Interdecadal changes in precipitation seasonal cycle over South America. Relationship with surface temperature

RUSTICUCCI M., PENALBA O

CLIMATE RESEARCH

INTER-RESEARCH

Lugar: Oldendorf/Luhe; Año: 2000 vol. 16 p. 1 - 15

0936-577X

The decadal variability in the structure of the annual precipitation cycle over SouthernSouth America (SSA) is analysed with the purpose of investigating whether the lower frequency variabilitylaid bare by annual data is also evident in the annual precipitation structure. Climatic analysisshows that the annual cycle plus the semi-annual cycle dominate the annual variability of precipitationin SSA and represent most of the physical factors responsible for the observed patterns. Thepercentage of variance explained by the annual cycle shows 2 local maxima, in northwesternArgentina and southern Chile, with opposite phases, summer and winter. The interdecadal analysisof the annual cycle shows 2 areas of relevant variability: one over the central east and the other overthe north east. In the first area, there is a positive trend in the variance explained by the first harmonic,indicating that precipitation tends to be better represented by an annual cycle, a fact thatmight indicate a climatic change in so far as this variable is concerned. In view of global warming,and as a first step towards quantifying the relationship between temperature and precipitation in theregion, correlation coefficients are evaluated. The correlation structure of the warmer period1943?52 generally shows a slight correlation pattern when compared to the 2 colder periods,1955?64 and 1966?75. In northwestern Argentina, the highest positive correlation coefficients arefound at the coldest times in 1955?64, and are probably related to an increase in cloudiness. Duringsummer months (November to February), there is an inverse relationship between precipitation andtemperature over most of SSA. That is, warmer/colder summers are associated with precipitationbelow/above the mean. In autumn and spring months, the correlation is positive in the eastern partof Argentina and Paraguay, and represents well the mechanism of maximum precipitation in thisarea, which is mainly the result of cyclogenesis. Winter correlation shows a weak positive patternover SSA and a negative correlation area to the east of the Cordillera de los Andes, which is moreintense in spring.