mention that the combined Pacific and Atlantic SST effects explain 53% of the variance in the Amazon precipitation, 25% from Niño 3.4 region, and 20% from the North Atlantic. However, in the legend of their Figure 11, they say that the variances of precipitation over Amazon explained by these three modes are 57.9% for North Atlantic SST, 30.4% tropical Pacific SST, and 7.8% for South Atlantic SST. These statistical findings may or may not hold for other wet years. In the case of the 2012 flood, the South Atlantic was cooler and the anomaly was stronger than in the North Atlantic during December through May Therefore, the 2012 situation cannot be explained by the statistics of Yoon and Zeng [2009]. [19] Figure 4 (bottom) shows the monthly net moisture convergence anomalies in the two basins, the whole Amazon Basin (TA1) and the Western Amazon Basin (TA2). In September 2011 the anomalies were negative and helped to reduce the precipitation of that month in relation to climatology, leading to very low level of the Negro River in October From October through February 2012 the anomalies were positive and high, contributing to excess rainfall in both basins. The positive anomalies continued in March, April, and May 2012, adding more water to the already existing large volume of water in the basin. Due to continued cool conditions in the Pacific (Niño 3.4) in April and May, the moisture convergence anomaly increased in May The result was the record flood at the end of May and the beginning of June [20] The five highest flood years prior to 2012 identified from the river level data are 1953, 1976, 1989, 1999, Figure 5 presents the five high-flood years composite (Figure 5a) and the distribution (Figure 5b) of the seasonal (October 2011 to April 2012) mean SST anomaly in the Pacific and in the Atlantic. The figure shows cooler surface waters in the equatorial Pacific (La Niña) accompanied by also cooler waters in the tropical South Atlantic off the east coast of Brazil. While the La Niña situation causes positive anomalies in the rainfall over the Amazon Basin through the Walker circulation with its ascending limb over the basin, the cooler waters in the South Atlantic favor subsidence over the eastern parts of Brazil adjoining the cooler waters. The compensating rising motions help strengthen the moisture convergence in the tropical interior of the continent (Figures 5c and 5d). While the mean anomaly of moisture convergence in the western Amazon Basin for the five wettest Figure 5. Seasonal (October through April) sea-surface temperature anomaly ( C): (a) , (b) composite of the , , , , seasons. Vertically integrated moisture convergence (color shaded, 10 5 kg m 2 s 1 ) and moisture flux (arrows, kg m 1 s 1 ) anomalies for (c) and (d) mean of five wettest seasons till

5 Figure 6. Moisture transport across the borders of TA1 and TA2. Left column (a, c, e): TA1. Right column (b, d, f): TA2. Top row (a, b): accumulated in the period October 2011 to May Middle row (c, d): climatology. Bottom row (e, f): anomaly. Units: 10 7 kg s 1. Net moisture convergence inside target area in 10 7 kg s 1 (top), and equivalent precipitation in mm/244 days (bottom) are given at the bottom-right corner. years till 2011 is positive (green and blue shades), the anomaly for the year is more intense, corresponding to the record flood situation. [21] Figure 6 presents the anomalies of the moisture transports across the four boundaries of the basins integrated over the period October 2011 through April 2012 and the net convergence of moisture over the basins. There was a substantial increase in the net moisture convergence in the whole basin (TA1) during the rainy season (Figure 6a) in relation to the climatology (Figure 6c). This increase (Figure 6e) was equivalent to a precipitation of 1.8 mm d 1. Over the Western Amazon Basin the increase was more pronounced, 2.8 mm d 1. The transport of moisture into the basin across the northern boundary of TA1 (equator) had a substantial increase. Across the other three boundaries also there was contribution for the anomaly of net convergence of moisture. For the western Amazon Basin, there were significant contributions from the northern and western boundaries. Apparently, cross-equator lower tropospheric moisture fluxes into the basins were important for the increased rainfall during the rainy season and postrainy season that caused the record floods. The moisture flux vectors (not presented) show that the moisture source for the basin is in the Atlantic. 4. Summary and Discussion [22] The Negro River water level at the Manaus Port in May 2012 has surpassed the previous record of 2009 by 20 cm and reached 29 m 97 cm and caused problems for the city of Manaus and the river-side communities of the Amazonas and Para states in Brazil. The annual range of water level is observed to have gradually increased since 1970 at an average rate of 10 cm yr 1. Severe droughts and floods have occurred more frequently in the past 40 years than in any other period since the beginning of the recorded river level measurements. This observation provides a strong argument in favor of climate change in the Amazon region. [23] In this paper, however, we discussed the conditions associated with the 2012 record floods in the Amazon Basin. Three conditions were associated with the record floods: [24] 1. The Niño 3.4 region was cooler by about 1 C from September 2011 through May Actually, the La Niña situation started in 2010 and, although weak in intensity, continued till May The Niño situation also was favorable for positive rainfall anomalies in the northern regions of South America, i.e., SST anomalies cooler by about 1 C, till January

6 [25] 2. A regression analysis (Figure 2) based on 26 years reanalysis data shows that about 60% of the variance of the rainy season precipitation in the Amazon Basin is associated with the variability of moisture convergence. The moisture convergence over the basin was about kg s 1 more intense than climatology from October 2011 through May 2012, most of the excess moisture convergence was caused by the moisture transport across the northern boundary into the basin. [26] Nigam and Ruiz-Barradas [2006] pointed out that the summer moisture-flux convergence over the Great Plains differs considerably between the global and regional reanalyses. That is, the model derived data (reanalysis) may have some systematic errors. A recent study by Zhong et al. [2012] showed that the seasonal cycle of moisture-flux convergence simulated by the National Center for Atmospheric Research Mesoscale Model, known as MM5 regional climate model and North American Regional Reanalysis model over the Great Lakes area was satisfactory. The present study shows the anomalies (i.e., the season minus climatology) and as such the effect of systematic model errors on our results is less. However, as is cautioned by Trenberth et al. [2001], the accuracy of the tropical analysis is somewhat limited. [27] 3. The tropical South Atlantic was cooler by 1 C from November 2011 through February The tropical North Atlantic remained normal or slightly warmer than normal. This situation is different from the one considered favorable for excess rainfall in the Amazon Basin by Yoon and Zeng [2009]. [28] It is known that the La Niña conditions in the Niño 3.4 and Niño regions cause subsidence over the equatorial Pacific, which is compensated by rising motion over the South American tropics and thus the convective activity increases. Due to cooler temperatures the subtropical high pressure center in the South Atlantic may have intensified, contributing to the intensification of trade winds. The evaporation over the warmer tropical North Atlantic increases and the moisture is transported by the trade winds westward into the South American tropics. Thus, the transport across the northern boundary of the basin increases, which contributes to increased moisture convergence over the basin. The effect of the South Atlantic over the Amazon Basin rainfall is not fully understood. In a dynamical perspective the excess rainfall in the rainy season can be viewed in the following manner. The convective activity over the warmer continent surrounded by cooler waters in the equatorial Pacific and the tropical South Atlantic, aided by increased moisture convergence, has increased, resulting in higher rainfall and floods. [29] Acknowledgments. The first author thanks Dr. Gloria Maria Mendes of CESTU, UEA and CNPq for support. The second author is a doctoral student supported by CAPES, Brazil. The third author thanks CNPq for support. References Marengo, J. A. (2004), Interdecadal variability and trends of rainfall across the Amazon Basin, Theor. Appl. Climatol., 78, doi: / s Marengo, J. A., C. A. Nobre, J. Tomasella, M. D. Oyama, G. S. Oliveira, R. Oliveira, H. Camargo, L. M. Alves, and I. Foster Brown (2008), The drought of Amazonia in 2005, J. Climate, 21, Marengo, J. A., J. Tomasella, W. R. Soares, L. M. Alves, and C. A. Nobre (2011), Extreme climatic events in the Amazon basin: Climatological and hydrological context of recent floods, Theor. Appl. 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