chapter - Atmospheric and Oceanic Science

Introduction
Meanwhile, the activities where climate is an input for decision making require
answers that cannot wait for the development of an impeccable and precise methodology
that today is not available. In particular, the main demand is in the estimate of
future climate conditions in the next two or three decades, which is the usual time
horizon for medium and long term planning. The purpose of this book is to offer a
conceptual and critical framework of the few tools available today and of its future
evolution. In the last part of this book (chapters 11 at 15), three of these tools are
analyzed, climatic scenarios, the use of the low frequency climatic variability and
the statistical treatment of extremes for non stationary series. Given the remarkable
importance of the climatic and hydrological trends of the last decades in the La Plata
basin, the three types of tools are illustrated with examples of this basin.
1.2. Climatic and hydrological trends in the La Plata basin
La Plata basin is an environment of great economic and demographic significance,
shared by 5 countries with a population surpassing the 200 millions. It
accounts for the generation of most of the electricity, the food and the exports of
these countries. In most of this immense basin of 3500000 km 2 , there are clear manifestations
of important climatic and hydrological trends that could likely be related
with the Global Climate Change. The most direct indication in such a sense is
the simultaneity of the beginning of these trends with the last trend of global heating
initiated in the 1970 decade, which as will be seen in chapter 9, is attributed to
the concentration increase of greenhouse gases.
Amongst sub-continental regions of the world, Southern South America has
shown the largest positive trend in precipitation during the last century (Giorgi
2002). This is regardless that the region includes subtropical Chile where there
were negative trends (Minetti and Vargas 1998; IPCC 2001). The increase in annual
precipitation in the last 40 years has been more than 10% over most of the region,
but in some places it has reached 30% or more (Castañeda and Barros, 1994;
Minetti et al. 2003). For example, in the West of the Buenos Aires province and in
part of the Argentine-Brazilian border, the mean annual precipitation increases
more than 200 mm.
In addition to the increase in the mean annual precipitation, episodes of heavy
rainfall are becoming more frequent. The frequency of precipitation events exceeding
100 mm in Central and Eastern Argentina three-folded during the last 40 years
(Barros 2004). This trend was also observed in Sao Paulo, Brazil, where the frequency
of heavy rainfalls has increased significantly (Xavier et al. 1992 and 1994)
especially during summertime. Extreme precipitations in Southeastern South
America (SESA) usually come from mesoscale convective systems (MCS)
(Velasco and Fritsch 1990). Rainfalls from MCS have a large destructive potential
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