Ana Graciela Ulke1, Karla Maria Longo2 and Saulo Ribeiro de Freitas3

1Departamento de Ciencias de la Atmosfera y los Oceanos, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires 2Divisao de Geofisica Espacial, Instituto Nacional de Pesquisas Espaciais,

Sao Jose dos Campos, Sao Paulo 3Centro de Previsao de Tempo e Estudos Climaticos, Instituto Nacional de Pesquisas

Espaciais, Cachoeira Paulista, Sao Paulo

1Argentina

2,3Brazil

1. Introduction

The Andes Mountains barrier and the interaction with the easterly trade winds, and the flow associated to the South Atlantic Subtropical High (SASH) are responsible of a key feature of the low-level atmospheric circulation and climate: the so called South American Low Level Jet (SALLJ). The SALLJ is a wind maximum immersed in a pole-ward and moist current with a cross stream mean dimension in the mesoscale, which has been identified as an efficient dynamical mechanism to transport heat and humidity from tropical to subtropical latitudes. The SALLJEX (South American Low Level Jet Experiment) field campaign provided a unique data set for the study and better understanding of the SALLJ (Vera et al., 2006). The SALLJ feeds and controls the life cycle of the mesoscale convective systems over an area that includes the Del Plata basin, and accounts for an important fraction of the precipitation in southern South America, thus influencing the water balance in the region (Nicolini et al., 2002; Saulo et al., 2000). The SALLJ has also being pointed as an important agent to transport and mix other biogeochemical components (Paegle, 1998).

The orographic control of the Andes favouring the poleward flow causes the persistency of the SALLJ all year round, being only episodically interrupted by mid-latitude transient systems arriving in the subtropical South America (SA) (James & Anderson, 1984; Nogues- Paegle et al., 1998). While during the summer this flow has a net poleward component, in the winter it has an eastward tendency up in the mid-latitudes, with an outflow toward the South Atlantic Ocean broadly ranging from 20° S to 40° S, strongly depending on the position of the SASH. Nogues-Paegle & Mo (1997) found an intraseasonal meridional seesaw of dry and wet conditions over tropical and subtropical South America during austral summer in which the South Atlantic Convergence Zone (SACZ) and the low-level stream intensify alternatively. Over the central and north bands of SA during the winter, the climate is strongly influenced by the northward motion of the Inter-tropical Convergence Zone (ITCZ) and the westward displacement of SASH, composing a scenario of a low levels high pressure system over the continent, with light winds and most of the convection being shifted to the northern part of the Amazon and very little precipitation.

This is the climatological scenario of the SA dry season that eases the Tropical Forest and Cerrado biomes anthropogenic crackdown, followed by the biomass burning. In fact, the vegetation fire activity had been since remote times incorporated, as a supposedly acceptable practice, by the local culture to expand pasture and crop lands and even as a regular agricultural harvest tool for some types of produce, such as sugar cane. Every year during the dry season hundreds of thousands of fire spots and the produced thick regional smoke plume, which covers an area of about 4-5 millions of square kilometres, have been detected by satellite observation over SA.

As the SALLJ drives an important mass exchange from the tropical Amazon to the sub-tropics it is predictable that this low level flow could as well play an important role intercommunicating regional climate changes in the Amazonian basin to the southern South American basins. This paper examines the mass exchange between the Amazon basin and the subtropical SA patronized by the SALLJ during the dry/burning season, when the transport of heat and moist occurs associated with the transport of biomass burning smoke aerosol particles.