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A well known atmospheric low frequency variability is the so-called El Niño-Southern Oscillation - ENSO, which is considered one of the most prominent sources of interannual variations in weather and climate around of the world. The major atmospheric and oceanic features associated with El Niño episodes are: predominance of positive anomalies of sea surface temperature (SST), weakness of the trade winds in the surface and low pressure with deep convection on the oriental Pacific and high pressure with subsidence movement on the western Pacific, Indonesia and Australia. La Niña events generally feature reversed atmospheric and oceanic patterns. South America is one of the continental areas around the world that is directly influenced by the ENSO cycle where several studies have documented the ENSO impacts (mainly the El Niño events) in the South American rainfall. Findings of these works indicate, in general, that the main areas of South America influenced by ENSO are located at the sections West (Peru and Ecuador), North and Northeast (Amazonian and Brazilian Northeast) and South-Southeast (Southern Brazil, Uruguay and Argentina). In analyses focused on southern Brazil and Southern South America (SSA), some results showed that the impact on rainfall in Summer is much weaker than in Spring. Other studies indicate that ENSO may have considerable signal in the interannual variability of the precipitation over La Plata basin. This signal varies along each of the ENSO phases, but is particularly strong during the Spring. Studies focused in the response to ENSO in smaller areas within La Plata basin show no evidence of a signal in rainfall during midsummer, but in late Summer and Autumn there is again a strong correlation between SSTs at El Niño regions 3 and 3.4, and Outgoing Longwave Radiation (OLR) over the Upper and Middle Paraná. The goal of this chapter is to offer a general view of the ENSO impact on the South America and particularly over the La Plata basin. 35 CHAPTER INTERANNUAL LOW FREQUENCY VARIABILITY: BACKGROUND ABSTRACT Tércio Ambrizzi 1 1 University of São Paulo, Brasil. III
3.1. Introduction Interannual low frequency variability: Background Most of the annual total rainfall observed over South America usually occurs during the austral Summer, December to February (DJF) and Autumn, March to May (MAM)...The large and synoptic meteorological systems that modulate the rainfall in the Summer are linked to the performance of the South Atlantic Convergence Zone (SACZ) (Casarin and Kousky, 1986; Figueroa et al., 1995; Nogués-Paegle and Mo, 1997), the Bolivian High and the upper tropospheric cyclonic vortices (Virji 1981; Kousky and Gan 1981; Kayano et al. 1997). The variability of precipitation in the subtropical plains of South America is also closely tied to the variability of the South American low level jet (SALLJ) that is a long, narrow, low level northerly wind current that flows to the East of the Andes Mountains year-round (Nogues-Paegle and Mo 1997; Saulo et al. 2000). The SALLJ supplies the warm, moist tropical air that fuel convection and precipitation in the subtropical plains of South America, being modulated by the El Niño- Southern Oscillation on interannual time-scales (Zhou and Lau 2001), frontal passages and SACZ on submonthly timescales, and boundary layer dynamics on diurnal timescales. In the subsequent period, MAM, the rainy season is located on the centre-East of the Amazonian and Northeast of Brazil, which is modulated by the migration to the South of Equator of Inter Tropical Convergence Zone - ITCZ (Hastenrath and Heller 1977; Moura and Shukla 1981; Nobre and Shukla 1996, Souza et al. 1998a). The so-called El Niño-Southern Oscillation - ENSO, is considered one of the most prominent sources of interannual variations in weather and climate around of the world (Trenberth and Caron 2000). ENSO, a phenomenon of planetary scale related to a strong and complex ocean-atmosphere coupling over the tropical Pacific basin (Cane 1992), has a cycle, with the El Niño (warm phase) manifesting in one extreme phase and the La Niña (cold phase) in the opposite extreme. The major atmospheric and oceanic features associated with El Niño episodes are: predominance of positive anomalies of sea surface temperature (SST), weakness of the trade winds in the surface and low pressure with deep convection on the oriental Pacific and high pressure with subsidence movement on the western Pacific, Indonesia and Australia. La Niña events generally feature reversed atmospheric and oceanic patterns (Kousky and Ropelewski 1989). These anomalous patterns occur over the tropical Pacific basin, including an extensive spatial area of the tropics (more than a third of the tropical belt around of the globe). A schematic view of these features is given by figure 4.1, where the ocean-atmosphere changes can be observed in a 3-D picture during a typical event of El Niño (Fig.3.1a), La Niña (Fig.3.1b) and a normal year (Fig.3.1c). Hence, ENSO triggers off changes in the general circulation of the atmosphere, resulting in climatic impacts in several continental areas located in the trop- 36
Page 1 and 2: CLIMATE CHANGE IN THE LA PLATA BASI
Page 3 and 4: INDEX FOREWORD . . . . . . . . . .
Page 5 and 6: 7.3. Methodology . . . . . . . . .
Page 7 and 8: 13.3. Regional validation of GCM fo
Page 9 and 10: 1.1. Dropping the hypothesis of sta
Page 11 and 12: Introduction as they pour water ove
Page 13 and 14: A better understanding of the obser
Page 15 and 16: References Introduction Barros, V.
Page 17 and 18: ABSTRACT The hydrologic cycle of th
Page 19 and 20: La Plata basin climatology Within t
Page 21 and 22: in this last description, although
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Page 25 and 26: La Plata basin climatology b. Upper
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Page 39 and 40: Interannual low frequency variabili
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Page 47 and 48: SUB-BASIN COUNTRY Argentina Bolivia
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Page 51 and 52: Physiography and Hydrology The Para
Page 53 and 54: Physiography and Hydrology The Pant
Page 55 and 56: Physiography and Hydrology forcings
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the central area of the high basin
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7.1. Introduction Several authors,
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Real evaporation trends a) PET > Pi
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a) b) c) Mean Temperature Real evap
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The positive trends of the mean tem
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d) e) f) Mean Temperature Real evap
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7.5. Discussion and final comments
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ABSTRACT The water, as resource, is
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puts in risk large number of person
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The main services and problems of w
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There are similar problems in diffe
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8.3.3. Energy a. Hydroelectric depe
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(chapters 12 and 13), because of th
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The main services and problems of w
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9.1. Introduction The emissions of
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9.4. Greenhouse effect The atmosphe
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Temperature anomalies (°C) 0.8 0.6
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Global climatic change at a global
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In view of the unavoidability of th
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Background on other regional aspect
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Considering the non-linear interact
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SST anomalies also have influence o
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ABSTRACT The purpose of this chapte
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Global climate models Mid-1970’s
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Global climate models Table 11.1. B
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Global climate models Table 11.2. M
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Although changes in weather and cli
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Socio-economic variables and also s
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Applicability in impact assessments
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of the global climate system to the
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Climate scenarios B2: Assumes a wor
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Climate scenarios The reliability o
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Climate scenarios From Table 12.2 i
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Climate scenarios Fig. 12.5. The mu
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Climate scenarios this would be rel
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Results of global circulation model
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13.2.3. Statistical downscaling The
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La Plata basin are also considerabl
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10 N EQ 10 S 20 S 30 S 40 S 50 S 10
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Regional climatic scenarios Fig. 13
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a. Pre-industrial experiment: Initi
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Regional climatic scenarios Kalnay,
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Low-frequency variability 14.1. Bac
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egime occurred during the periods o
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Fig. 14.2. As Fig. 14.1, except for
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14.4. SST composites Low-frequency
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Low-frequency variability during th
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Fig. 14.10. As Fig. 14.9, except fo
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Low-frequency variability In genera
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Low-frequency variability Mantua, N
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15.1. Introduction Statistical anal
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Statistical analysis of extreme eve
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CURRÍCULA OF THE AUTHORS Vicente B
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Curricula of the autors Rubén Mari
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Proyect: SGP II 057: “Trends in t
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