CHAPTER 3C SATYAMURTY, NOBRE, AND SILVA DIAS Chapter 3C South America PRAKKI SATYAMURTY AND CARLOS A. NOBRE Centro de Previsiio de Tempo e Estudos Climdticos, Instituto Nacional de Pesquisas Espaciais, Siio Paulo, Brazil PEDRO L. SILVA DIAS Department de Ciencias Atmosfericas, Instituto Astronomico e Geofisico, Universidade de Sao Paulo, Sao Paulo, Brazil 3C.1. Introduction 3C.2. Regional circulation features Extending meridionally from WON to 6O o S, South America (SA) presents features of tropical, subtropical, and extratropical weather and climate. This continent lies between the two large oceans, the Pacific and the Atlantic, and as such there is a great influence of these oceans on the meteorology of this landmass. The regional circulation characteristics over SA can only be appreciated by referring to the effects of these two oceans. An important and distinct geographical feature of the continent is the presence of a steep and narrow mountain range extending all the way from the northern tip to the southern tip along the west coast. Another important feature is the tropical Amazon jungle, occupying about 35% of the total continental area and 65% of the tropical area. With one of the world's most humid climates, this tropical forest makes the continent very unique. This region also contains some of the infamous deserts and arid regions, such as the Atacama desert in northern Chile and northeastern Brazil (NEB). The regional atmospheric circulation over SA presents many interesting characteristics, such as the Bolivian high, the SACZ, the Chaco low in summer, and the cold surges into the equatorial region known as friagens in winter. The tropical eastern South Pacific and the tropical South Atlantic adjoining this continent are the only tropical oceans free of tropical storms such as hurricanes and typhoons. In the tropical Atlantic Ocean, the ITCZ presents a west-southwest to east-northeast stretching with a reasonably wide meridional seasonal migration at its southwestern end. South America is also a region of strong meridional interaction between the Tropics and the extratropics. South America's tropics and subtropics suffer a large direct influence of the El Nmo (EN) and the Southern Oscillation (SO) phenomena. Early this century, Sir Gilbert Walker found a very significant correlation between SO and the Ceara (Brazil) rainfall, which has led to the investigation of the effects of global circulation anomalies in the Tropics over the climatic variability in many regions of the world. More will be said later about the impact of ENSO on SA. An attempt is made to give a broad description of the South American regional circulation features and their role in the meteorology and climatology of the continent. Large seasonal changes are seen in the overall characteristics of the regional circulation over South American tropics and subtropics. They are presented with the aid of the climatological (30-yr means of the period 1961-90) monthly charts of the MSLP, the 850-hPa vector winds, the OLR, and the 200-hPa vector winds for January, April, July, and October, representing the four austral seasons-summer, fall, winter, and spring-respectively. These charts are obtained from NCEP (V. Kousky 1996, personal communication). The MSLP for the four austral seasons is shown in Fig. 3C.l. The subtropical high pressure centers in the eastern South Pacific and the South Atlantic Oceans lie on either side of SA, and relative low pressures prevail over the continent in all seasons. The MSLP over the continent is lowest in summer (January) and highest in winter (July). The Pacific high is stronger in summer and the Atlantic high is stronger in winter, and they are about the same intensity in the transition seasons. The two highs are farthest from the continent in summer, when most of the tropics and sUbtropics of South America have their rainy season. It can also be seen over the continent that there is a general low pressure belt around the Equator, known as the equatorial trough, which merges with the ITCZ over the oceans. The position and intensity of this trough has a strong bearing on the rainy season in the northern Amazon, Guyanas, Venezuela, and Colombia. The subtropical high in the Atlantic migrates from 15°W, 27°S in August to 5°W, 33°S in February and, in association with the southward migration of the ITCZ in February to May, determines the rainy season in the adjacent NEB region (Hastenrath 1991). An important feature in the South Atlantic is that the subtropical high in summer is weaker than in 119 D. J. Karoly et al. (eds.), Meteorology of the Southern Hemisphere © American Meteorological Society 1998 120 METEOROLOGICAL MONOGRAPHS SLP JAN (a) (b) -C"' VOL. 27, No. 49 SLP APR 1l1li : lOS ............ . EQ . 1011 IOSt--,---.--: 20S 1 - - - -__ JOS SLP JUL (c) (d) SLP OCT EO lOS 20S JOS ~.:-;c 4OS~ IOfIOW lOOW ,OW I10W 7r1fI &OW FIG. 50W lOW 3C.1. MSLP (hPa) distribution for January, April, July, and October. winter, while all other subtropical highs, in general, are stronger in summer. As the subtropical high in the South Atlantic migrates southward in early summer, convection develops along the SACZ (described later in this section), intruding into the high. As a result, the subtropical high in summer is frequently split, and in the average it is weaker. The lower tropospheric mean circulation for the four austral seasons are shown in Fig. 3C.2. In general the trade winds, coming from the Atlantic region north of the sUbtropical high pressure center, flow as far west as the Andes in Colombia and Peru, where they are blocked by the steep and high topography and tum gradually northerly and northwesterly east of the