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Jose A. Marengo and Stefan Hastenrath

Abstract

The mechanisms of climate anomalies in the Amazon basin were explored from surface climatological and hydrological series, upper-air, and satellite observations. The paper is focused on the March–April rainy season peak in the northern portion of Amazonia. Case studies for the moderately wet year 1986 (WET), showed a relatively far-southerly location of the Atlantic near-equatorial trough, and embedded intertropical convergence zone (ITCZ); strong ascending motion and vigorous convection over the Amazon basin, contrasting with pronounced subsidence off the west coast of South America,, and weak subtropical westerly jets (STWJ) in both hemispheres. In contrast, the extremely dry El Niño year 1983 (DRY), featured a more northward located ITCZ; subsidence over the Amazon basin; ascending motion and convective rainfall to the west of the Andes; and strong STWJ.

In synthesis from these analyses, some major mechanisms of extreme rainfall events in northern Amazonia stand out, but only for the late austral summer, when the ITCZ in the tropical Atlantic-South American sector attains its southernmost position, as the intense summertime convection over Amazonia is an important component of the ITCZ. Thus, ascending motion over the northern part of the Amazon basin with an anomalously far-southward displaced ITCZ appears compatible with subsidence to the west of the Andes during the high phase of the Southern Oscillation (SO), which is defined by anomalously high/low pressure at Tahiti/Darwin. In contrast, ascending motion and convection over the easternmost equatorial Pacific, as is common during extreme events of the low-SO phase, require compensatory subsidence, and this may interfere with convection to the east of the Andes. However, hydrometeorological anomalies in Amazonia are not prevailingly related to the SO.

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José A. Marengo, Carlos A. Nobre, and Alistair D. Culf

Abstract

Meteorological observations from the Anglo–Brazilian Amazonian Climate Observation Study (ABRACOS), together with the global reanalysis from the National Center for Environmental Prediction (NCEP) and satellite images, have been used to study the spatial extent and intensity of cold surges (known locally as “friagens”) in the Amazon basin. Case studies are presented of two of the strongest events of the 1994 winter season: 26 June and 10 July 1994. In both events, daily minimum temperatures in southeastern Brazil dropped to near or below 0°C, while at the same time minimum temperatures in southern Amazonia (Ji-Paraná site) were almost 8°C below average. Air temperature and humidity also fell in central and western Amazonia (Manaus and Marabá sites, respectively), although the fact that these reductions were less substantial than those farther to the south indicates that the cold air is greatly modified as it moves across Amazonia.

In Ji-Paraná the largest drops in minimum temperature coincided with strong winds from the south, implying that cold advection was the main mechanism for the falling temperatures. In contrast, there was no increase in wind speed at Manaus and Marabá during the days with reduced temperatures. At these sites, cooling was due to a reduction of the maximum temperature caused, at least partially, by increased cloudiness rather than by a lowering of minimum temperatures as at Ji-Paraná. With regard to the depth of the atmospheric boundary layer (ABL), it is observed that during the passage of the cold air in southern Amazonia, the ABL was cooler and shallower than during the pre- and postfriagem days. The friagens presented are of 5- to 6-day duration, including the passage of the cold front, but the period with cold temperatures lasts between 2 and 3 days.

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Jose A. Marengo, Wagner R. Soares, Celeste Saulo, and Matilde Nicolini

Abstract

A climatology of the South American low-level jet east of the Andes (SALLJ) is developed using the 1950– 2000 circulation and moisture fields from the NCEP–NCAR reanalyses and available upper-air observations made in Bolivia and Paraguay since 1998. Upper- and low-level circulation fields were derived for seasonal means and SALLJ composites during the warm and cold seasons. The Bonner criterion 1 was applied for sites in central Bolivia and downstream near northern Paraguay, to determine the spatial and temporal characteristics of the SALLJ. On the circulation characteristics, SALLJ composites during the warm season show the enhanced low-level meridional moisture transport coming from equatorial South America as well as an upper-level wave train emanating from the west Pacific propagating toward South America. The intensification of the warm season SALLJ follows the establishment of an upper-level ridge over southern Brazil and a trough over most of Argentina. The circulation anomalies at upper and lower levels suggest that the intensification of the SALLJ would lead to an intensification of the South Atlantic convergence zone (SACZ) later on and to the penetration of cold fronts with an area of enhanced convection ahead at the exit region of the SALLJ.

Regarding the time variability, the SALLJ seems to occur all year long, bringing tropical moist air masses from the Amazon into southern Brazil–northern Argentina more frequently in the warm season, and bringing tropical maritime air, which is less humid than the tropical air masses coming from the subtropical Atlantic high, more frequently during the cold season. SALLJs are detected mostly during the warm season to the north of ∼20°S, while to the south the SALLJs seem to occur all year long. The diurnal cycle shows that SALLJs are more frequent and intense between 0600 and 1200 UTC for the warm season north of 20°S, while at the region downstream the maximum is detected between 0000 and 0600 UTC during the cold season. At interannual time scales, even though there is a weak tendency for stronger and more frequent warm season SALLJ episodes in years with anomalously warm surface waters in the tropical Pacific, it cannot be affirmed with a large degree of certainty that there is a strong relationship between the occurrence of El Niño events and the number and/ or intensity of SALLJ episodes. However, the 1998 El Niño featured more frequent and intense warm season jet episodes than occurred during the 1999 La Niña, and this has been demonstrated by the reanalyses, the available Pan American Climate Studies-Sound Network (PACS-SONET) upper-air observations, and by other studies using independent datasets and regional modeling.

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Jose A. Marengo, Brant Liebmann, Vernon E. Kousky, Naziano P. Filizola, and Ilana C. Wainer

Abstract

Onset and end of the rainy season in the Amazon Basin are examined for the period 1979–96. The onset and end dates are determined by averaging daily rainfall data from many stations, and then constructing 5-day averages (pentads). Onset (end) is defined as the pentad in which rainfall exceeds (falls below) a given threshold, provided that average rainfall was well below (above) the threshold for several pentads preceding onset (end), and well above (below) the threshold for several pentads after onset (end). For the criteria chosen, the climatological onset progresses toward the southeast, arriving in mid-October, and then toward the mouth of the Amazon, arriving near the end of the year. The end dates are earliest in the southeast and progress toward the north, but withdrawal is slower than onset. The onset dates, however, are quite sensitive to changes in the threshold. If the threshold is doubled, for example, the sense of onset is reversed, with onset occurring toward the northwest. Changes in threshold do not change the direction of the progression of the end of the rainy season.

The central Amazon shows the largest variation in the date of onset. In several years, onset in the southeast occurs before that in the central Amazon, but onset near the mouth is always latest. There is an unexpectedly low relationship between the length of the rainy season and total accumulation. Likewise, there is little relationship between the onset (and end) date and the total accumulation.

Composites of outgoing longwave radiation and the low-level wind field show that in the central Amazon, onset is associated with an anomalous anticyclone and enhanced trade winds in the Atlantic. Near the mouth of the Amazon, however, onset is associated with large-scale northerly anomalies, and the zonal component of the trade winds is reduced.

There is an apparent association between sea surface temperature anomalies in the tropical Atlantic and Pacific and the pentads of onset and end of the rainy season in the northern and central Amazon, and near its mouth. The sense is that a warm Pacific and cold Atlantic result in a delayed onset and early withdrawal. Although the strong El Niño of 1982/83 and La Niña 1988/89 were examples of a delayed and early onset, respectively, the relationships it still holds these years are not considered.

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Niklas Boers, Henrique M. J. Barbosa, Bodo Bookhagen, José A. Marengo, Norbert Marwan, and Jürgen Kurths

Abstract

Based on high-spatiotemporal-resolution data, the authors perform a climatological study of strong rainfall events propagating from southeastern South America to the eastern slopes of the central Andes during the monsoon season. These events account for up to 70% of total seasonal rainfall in these areas. They are of societal relevance because of associated natural hazards in the form of floods and landslides, and they form an intriguing climatic phenomenon, because they propagate against the direction of the low-level moisture flow from the tropics. The responsible synoptic mechanism is analyzed using suitable composites of the relevant atmospheric variables with high temporal resolution. The results suggest that the low-level inflow from the tropics, while important for maintaining sufficient moisture in the area of rainfall, does not initiate the formation of rainfall clusters. Instead, alternating low and high pressure anomalies in midlatitudes, which are associated with an eastward-moving Rossby wave train, in combination with the northwestern Argentinean low, create favorable pressure and wind conditions for frontogenesis and subsequent precipitation events propagating from southeastern South America toward the Bolivian Andes.

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Jose A. Marengo, Lincoln M. Alves, Wagner R. Soares, Daniel A. Rodriguez, Helio Camargo, Marco Paredes Riveros, and Amelia Diaz Pabló

Abstract

Two simultaneous extreme events affected tropical South America to the east of the Andes during the austral summer and fall of 2012: a severe drought in Northeast Brazil and intense rainfall and floods in Amazonia, both considered records for the last 50 years. Changes in atmospheric circulation and rainfall were consistent with the notion of an active role of colder-than-normal surface waters in the equatorial Pacific, with above-normal upward motion and rainfall in western Amazonia and increased subsidence over Northeast Brazil. Atmospheric circulation and soil moisture anomalies in the region contributed to an intensified transport of Atlantic moisture into the western part of Amazonia then turning southward to the southern Amazonia region, where the Chaco low was intensified. This was favored by the intensification of subtropical high pressure over the region, associated with an anomalously intense and northward-displaced Atlantic high over a relatively colder subtropical South Atlantic Ocean. This pattern observed in 2012 was not found during other wet years in Amazonia such as 1989, 1999, and 2009. This suggests La Niña as the main cause of the abundant rainfall in western Amazonia from October to December, with wet conditions starting earlier and remaining until March 2012, mostly in northwestern Amazonia. The anomalously high river levels during the following May–July were a consequence of this early and abundant rainy season during the previous summer. In Northeast Brazil, dry conditions started to appear in December 2011 in the northern sector and then extended to the entire region by the peak of the rainy season of February–May 2012.

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José A. Marengo, Carlos A. Nobre, Javier Tomasella, Marcos D. Oyama, Gilvan Sampaio de Oliveira, Rafael de Oliveira, Helio Camargo, Lincoln M. Alves, and I. Foster Brown

Abstract

In 2005, large sections of southwestern Amazonia experienced one of the most intense droughts of the last hundred years. The drought severely affected human population along the main channel of the Amazon River and its western and southwestern tributaries, the Solimões (also known as the Amazon River in the other Amazon countries) and the Madeira Rivers, respectively. The river levels fell to historic low levels and navigation along these rivers had to be suspended. The drought did not affect central or eastern Amazonia, a pattern different from the El Niño–related droughts in 1926, 1983, and 1998. The choice of rainfall data used influenced the detection of the drought. While most datasets (station or gridded data) showed negative departures from mean rainfall, one dataset exhibited above-normal rainfall in western Amazonia.

The causes of the drought were not related to El Niño but to (i) the anomalously warm tropical North Atlantic, (ii) the reduced intensity in northeast trade wind moisture transport into southern Amazonia during the peak summertime season, and (iii) the weakened upward motion over this section of Amazonia, resulting in reduced convective development and rainfall. The drought conditions were intensified during the dry season into September 2005 when humidity was lower than normal and air temperatures were 3°–5°C warmer than normal. Because of the extended dry season in the region, forest fires affected part of southwestern Amazonia. Rains returned in October 2005 and generated flooding after February 2006.

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Iracema F. A. Cavalcanti, José A. Marengo, Prakki Satyamurty, Carlos A. Nobre, Igor Trosnikov, José Paulo Bonatti, Antonio Ocimar Manzi, Tatiana Tarasova, Luciano P. Pezzi, Cassiano D'Almeida, Gilvan Sampaio, Christopher C. Castro, Marcos B. Sanches, and Hélio Camargo

Abstract

The Center for Weather Forecasting and Climate Studies–Center for Ocean–Land–Atmosphere Studies (CPTEC–COLA) atmospheric general circulation model (AGCM) is integrated with nine initial conditions for 10 yr to obtain the model climate in an ensemble mode. The global climatological characteristics simulated by the model are compared with observational data, and emphasis is given to the Southern Hemisphere and South America. Evaluation of the model's performance is presented by showing systematic errors of several variables, and anomaly correlation and reproducibility are applied to precipitation. The model is able to simulate the main features of the global climate, and the results are consistent with analyses of other AGCMs. The seasonal cycle is reproduced well in all analyzed variables, and systematic errors occur at the same regions in different seasons. The Southern Hemisphere convergence zones are simulated reasonably well, although the model overestimates precipitation in the southern portions and underestimates it in the northern portions of these systems. The high- and low-level main circulation features such as the subtropical highs, subtropical jet streams, and storm tracks are depicted well by the model, albeit with different intensities from the reanalysis. The stationary waves of the Northern and Southern Hemispheres are weaker in the model; however, the dominant wavenumbers are similar to the observations. The energy budget analysis shows values of some radiative fluxes that are close to observations, but the unbalanced fluxes in the atmosphere and at the surface indicate that the radiation and cloud scheme parameterizations need to be improved. Besides these improvements, changes in the convection scheme and higher horizontal resolution to represent orographic effects better are being planned to improve the model's performance.

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Jose A. Marengo, Luiz E.O.C. Aragão, Peter M. Cox, Richard Betts, Duarte Costa, Neil Kaye, Lauren T. Smith, Lincoln M. Alves, and Vera Reis
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Brant Liebmann, Suzana J. Camargo, Anji Seth, José A. Marengo, Leila M. V. Carvalho, Dave Allured, Rong Fu, and Carolina S. Vera

Abstract

Rainfall in South America as simulated by a 24-ensemble member of the ECHAM 4.5 atmospheric general circulation model is compared and contrasted with observations (in areas in which data are available) for the period 1976–2001. Emphasis is placed on determining the onset and end of the rainy season, from which its length and rain rate are determined.

It is shown that over large parts of the domain the onset and ending dates are well simulated by the model, with biases of less than 10 days. There is a tendency for model onset to occur early and ending to occur late, resulting in a simulated rainy season that is on average too long in many areas. The model wet season rain rate also tends to be larger than observed.

To estimate the relative importance of errors in wet season length and rain rate in determining biases in the annual total, adjusted totals are computed by substituting both the observed climatological wet season length and rate for those of the model. Problems in the rain rate generally are more important than problems in the length.

The wet season length and rain rate also contribute substantially to interannual variations in the annual total. These quantities are almost independent, and it is argued that they are each associated with different mechanisms.

The observed onset dates almost always lie within the range of onset of the ensemble members, even in the areas with a large model onset bias. In some areas, though, the model does not perform well. In southern Brazil the model ensemble average onset always occurs in summer, whereas the observations show that winter is often the wettest period. Individual members, however, do occasionally show a winter rainfall peak. In southern Northeast Brazil the model has a more distinct rainy season than is observed. In the northwest Amazon the model annual cycle is shifted relative to that observed, resulting in a model bias.

No interannual relationship between model and observed onset dates is expected unless onset in the model and observations has a mutual relationship with SST anomalies. In part of the near-equatorial Amazon, there does exist an interannual relationship between onset dates. Previous studies have shown that in this area there is a relationship between SST anomalies and variations in seasonal total rainfall.

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