Interdecadal changes in the precipitation seasonal cycle over Southern South America and their relationship with surface temperature

摘要

The decadal variability in the structure of the annual precipitation cycle over Southern South America (SSA) is analysed with the purpose of investigating whether the lower frequency variability laid bare by annual data is also evident in the annual precipitation structure. Climatic analysis shows that the annual cycle plus the semi-annual cycle dominate the annual variability of precipitation in SSA and represent most of the physical factors responsible for the observed patterns. The percentage of variance explained by the annual cycle shows 2 local maxima, in northwestern Argentina and southern Chile, with opposite phases, summer and winter. The interdecadal analysis of the annual cycle shows 2 areas of relevant variability: one over the central east and the other over the 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 that might 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 the region, correlation coefficients are evaluated. The correlation structure of the warmer period 1943-52 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 are found at the coldest times in 1955-64, and are probably related to an increase in cloudiness. During summer months (November to February), there is an inverse relationship between precipitation and temperature over most of SSA. That is, warmer/colder summers are associated with precipitation below/above the mean. In autumn and spring months, the correlation is positive in the eastern part of Argentina and Paraguay, and represents well the mechanism of maximum precipitation in this area, which is mainly the result of cyclogenesis. Winter correlation shows a weak positive pattern over SSA and a negative correlation area to the east of the Cordillera de los Andes, which is more intense in spring. [References: 35]