Recent Precipitation Trends in Southern South America East of the Andes: An Indication of Climatic Variability

Abstract

Positive trends in precipitation were observed during 1916–1991, especially since the fifties, over most of the Argentine territory. The seasonal variation of the climatic parameters, including precipitation between 1956–1991, can be summarized as a displacement of the positive nucleus of precipitation to the northeast from summer to winter and a less systematic return to the southwest from winter to summer. Correlation studies between annual precipitation and hemispheric indices show that the correlation with the mean meridional gradient of temperature (MMTG) is of the same importance and in some areas greater than the correlation with the Southern Oscillation Index (SOI). Furthermore, the correlation field strongly suggests that the precipitation trends observed in the last 35 years are due to the decrease of the MMTG. In fact, the MMTG decreases around 1.5° C during that period. According to the theory of baroclinic instability this implies a displacement to higher latitudes of the planetary circulation systems. A displacement of 3° in latitude to the south has been reported by Gibson (1992) for the mean position of the maximum wind at 200 hPa in the Southern Hemisphere. Since 1976 a similar displacement of the Atlantic Subtropical High also can be inferred from data of the Atlantic coast. The shift to the south of the general circulation features produces trends in precipitation because of the close connection between precipitation and the latitude of the circulation systems. The data show relationship between the precipitation field and the latitude of the maximum wind speed at the altitude of 200 hPa. Consequently, the observed trends in the precipitation fields could be explained largely by a 5° displacement to the south of this latitude of maximum wind during the last 35 years. This means that in the study area an important component of the global circulation system changed its position in a statistically provable manner.