Characteristics of Amazonian climate: Main features

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Amazonia and Global ChangeGeophysical Monograph Series 186Copyright 2009 by the American Geophysical Union.10.1029/2008GM000720

Characteristics of Amazonian Climate: Main Features

Carlos A. Nobre, Guillermo O. Obregón, and José A. Marengo

Centro de Ciências do Sistema Terrestre, Instituto Nacional de Pesquisas Espaciais, Cachoeira Paulista, Brasil

Rong Fu

Jackson School of Geosciences, University of Texas at Austin, Austin, Texas, USA

German Poveda

Escuela de Geociencias y Medio Ambiente, Universidad Nacional de Colombia, Medellin, Colombia

This chapter summarizes our current knowledge on the mean climatological features of Amazonia. Significant uncertainties remain in our understanding of the complex dynamics of climate and climate variability in that region, which are due, in part, to the lack of observational data. The strong seasonality of the rainfall and the relatively rapid transition between the wet and dry season associated with onset of the rainy season is related to the establishment of the South America Monsoon System (SAMS). The SAMS is controlled by large-scale thermodynamic conditions influenced by the near-equatorial sea surface temperature (SST). It has been suggested that land-surface dryness in the dry season is the main cause of the delay in the onset of the subsequent wet season. The 30- to 60-day oscillation is the major mode of intraseasonal variability. Interannual variability of the hydroclimatic system is strongly related to El Niño–Southern Oscillation. More generally, tropical Pacific and Atlantic SSTs control rainfall variability in Amazonia, and SW Atlantic SST anomalies influence the variability of the South Atlantic Convergence Zone (SACZ). Land surface-atmosphere interactions have been proposed as a possible dynamical mechanism for the unexplained variance at the annual and interannual timescales. At decadal and interdecadal timescales, rainfall variability is related to the Pacific Decadal Oscillation mainly over the southern portions, and linked to the North Atlantic Oscillation. At paleoclimate timescales, there is large uncertainty on major aspects of rainfall variability over tropical South America. For instance, there remains uncertainty on the basic character of rainfall anomalies over Amazonia, whether drier or wetter, during the Last Glacial Maximum, and paleoclimate reconstructions still suffer from lack of data.

1. INTRODUCTION

The Amazon basin is one of the three quasi-permanent centers of intense convection embedded in the equatorial trough zone. It plays a pivotal role in the functioning of the

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global climate. The forests of Amazonia play a critical role in regulating climate at both regional and global levels. Through intense evapotranspiration, the tropical forests pump latent heat into the atmosphere to balance the strong surface radia-tive heating. The strong and extensive tropical convection over the continent during the Southern Hemisphere summer transports the latent heat to the upper troposphere and dis-tributes it to the temperate zones. In doing so, the forest and convection together cool Amazonia, while also providing a strong tropospheric heat source for the global atmospheric circulation. Thus, the release of latent heat is a large source of heating in the region and is responsible for the regional circulation characteristics in austral summer [Silva Dias et al., 1987]; it also can generate significant circulation anoma-lies in the Northern and Southern Hemispheres as telecon-nection patterns [Grimm and Silva Dias, 1995].

The complex interactions among climate variables in the Amazon basin have important implications for potential cli-mate change, both globally and locally. Because Amazonia is a data-sparse region, the climate variables are poorly quan-tified; significant uncertainties remain in our understanding of the different processes that underlie the dynamic mecha-nisms of the climate and its variability over a wide range of temporal and spatial scales.

Tropical convection is the main characteristic of climate over the Amazon basin. It is mainly modulated by large-scale atmospheric features including the Hadley circulation and the Intertropical Convergence Zone (ITCZ), the Walker circulation, the 40- to 60-day intraseasonal oscillation, and atmospheric waves, but also by meteorological processes such as the penetration of extratropical cold fronts [Santos de Oliveira and Nobre, 1986]. Also, the importance of land surface-atmosphere feedbacks on Amazonian hydroclima-tology cannot be overstated. Regional patterns of rainfall de-pend largely on the water and energy surface budgets driven by seasonal and diurnal cycles of solar heating, which in turn modulate the recycling of precipitation which can account for approximately 25–30% of rainfall in Amazonia [Elta­hir and Bras, 1996]. Important roles are also played by the sea surface temperature (SST) of the tropical Atlantic Ocean [Dickinson, 1987] on the eastern part of the basin and the forcing of the Andes along the western part.

Two of the most distinctive characteristics of the upper level atmospheric circulation in the Southern Hemisphere summer over the tropical South America are the well- defined anticyclone centered over Bolivia, the “Bolivian High” [Kreuels et al., 1975; Virji, 1981] and a trough near the coast of northeast Brazil [Kousky and Gan, 1981]. The South Atlantic Convergence Zone (SACZ) is another impor-tant feature of the summer circulation in the South America region. It is a wide and long convergence zone following a

northwest-southeast orientation from Amazonia to the sub-tropics near the coast of southeast Brazil, projecting into the adjacent South Atlantic Ocean [Kodama, 1992, 1993]. In the lower troposphere, the trade winds from the equa-torial Atlantic penetrate into Amazonia and then turn anti-clockwise east of the Andes Cordillera to flow southward and southeastward to 15°S, where the flow then becomes cyclonic in the central parts of the continent forming a low near 20°S.

All of the above mentioned characteristics of the observed circulation over tropical and subtropical South America dur-ing the austral summer form the South American Summer Monsoon (SASM). Many of these features that dominate the regional circulation are recognized as the typical monsoon characteristics [Zhou and Lau, 1998; Vera et al., 2006]. The SASM main features are best developed during the Decem-ber–February summer months and include a large-scale land-ocean temperature gradient, low pressure over the interior of the continent (Chaco Low) and high pressure (Bolivian High) with anticyclonic circulation aloft, a vertically over-turning circulation with a rising branch over the interior of the continent and sinking motion over the ocean, and intense moisture influx to the continent at low levels responsible for strong seasonal precipitation changes, as well as a moisture outflow from the Amazon region to La Plata basin, referred to as the South American low-level jet east of the Andes [Marengo et al., 2004].

2. SPATIAL DISTRIBUTIONS OF CLIMATIC VARIABLES

2.1. Temperature

At seasonal timescales, mean air temperature does not vary a great deal in most of the region with the exception of southern Amazonia (Rondônia, Mato Grosso). This behavior is due to the high values of incident solar radiation through-out the year. Mean air temperature values are between 24° to 26°C with annual amplitude of 1° to 2°C. In southern Ama-zonia, the annual cycle of temperature is more pronounced due to solar forcing and also due to the penetration of extra-tropical cold fronts. Here, the annual amplitude can reach 3° to 4°C. More important is the large amplitude in the diurnal cycle of temperature and solar heating for the triggering of convection and the development of intense storms over the region [Fu et al., 1999].

2.2. Precipitation

A large number of analyses of the spatial and temporal distribution of rainfall over Brazil and South America can be

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used to describe the main features of precipitation [Schwerdt­feger, 1976; Ratisbona, 1976; Caviedes, 1981; Salati, 1987; Horel et al., 1989; Figueroa and Nobre, 1990; Rao and Hada, 1990; Marengo, 1992, 1995; Rao et al., 1996; Marengo and Nobre, 2001; Marengo, 2005].

In the northern part of the basin, the spatial and seasonal rainfall distribution shows a significant heterogeneity. The southern part has distinct dry and wet seasons, with a maxi-mum of precipitation occurring in the austral summer. To-tal annual rainfall shows two maxima located, respectively, around the mouth of the Amazon River and over the western part of the basin. The maximum of annual rainfall, located over northwestern Amazonia with an annual total of over 3000 mm is associated with low-level convergence of east-erly moisture flow, likely a result of the presence and also to the concave shape of the Andes to the west of that maxi-mum [Nobre, 1983]. The high rainfall over this region may be understood as the response of the dynamical fluctuation of the center of quasi-permanent convection [Marengo and Hastenrath, 1993], in combination with the large amount of local evapotranspiration contributing to precipitation recy-cling. The second precipitation maximum located over the mouth of the Amazon River has been associated with the ITZC [Hastenrarth and Heller, 1977] and local circulations related to instability lines, which appear along the coast mainly during late afternoon forced by the sea breeze circu-lation [Cohen et al., 1995].

During the austral spring, precipitation increases over the Amazon basin, and a NW-SE band of precipitation devel-ops linking tropical convection in the west of the basin to precipitation activity in the extratropics. In the austral sum-mer season, there is a marked maximum centered at about 10°S. That maximum is prolonged to the SE to form SACZ. Most of tropical and subtropical South America receives more than 50% of its total annual rainfall in austral sum-mer [Figueroa and Nobre, 1990] in the form of convective rainfall with strong diurnal variation. In southern Amazonia, daily rainfall amounts are of the order of 10 mm d–1 on av-erage over vast regions, reaching over 30 mm d–1 in heavy rainfall episodes.

2.3. Winds and Geopotential Height

The distinctive aspect of the upper level circulation over tropical South America during the austral summer is the Bolivian High (Plate 1a). Its genesis is strongly related to latent heat released over the regions of high precipitation in Amazonia [Silva Dias et al., 1983; Figueroa et al., 1995; Seluchi et al., 1998, among many others]. Downstream of the Bolivian High to the east, there is an upper level trough off the east cost of northeast Brazil, associated with de-

scending motion and is part of the mechanism that leads to the very low rainfall over that region. Southern Amazonia and Bolivia Altiplano are strongly heated during the austral warm season resulting in enhanced tropospheric zonal tem-perature gradients and enhanced upper tropospheric meridi-onal flows in the vicinity of both coasts of South America. In addition, the mechanical blocking of the cross-Andes flow and lee genesis also contribute to strong pressure gradients on the east side of the Andes and lower level meridional flow, leading to strong episodes of low-level jets following strong zonal wind over the Andes [Campetella and Vera, 2002; Wang and Fu, 2004].

At the opposite extreme of the annual cycle during aus-tral winter (Plate 1b), upper level westerly flow extends un-impeded across South America. During both extremes of the annual cycle, easterly flow at 850 hPa extends into the Amazon basin and then turns toward the south as it ap-proaches the Andes mountains. These features of both the upper and lower level circulation over tropical South America, which are closely linked to the maximum precipi-tation over Amazonia and the SACZ, encompass the main characteristics of other tropical monsoonal circulations and are currently known as South American Monsoon System (SAMS) [e.g., Zhou and Lau, 1998; Marengo et al., 2004; Vera et al., 2006]. The typical low-level wind reversal toward prevailing westerlies of other monsoonal circula-tions is not seen in the SAMS because the Andes Cordill-era prevents that from happening. However, the reversal of meridional winds from southerly in winter to northerly in summer is similar to other monsoon systems, and it strongly influences moisture transport and distribution of the rainfall [Wang and Fu, 2002].

The main source of moisture over the Amazon basin is the tropical Atlantic Ocean through a persistent northeasterly flow most of the year. To the south, the flow turns south-ward, supplying moisture to the higher latitudes of South America. A large part of the southward moisture transport is carried out by a northerly low-level jet, with a maximal wind speed on the order of 15 m s–1 at 850 hPa, at approxi-mately 17°S and 62°W. This low-level jet is responsible for the transport of water vapor and heat from the Amazon to Paraguay, northern Argentina, and southern Brazil [Nogues­Paegle and Mo, 1997; Marengo et al., 2004].

The upper level circulation over South America dur-ing austral winter is characterized by weak winds over the tropics, while the subtropical westerly jet is stronger and located equatorward in comparison to its summer position, consistent with the descending branch of the Hadley-type circulation over that area. At lower levels, a northward- displaced near-equatorial low-pressure trough characterizes the circulation. A northward cross-equatorial flow turning

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clockwise is found over the tropical Atlantic Ocean. Near the surface, during wintertime, surges of cold high-latitude air, known locally as “friagens,” move across southeastern Brazil and Amazonia from the south, greatly modifying the atmospheric structure and climatic conditions. “Friagens”

can produce severe frost in areas of southern Brazil and sub-stantial cooling in the Amazon basin [Hamilton and Tarifa, 1978; Marengo et al., 1997a, 1997b]. The cold air heading these thrusts of high-latitude air may reach as far north as the equator. The events are relatively common in Amazonia

Plate 1. Upper level circulations (200 hPa) over tropical South America: (a) austral summer (December–February) and (b) austral winter (June–August).

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producing high convective nebulosity during fall and spring [Oliveira, 1986]. In winter, cold front penetration can pro-duce cooling in the region, especially southern and western Amazonia, and temperatures can drop to very low values, as during the strong polar outbreak of June 1994, where tem-peratures in Rio Branco, Acre, fell to 11°C.

2.4. Surface Pressure

The spatial distribution of the sea level pressure over the Amazonia is almost constant throughout the year, due to its tropical position. The maximum values are observed in win-tertime and the minimum in summertime. During this latter season, low values of pressure south of Amazonia extend to subtropical areas east of the Andes.

2.5. Solar Radiation and Cloudiness

Top-of-the-atmosphere solar radiation in Amazonia be-tween 5°N and 10°S varies from a maximum value of 36.7 MJ m–2 d–1 in December–January to a minimum value of 30.7 MJ m–2 d–1 in June–July [Salati and Marques, 1984]. At the surface, incident solar radiation is about 16–18 MJ m–2 d–1. The seasonal cycle of incident solar radiation at the surface in central Amazonia shows maximum values in September/October and minimum values in December/February [Culf et al., 1996]. This temporal distribution is controlled mostly by cloudiness related to Amazonian convection [Horel et al., 1989].

The outgoing longwave radiation (OLR), which describes upwelling infrared emission from the Earth, can also be used to estimate the depth of convective clouds. Low values of OLR denote areas of deep convection, abundant rainfall, and enhanced upper level divergence [Liebmann et al., 1998]. The Amazonia exhibits a strong annual cycle in OLR [Horel et al., 1989; Kousky, 1988], with a minimum during the rainy season of austral summer.

3. TIME EVOLUTION

3.1. Annual March of Temperatures and Precipitation

As mentioned above, the values of the mean air tempera-ture over Amazonia are between 24° and 26°C, and the yearly amplitude is between 1° to 2°C [Williams and Sátori, 2004], using climatological data of mean air temperature from INMET [1992], it was found that the annual cycle is predominant, and only few stations within 5° of the equator showed double peak in the annual march of temperature.

The annual march of rainfall shows also only one peak [Hsu and Wallace, 1976; Figueroa and Nobre, 1990; Obregón,

2003], except for some elevated zones in the Andes. The rainy season over most of the Amazon basin located in the SH is between November and March, with a peak in December– February (DJF), and the dry season is from May to Septem-ber. On the northern portions of the basin, there is a reversal of this phase, whereby the rainy season occurs from May through October and the driest period from December through February. These results have large-scale manifestations in the seasonal records of river discharge with a single broad an-nual maximum for the Amazon River [Richey et al., 1989; Amerasekera et al., 1997]. Hydroclimatological feedbacks between the Andes and the Amazon River basin are discussed by Poveda et al. [2006]. Also, Vizy and Cook [2007] present evidence of the strong coupling of rainfall between the Andes and Amazonia ever since the Last Glacial Maximum (LGM).

3.2. Onset of the Rainy Season and of the South America Monsoon System

The transition between wet and dry seasons is short in Amazonia. The onset of the wet season occurs normally within the period of a single month. The transition from the wet to the dry season takes longer than a month. The onset of the rainy season in most of the Amazon basin, which is closely associated to the establishment of the South Ameri-can Monsoon System, occurs as a rapid shift of the area of intense convection between the northwestern extreme of the continent and latitudes south of the equator, around mid- October [Kousky, 1988; Horel et al., 1989; Vera et al., 2006; Marengo et al., 2001; Liebman and Marengo, 2001]. The demise of the SAMS occurs typically from April to May.

In the Amazon basin, Marengo et al. [2001], using pentad averages of gauge-based rainfall observations, did not find a large wind signal related to the onset, which suggests that the onset is controlled by large-scale thermodynamic condi-tions, especially in the southern Amazonia [Fu et al., 1999]. They did, however, find relationships between tropical SST and the onset and end of the rainy season in central Amazo-nia and near the mouth of the Amazon River. SST anoma-lies were not found to be related to the timing of the onset in southern Amazonia. Both of these findings are consistent with the arguments of Fu et al. [1999] that, near the equa-tor, SST influence on onset may be important because the contrast between land and sea temperatures is small. On the other hand, the relationship between tropical Pacific and At-lantic SSTs and rainfall confined to the equatorial region of Brazilian Amazonia are found during the transition season between wet and dry regimes or, entirely, within the dry sea-son [Liebmann and Marengo, 2001]. This argument implies that the SST seems to control the seasonal totals through the timing of the rainy season onset or end.

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In addition to the influence of tropical SST, Fu and Li [2004] suggest that the land surface dryness, represented by the Bowen ratio, during the dry season appears to be a main cause of the strong delay in subsequent wet season onset. This is because an increase of evapotranspiration during late dry season, associated with the seasonal increase of leaf area index or greenness of the rainforest is critical for initiating the transition from dry to wet season. Finally, changes of fre-quency and intensity of the “friagens” during the transition season can also affect the onset dates [Li and Fu, 2006].

Over much of tropical and subtropical South America, more than 50% of the annual precipitation falls during the summer months, associated with the establishment of the SASM. As a monsoon system, the SASM is dynamically and geographically different from the maritime ITCZ, although the latter is sometimes erroneously invoked to explain the seasonal march of precipitation over the South American continent [Vuille and Werner, 2005]. On interannual and longer timescales, summer precipitation shows significant variations in intensity and spatial extent, which are still not very well understood. This variability is caused by a number of factors influencing the SASM during both the developing and mature stage, including tropical Atlantic SST [Mechoso et al., 1990; Hastenrath and Greischar, 1993; Marengo and Hastenrath, 1993; Vuille et al., 2000a], the El Niño–Southern Oscillation (ENSO) [Aceituno, 1988; Vuille, 1999; Garreaud and Aceituno, 2001; Paegle and Mo, 2002; Grimm, 2003, 2004; Lau and Zhou, 2003], land surface conditions such as soil moisture or vegetation cover through their influence on precipitation [e.g., Poveda et al., 2001; Oyama and Nobre, 2003; Koster et al., 2004; Poveda and Salazar, 2004; Xue et al., 2006], which, in turn, can feed back onto the ocean-at-mosphere dynamics of the tropical Atlantic Ocean [Poveda and Mesa, 1997; Wang and Fu, 2007] and interactions with the extratropical circulation [e.g., Garreaud and Wallace, 1998; Seluchi and Marengo, 2000; Chou and Neelin, 2001; Marengo et al., 2004]. The relative importance of the vari-ous factors contributing to SASM variability, however, is often difficult to determine, as many components such as tropical Pacific and Atlantic SST are dynamically coupled with each other [Enfield, 1996; Uvo et al., 1998; Vuille et al., 2000b; Pezzi and Cavalcanti, 2001; Giannini et al., 2001; Ronchail et al., 2002].

4. MECHANISMS OF AMAZONIAN CLIMATE VARIABILITY

4.1. Solar Forcing of the Seasonal Climate

While the annual cycle of solar radiation is undoubtedly the key factor in forcing convection in this region, there re-

main gaps in our understanding of the spatial variability of rainfall within the Amazon basin itself.

The impact of local insolation on precipitation can be explained by balancing net energy input at the top of the atmospheric column with the export of energy by the diver-gent circulation that accompanies convection [Biasutti et al., 2004]. Also, increased insolation reduces the stability of the atmosphere in the convection centers, but not in mon-soon regions. The annual cycle can be thought of as being forced locally by the direct action of the sun and remotely by circulations forced by regions of persistent precipitation organized primarily by SST and, secondarily, by land pro-cesses [Biasutti et al., 2003]. The annual cycle dominates convection over Amazonia [Horel et al., 1989], with con-vection to a first approximation following (or lagging) solar insolation.

4.2. Interannual Climate Variability

Although the annual cycle dominates the climate variabil-ity over Amazonia, interannual variability is quite remark-able, as revealed by historical records of the Amazonian rivers [Molion and de Moraes, 1987; Richey et al., 1989; Marengo, 1992, 1995; Guyot et al., 1998; Marengo et al., 1998] showing that interannual variability of precipitation in Amazonia is very significant, which is dynamically linked with consistent anomalies in the whole set of variables of the surface water and energy balances over the Amazon basin. Most studies of interannual variability of Amazonian rain-fall have focused attention on anomalies associated with the ENSO phenomenon [Aceituno, 1988; Ropelewski and Halp­ert, 1987, 1989; Rao and Hada, 1990; Figueroa and Nobre, 1990; Obregón and Nobre, 1990; Marengo, 1992, 1995; Marengo and Hastenrath, 1993; Rao et al., 1996; Poveda and Mesa, 1997; Marengo and Nobre, 2001; Fu et al., 2001; Poveda and Salazar, 2004; Poveda et al., 2006]. The role of land surface-atmosphere interactions and, particularly, that of soil moisture and evapotranspiration have been proposed as possible dynamical mechanisms for the unexplained vari-ance of hydroclimatological processes at annual and interan-nual timescales over Amazonia [Poveda and Mesa, 1997; Makarieva and Gorshkov, 2007].

Precipitation reduction in tropical South America dur-ing El Niño is also consistent with the development of an anomalous position and direction of the Hadley cell over the equatorial region. Implicit here is the existence of a positive feedback effect between the tropical precipitation and the Hadley circulation [Numaguti, 1993; Kiehl, 1994]. Nega-tive anomalies in tropical South American precipitation during El Niño are also associated with negative anomalies in soil moisture at interannual timescales [Nepstad et al.,

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2004; Jipp et al., 1998; Poveda and Mesa, 1997; Poveda et al., 2001]. The work by Poveda et al. [2006] discusses the whole suite of hydroclimatic anomalies associated with the occurrence of El Niño in tropical South America, including Amazonia.

4.3. Intraseasonal Variability

Over tropical South America, the principal mode of cli-matic fluctuations in the intra-annual spectral band, is in the 30- to 60-day oscillation, as manifested in the OLR anom-aly patterns at 250 hPa for the period 1979–1990 [Mo and Kousky, 1993].

4.4. Tropical Pacific and Atlantic Ocean

The correlation between precipitation in Brazilian Ama-zonia and SSTs over the Pacific and Atlantic has been doc-umented since the early 20th century. The impact of each ocean on variability (frequency and intensity) of the wet/dry season over the Amazon basin and the underlying mecha-nisms are gradually being clarified. However, at the outset, it must be emphasized that the combined tropical Pacific and Atlantic SST variability explains little more than 50% of interannual precipitation variance over Amazonia and not much is known about other mechanisms, internal or external to the region, responsible for the remaining unexplained in-terannual variability.

The influence of the tropical Pacific is mainly through perturbations of a Walker-like cell mechanism. El Niño epi-sodes with warm Equatorial Pacific SSTs are associated with a weakening of the cell, with subsidence and reduced cloudi-ness and rainfall over northern, central, and eastern Amazonia. A detailed discussion of mechanisms acting on hydroclima-tological anomalies over the region during El Niño is given by Poveda et al. [2006; see their Figure 8]. During La Niña, generally opposite conditions prevail, and rainfall and river discharge are above the average. ENSO could also influence rainfall in southeastern Amazonia through an anomalous atmospheric wave train from the south tropical Pacific to subtropical South America forced by SST and atmospheric heating anomalies in the South tropical Pacific [Kalney et al., 1986; Liebmann et al., 1999; Fu et al., 2001].

Observational, conceptual, and atmospheric and coupled atmospheric-ocean models show evidence that the tropical Atlantic strongly influences interannual climate variability of the Americas [Hastenrath and Heller, 1977; Moura and Shukla, 1981; Hastenrath et al., 1984; Hastenrath, 1990; No­bre and Shukla, 1996]. Although the role of the tropical Pacific has been emphasized in studies of the association between SST and Amazonian rainfall, the Atlantic Ocean strongly

influences rainfall. Marengo [1992] and Rao et al. [1996] showed that increased rainfall in the Amazon basin is associ-ated with an increase of water vapor transport from the Atlan-tic. In particular, the eastern region of Amazonia is strongly influenced by the atmospheric and oceanic condition of the tropical Atlantic [Molion, 1993; Nobre and Shukla, 1996].

The influence of the tropical Atlantic SSTs over Amazo-nian rainfall is associated with Hadley-like cell perturba-tions. Positive rainfall anomalies in northern Amazonia are concomitant with: (a) anomalously warm waters in the tropi-cal North Atlantic, (b) cold surface waters in the equatorial South Atlantic, (c) weak northeast trades, which entails a reduced influx of moisture coming from the Atlantic toward the Amazon basin. Consequently, the ITCZ is located anom-alously to the north of its average position.

5. MECHANISMS CONTROLLING DROUGHTS IN AMAZONIA: PACIFIC VERSUS

ATLANTIC CONTROLS

As mentioned above, droughts in Amazonia are usually associated with El Niño or warming in the tropical North At-lantic, and El Niño-caused droughts are most pronounced in the central and northern areas of the basin, as in 1926, 1983, and 1998. Previous studies [Poveda and Mesa, 1997; Marengo et al., 1998, 2008a, 2008b; Ronchail et al., 2002; Poveda and Salazar, 2004, among many others] have identified negative rainfall anomalies in Amazonia associated with ENSO events and to SST anomalies in the tropical Atlantic as well. The studies have linked some of the major droughts in Amazonia to (a) the occurrence of intense El Niño events, (b) strong warming in the surface waters of the tropical North Atlantic during the Northern Hemisphere summer-autumn season, or (c) both. Very intense El Niño events have been associated with the extreme droughts in 1925–1926, 1982–1983, and 1997–1998, and the last two also experienced intense warm-ing in the tropical North Atlantic along with warming in the equatorial Pacific. There is evidence of extensive droughts, and perhaps widespread fires, linked to paleo-ENSO events occurring in the Amazon basin in 1,500, 1000, 700, and 400 B.P., and these events might have been substantially more severe than the 1982–1983 and 1997–1998 ones [Meggers, 1994]. The best documented case of an earlier drought event in Amazonia linked to El Niño event was during 1925–1926 [Sternberg, 1987; Williams et al., 2005]. Rainfall in central-northern Brazilian Amazonia and southern Venezuela in 1926 was about 50% lower than normal.

Contrary to the above droughts, the droughts of 2005 as well as those in 1963–1964 and in 1979–1981 did not occur associated with El Niño events. While several studies analyze the droughts of 1982–1983 [e.g., Marengo et al., 1998], and

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1997–1998 [e.g., Nepstad et al., 1999] and 2005 [Marengo et al., 2008a, 2008b; Zeng et al., 2008] and their impacts on climate, hydrology, and fires in Amazonia, there are only casual references to the drought event of 1963–1964. The combined effects of tropical Pacific and Atlantic SSTs ex-plain 53% of the Amazon basin rainfall variability, with comparable contribution from the Pacific and the Atlantic [Uvo et al., 1998], suggesting that the effect of other sources of variability, such as land surface processes and variability of frequency of transients from the South Atlantic may be also important in the interannual rainfall variability in the region. For example, these processes could contribute to the interannual rainfall anomalies through changing wet season onset dates [Fu and Li, 2004; Li and Fu, 2006].

5.1. Ocean Modulation of Coastal Climate

Sea-breeze circulations and squall lines that propagate over Amazonia from the Atlantic coast constitute a complex system where scale interactions range from the large-scale environmental characteristics, to the mesoscale and cloud-scale circulations. Such systems are the key rain-producing mechanisms explaining the precipitation maximum over the Atlantic coast. At the mesoscale, the propagating and non-propagating squall lines are basically initiated by the sea breeze circulation [Kousky, 1980], while the cloud-scale circulations maintain the squall-line propagation in a quasi-steady state. For example, the mesoscale circulation associ-ated with the maritime breeze may organize the convection into coastal squall lines, which are responsible for a signifi-cant portion of the precipitation over the eastern Amazonia [Garstang et al., 1994; Cohen et al., 1995]. The interaction between the large-scale circulation and the maritime breeze circulation also determines the time of day for the precipi-tation events on the northeastern coast of South America [Kousky, 1980; Negri et al., 2000].

5.2. Atmospheric and Oceanic Controls of the SACZ

The SACZ connects atmospheric processes at low lati-tudes with those of the subtropics and mid-latitudes and, thus, is a mechanism allowing variability due to mid-latitude internal atmospheric dynamics to influence directly Amazo-nian climate variability and vice versa. Tropical convection is a key factor on both the onset and the maintenance of the SACZ through the latent heat release in the Amazon region [Kodama, 1992; Liebmann et al., 1999]. Several studies have analyzed the influence of the ocean surface temperature anomalies on SACZ variability. Barros et al. [2000] indi-cated that warm (cold) SST in the region between 20–40ºS and west of 30ºW is followed by a southerly (northerly)

displacement of the SACZ. According to Robertson and Mechoso [2000], interannual SACZ variability is accom-panied by SST anomalies with atmospheric forcing in the southwest Atlantic, with a dipole structure at about 40ºS. This variability is independent of the ENSO. Doyle and Barros [2002] suggested a positive feedback in the interan-nual scale between positive (negative) SST anomalies in the western part of the subtropical Atlantic and weak (intense) intensity of the SACZ that intensifies the SACZ low-level circulation. To understand the coupling between the SACZ and the South Atlantic, Chaves and Nobre [2004] carried out a series of experiments with atmospheric and ocean models. These results suggest that negative SST anomalies, gener-ally observed in the SACZ region, represent a response of the ocean to the atmospheric forcing.

5.3. Decadal and Interdecadal Variability: Relationship to PDO and NAO

Historical records of rainfall in Amazonia show decadal and interdecadal variability [Dias de Paiva and Clarke, 1995; Chu et al., 1995; Zhou and Lau, 2001; Matsuyama et al., 2002; Marengo, 2004; Botta et al., 2002; Chen et al., 2003]. At the regional scale, a slight decreasing trend is observed over northern Amazonia, while the southern part presents a positive trend, and apparent climate shifts were identified around the middle of the decades of the 1940s and 1970s [Marengo, 2004]. After 1975, the northern/southern part of Amazonia shows relatively less/more rainfall when it is compared with the former period [Obregón and Nobre, 2003, Marengo, 2004].

These climate shifts are related to the change in both at-mospheric and oceanic circulation over the North Pacific which took place in 1975–1976 associated with the Pacific Decadal Oscillation (PDO). During the period after 1975, apparently associated with the positive phase of PDO, there was less rainfall over northern Amazonia linked to more fre-quent and intense El Niños (1982–1983, 1986–1987, 1990–1991, 1997–1998).

Another important potential mechanism of long-term cli-mate variability in Amazonia is the North Atlantic Oscilla-tion (NAO). Annual variation of rainfall seems to be linked, at least indirectly, to the NAO because strong Atlantic trades bringing moisture into Amazonia are associated with south-ward-displaced ITCZ, which is in turn related to an anoma-lous distribution of Atlantic SST [Moura and Shukla, 1981; Nobre and Shukla, 1996]. Rajagopalan et al. [1999] present statistical evidence that SSTs in the subtropical South At-lantic are associated with variations in the NAO. This con-nection might work via the impact of South Atlantic SSTs on Amazonian rainfall; the latter influencing the NAO via

NOBRE ET AL. 157

atmospheric teleconnections or changes in the Hadley cell [Robertson et al., 2000]. There are some observational evi-dence linking Amazon River discharge and NAO index. The correlations are highest at interannual (5–6 years) and inter-decadal timescales, and the NAO leads river discharge by about 9 months [Obregón and Nobre, 2004].

Considering that deforested areas in Amazonia are located mostly over the southern portions of the basin and that this region presents a long-term, positive trend in rainfall, it is possible to conjecture that this trend is more likely to be as-sociated with the interdecadal variability related to the PDO, to a lesser extent to the NAO and less to deforestation.

5.4. Possible Influence of Biomass Burning Aerosols on the Monsoon Transition

In addition to a large number of studies addressing the influences of biomass burning aerosols on surface radia-tion budget, clouds and rainfall [e.g., Artaxo et al., 2002; Andreae et al., 2004], summarized in chapters B5 and B6 of this book, several recent studies have explored the impacts of biomass burning aerosols on the climate variability of the monsoon transition. For example, Zhang et al. [2008] and Liu [2005] have suggested that the radiative effect, including both direct and semidirect effects, of the biomass burning aerosols could delay and weaken the transition to the sum-mer monsoon circulation based on regional climate model simulations forced by aerosol radiative forcing estimated from observations from the Smoke Aerosols, Clouds, Rain-fall and Climate (SMOCC) field campaign and the Moderate Resolution Imaging Spectroradiometer (MODIS), respec-tively. An analysis of the MODIS aerosols and cloud data by Yu et al. [2007] also suggests that the aerosol-cloud relation-ship changes interannually with climate condition, such that it could amplify or reinforce the original climate anomalies. In particular, during an anomalously dry transition season, warm cloud fraction decreases with aerosol optical depth. Whereas in a normal and relatively wet transition season, warm cloud fraction increases with aerosol optical depth. Although whether these aforementioned results could be generalized remains unclear, they nevertheless suggest a po-tentially significant impact of biomass burning aerosols on climate variabilities of the monsoon circulation transition.

6. PALEOCLIMATE IN AMAZONIA DURING THE LAST GLACIAL MAxIMUM: WAS THE AMAZON

DRIER DURING THE LGM?

The short duration of instrumental records and the pau-city of proxy records, means that little is known about either the character or the causes of longer (millennial to orbital)

timescale variations in tropical climate and their possible global teleconnections. There is, however, a small variety of paleoclimatic time series available for tropical South Amer-ica. Many of these exist for the Altiplano of Bolivia, Peru, and Colombia, where the most important archives include sediment cores from fluvial deposits, lacustrine sediments and pollen, and salar (salt flat) environments, as well as ice cores from tropical glaciers. These records reveal large- amplitude climate changes with a range of periodicities.

Based on the postulate that the ocean-atmosphere interac-tions that influence modern interannual climatic variability in tropical South America also influenced climatic variability on millennial and orbital timescales, climate-sensitive tropical ecosystems can provide important information that may help us to fill the gaps in our knowledge concerning the evolution of rainforests during periods of full glaciation. Small changes in precipitation in the Amazon basin have immediate conse-quences for the survival of the Andean cloud forest because its dominant source of moisture today is the Atlantic Ocean.

Research on plant communities [Colinvaux et al. 2000] and pollen records from the Amazon fan [Haberle and Mas­lin, 1999] have concluded that Amazonia was drier during the LGM. Mourguiart and Ledru [2003], studying a 40,000-year lacustrine record from the Eastern Cordillera in Bolivia in an endemic species-rich and ecologically threatened region found a dry LGM, indicating a drastic decrease of the Ama-zonian moisture source. To explain this aridity, they infer steep temperature gradients between the pole and equator in both hemispheres that would have reduced considerably the size and displacement of the ITCZ and the austral summer precipitation. This major change in water supply induced a dramatic reduction in species diversity and suggests that the Andean cloud forest did not provide refugia for tropical low-land taxa during full glacial times.

In contrast, sedimentary records of Lake Titicaca reveal that the Altiplano was wetter during the LGM than in mod-ern climate [Baker et al., 2001] and that has led to the in-ference that the Amazon was also wetter. They make use of three physical mechanisms to explain LGM wetness in Amazonia. First, wet season insolation was at a maximum in the southern tropics at 20,000 years B.P. [Baker et al., 2001]; thus, the SASM was maximized. Second, during the LGM, zonal (cold in the east [Bard et al., 2000], warm in the west [Rühlermann et al., 1999]) and meridional (cold in the north, warm in the south [Mix et al., 1999]) SST gradients in the equatorial Atlantic were favorable for enhanced SST forcing of the northeast trades and atmospheric advection of water vapor into Amazonia [Baker et al., 2001]. Third, lower equatorial Atlantic SST favored increased gradients between land and sea-surface temperature during the austral summer, also enhancing water vapor transport into Amazonia.

158 CHARACTERISTICS OF AMAZONIAN CLIMATE

There are still many unresolved issues to determine whether Amazonia was drier and may have seen a decrease in the areas covered by forests, or wetter and fully covered by forests. One fundamental question, of course, is to see whether paleoclimatic data from the Altiplano is relevant to reveal the paleoclimate of Amazonia. It is necessary to believe that large-amplitude precipitation changes are syn-chronous and of the same sign for the Altiplano and Ama-zonia [Baker et al., 2001]. For instance, it is possible that the LGM was wetter than today in Amazonia, whereas most recent studies have concluded that Amazonia (and even the global tropics) were drier [Betancourt et al., 2000; Argollo and Mourguiart, 2000; Heine, 2000; Thompson et al., 2002]. Whereas such millennial variability of the North Atlantic re-gion is synchronous with millennial changes in tropical South America [Baker et al., 2001], recent studies have concluded that the Younger Dryas, for example, was only a northern hemisphere event [Bennett et al., 2000]. Interestingly, a re-cent study concluded just the opposite: that Amazonia was drier during the Younger Dryas [Maslin et al., 2000]. To add more complexity to the supposed persistent relationship be-tween Atlantic SST anomaly patterns and rainfall variability over the Altiplano, a detailed modern climatological study [Vuille et al., 2000b] concluded that rainfall variability in the central Andes is not correlated with Atlantic SSTs.

Recently, the climate of the LGM over South America with-out remote influences was simulated using a Regional Climate Model [Cook and Vizy, 2006]. Results showed that in the Am-azon basin during the LGM, the calculated rainfall was 25–35% lower than in the present day simulations. In that model simulation, the 2- to 4-month delay in the onset of the rainy season was due to a drier low-level inflow from the Atlantic onto South America in comparison to present day climate.

We cannot overlook the role of deforestation on Amazo-nian climate or the diverse roles of land use/land cover change on precipitation [Pielke et al., 2007]. Most of these studies find that deforestation leads to significant reduction in rainfall over Amazonia [Werth and Avissar, 2002; Oyama and Nobre, 2003; Sampaio et al., 2007; Sampaio, 2008]. All these land use changes would weaken the hydrological cycle in Amazo-nia and globally. The detailed analysis of Amazonian defor-estation and climate is presented in chapter B7 of this book.

Acknowledgment. The contribution of one of the authors (G. Poveda) is part of the GRECIA Program, funded by COLCIEN-CIAS of Colombia.

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