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The ENSO memory and its impacts on large French Guiana streamflows

Congress: 2008

Keyword(s): Non-stationary statistics, cyclostationarity, climate impact, inertia.
AbstractENSO is well known to have a worldwide impact, in particular on streamflows. As an illustration, we found a significant and high correlation (~0.47, 5-month time delay) between ENSO activity and discharges of the main French Guiana rivers. However, this classical calculation does not take into account for climatic and hydrological modulations extended in time. Most climate fluctuations may be modulated by a variety of periodic or quasi-periodic deterministic forcings (e.g. diurnal, seasonal or Milankovitch cycles). These process modulations often induce cyclostationary defined as periodic correlations. I propose here that quasi-periodic correlations can appear when the inertia (memory) of a process is modulating its pure cyclostationary property. This effect of the phenomenon memory leads to cyclostationary behaviour “in average” both in time and frequency domains, called Extended CycloStationarity (ECS) and being rigorously defined in this work. A new statistical tool has been developed to analyse ECS in a time series, with the help of a comfortable time-frequency visualisation technique (called DXM). This tool has been applied to the quantification of ENSO interannual influences on French Guiana streamflows, taking into account for the possible phenomenon memory. ECS property searched into the ENSO-SOI and North/South Atlantic SST indices perfectly confirms the well-known interannual influences on South-America hydrology (5-month time delay for ENSO and 3-month time delay for Atlantic) and their evolutions discussed in literature. Yet, these significant quasi-influences are associated to new delayed influences with more than 99 % confidence level: respectiveley ~20 and 40 months for ENSO and atlantic ocean influences on French Guiana river discharges. These delayed influences lead to the hypothesis that ENSO has not an instantaneous influence on French Guiana river discharges, rather than it influences the Atlantic ocean, this Atlantic ocean influencing in return French Guiana river discharges. This result, if confirmed, highlights the role of ENSO memory and cyclostationnarity in climate impacts and global changes. Well-known impacts of ENSO phenomenon could then be attributed not only to its intrisic chaotic behaviour, but also to its seasonal modulations and memory.
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