chapter - Atmospheric and Oceanic Science

which parameters of the GEV distributions show statistically-significant evidence
of trend, using (for example) statistical methods described by Coles (2001). Other
alternatives are statistical analysis of rainfall intensities that exceed some threshold
value (POT: “peaks-over-threshold” analysis) along lines described by Clarke
(2003) and Coles (2001).
Rainfall records are in general longer than discharge records in rivers, and are
free from some of their complications (increased abstraction; impounding by reservoirs;
changing relationships between river level and discharge; effects of sediment
deposition and removal; etc). However, similar analyses are also required for runoff
data. These will require not only a station-by-station analysis of individual runoff
records from selected, good-quality sites, but also a spatial analysis to take account
of spatial correlation between flood flows. Appropriate statistical tools for such
analyses are not well developed (for example, methods for fitting GEV distributions
to spatially-correlated flows that also contain time-trends need to be explored).
1.6. Interdecadal variability
Introduction
Superimposed to the climatic trends forced by changes in the concentrations
of greenhouse effect gases, there are variations of interdecadal scale that hinder the
climatic projections for the lapse that they are most required, the next 30 years. The
MCG that have shown certain ability to simulate the global climatic trends of the
last one hundred years, have not been that successful in describing the interdecadal
fluctuations for the same period, Thus, it cannot be guaranteed that model simulations
will capture the future interdecadal variability. This is because some of these
fluctuations are caused by the internal variability of the system, and are unpredictable.
In spite of it, in many cases, when these fluctuations reach certain magnitude,
their later evolution can be assessed.
In certain cases, regional climate variables as precipitation or temperature can
be related to explanatory variables (such as variables derived from sea-surface temperature
analysis or large scale or planetary circulation indexes) and when these
variables undergo low-frequency (decadal variability), they may have certain
potential for developing hydrological scenarios of the next ten to twenty years in
La Plata basin. Remarkable results in this sense were reported by Robertson et al.
(2001) for the summer season streamflow of the Paraná at Corrientes and for the
whole region by Kayano (2003). These methods are based on the low frequency
change of the sea surface temperature in regions that are related to La Plata basin
precipitation and one example is developed in chapter 14.
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