LARGE AND LOCAL SCALE ATMOSPHERIC VARIABLES ASSOCIATED TO HYDROLOGICAL BASINS IN THE

TROPICAL REGION: THE RIO DOCE BASIN (1973 – 2003) – STATE OF MINAS GERAIS – BRASIL

Fulvio Cupolillo – Magda Luzimar de Abreu

The abundance of solar energy reaching southeastern Brazil, over the year, creates a mild winter, dominated by the impact of large scale systems, such as cold fronts and the South Atlantic Subtropical High (ASAS – Alta Subtropical do Atlântico Sul), a ubsident branch of the Hadley cell, originated from the General Atmospheric Circulation.

The ASAS has its geographic center at the South Atlantic and originates, during the dry season, warm continental, subsiding and stable atmosphere, preceding the advance frontal systems towards the region.

In Minas Gerais State the ASAS is identified by clear days, high maximum temperatures and weak winds, predominantly from east and northeast. The occurrence of cold fronts (FF – Frentes Frias) in the region, coming from the southern part of South American, reaches the State with low nebulosity and few rain, mostly due to the dry air mass related to the ASAS effect (figure 1), that induces low relative humidity (Franca, 2009).

The cold air mass (Polar Air Mass, Massa de Ar Polar – MPA, characterizing the South Atlantic Polar High – Alta Polar do Atlântico Sul – APAS) following the frontal system, causes temperature decrease, atmospheric stability and stratiform clouds, due to the moist, ocean air, advection and to the strong thermal inversion. over these circumstances, south, southeast and southwest winds predominate. These dynamic elements act in the same way over the Rio Doce basin, although the proximity from the Brazilian coastline results in a wetter winter, with stratiform nebulosity, but low amounts of rainfall.

At the end of spring and during summer (the rainy season), the ASAS, is displaced to the ocean, diminishing its action over the continent (warmed by the intense solar radiation at this time of the year). Such warming results in the formation of convective systems over the central Brazil and western Minas Gerais State. Therefore centers of low atmospheric pressure predominate at the surface and undergoes to the east, reaching out the coastline. By the 70’s Edmon Nimer identified these convective patterns, naming them Tropical Instabilities (Instabilidades Tropicais – IT).

Under these conditions west winds predominate originating the Continental Equatorial Mass (Massa Equatorial Continental – MEC), causing intensity convective storms.

Nowadays it is known that the formation of the IT is associated to the high level anticyclonic circulation system named Bolivian High (Alta da Bolívia - AB). Jones and Horel (1990), Silva Dias and Marengo (2002), Vianello and Maia (1986), Prates (1994), the Climanálise Especial (1996) and Cupolillo (1997) state that the AB is a high tropospheric circulation which occurs during in the rainy season over South America. This atmospheric phenomenon causes strong convective activity and intense rainfalls over tropical region of the continent. It is mainly supported by the condensing latent heat released during the water vapor condensation.

The frontal systems are also present in the wet season. The cold fronts are fed by the low level moisture convergence and join to the ITs, originating abundant precipitation over central and southeastern Brazil. The episodes extending over seven days are characterized by the stationary condition of the frontal system which oscillates between warm (predominating winds from north and northwest) and cold (predominating southwest winds) fronts. This association results in events of intense and continuous precipitation that accumulates large amounts of rain. Under these circumstances, it is observed the phenomenon known as the South Atlantic Convergence Zone (Zona de Convergência do Atlântico Sul – ZCAS) (CLIMANÁLISE ESPECIAL, 1996; ABREU, 1998; QUADRO, 1993; CONRADO, 1992).