There has been an overall increase of rainfall over Argentine, Paraguay and southern Brazil between 1916-1991, especially since 1950 (1). For instance, the average of seven places in the region called the 'humid pampa' (including Buenos Aires) rose from about 850 mm/a to about 1150 mm/a. In general, annual variations during the period have been in harmony with those of the Southern Oscillation Index (Section 12.7) and (during the past 35 years) with the mean meridional temperature difference between subtropical and high latitudes. The increase of rainfall has been accompanied by a poleward displacement by 2-5° of latitude of the belt of the highest meridional temperature gradient. Such an explanation is supported by a 3° poleward displacement for the mean latitude of the maximum wind at 200 hPa elevation (Section 12.F), and a similar shift of the average latitude of the South Atlantic anticyclone (2).
(1) Barros, V., M.E. Castaneda & M. Doyle 1996 Recent precipitation trends in southern South America to the east of the Andes. Publication of Dept. Atmospheric Sciences, Univ. of Buenos Aires, and Proc. Latin-American Workshop on Greenhouse Gas Emission, Brazil.
(2) Gibson, T.T. 1992. An observed poleward shift of the Southern hemisphere subtropical wind maximum - a greenhouse symptom? Internat. J. Climatology 12, 637-40.