ABSTRACT:The variability of Seine River flow (France) was investigated using daily time series over the period of . The study aimed at characterizing the dominant modes explaining the variability of flow and at investigating their possible link with the North Atlantic Oscillation (NAO) as the dominant climate regime in the region. The Seine River flow was affected by a statistically significant increasing trend across the period. The hydrologic regime of the Seine River was found to be highly variable and seem to occur later in the year, approximately, since the end of the 1980s. Annual minimum, maximum and mean flow time series exhibited an obvious change around 1970. Continuous wavelet transform (CWT) of flow revealed energetic peaks highly localized in time, defining three time periods : before 1970, between 1970 and 1990 and after 1990. Two interannual modes (17 and 5-9 years) occur, respectively, around 1970 and in the early 1990s. The same observations on precipitation CWT ensure a climatic origin to these interannual modes. The annual oscillation was always strongly represented in flow and was affected by an increase in power from 1990 until the end of the time series. Comparison between NAO and annual flow anomalies revealed similar trend behaviour characterized by a change around 1970 towards more positive NAO indices and positive flow anomalies. In addition, CWT of NAO revealed similar modes of variability around 17 and 5-9 years as flow interannual modes. This highlighted a possible link between NAO and hydrometeorological processes in the region. The flow fluctuations (interannual modes and trend) expressing this potential link would explain approximately 23% of total variance of Seine River flow and would even reach 35% of the variance after removal of the annual cyclicity. These results would then emphasize the potential strong control of NAO on the variability of Seine River discharge.

Abstract:Intertidal mudflats play a critical role in estuarine exchange, connecting marine and continental supplies of nutrients and sediments. However, their complex morphodynamics, associated with a wide range of physical and biological processes, are still poorly understood and require further field investigation. In addition, mudflats are challenging areas for Structure-from-Motion (SfM) photogrammetric surveys. Indeed, the mudflats generally hold back residual tidal water, which can make stereo restitution particularly difficult because of poor correlations or sun-glint effects. This study aims to show the potential of light UAVs (Unmanned Aerial Vehicles) for monitoring sedimentary hydrodynamics at different spatial scales in a silty estuary. For each UAV mission an orthophotograph and a Digital Elevation Model (DEM) are computed. From repeated surveys the diachronic evolution of the area can be observed via DEM differencing. Considering the ground texture in such a context, the stereo restitution process is made possible because of the high spatial resolution of the UAV photographs. Providing a synoptic view as well as high spatial resolution (less than 4 cm), the UAV dataset enables multi-scale approaches from the study of large areas to the morphodynamics of smaller-scale sedimentary structures and the morphodynamics impact of plant ground cover.

[1] In this paper, the temporal dynamics of precipitation in northwestern France in relation to the dominant climatic pattern in Europe were investigated. The general trends and the nonstationary behavior of the North Atlantic Oscillation (NAO) were described using fractal analysis and Fourier spectral and continuous wavelet analysis of a NAO index time series over the 1865-2004 period. The 44-year and 8-year multidecadal components showed a clear increasing power during the second half of the last century. The possible link between rainfall variability and the NAO was then investigated. The links between the NAO and rainfall were not as obvious, as NAO-like components were not easily retrieved in the precipitation records: Relationships between the NAO and rainfall were very likely at certain timescales but were not systematically very obvious. For instance, the characteristic quasi-biennal oscillation (QBO) of the NAO was barely detected in precipitation, but a %6-year fluctuation beginning in the 90s was found to be statistically significant at a 95% confidence limit After investigating a possible link with the Southern Oscillation Index, the occurrence of this fluctuation in the beginning of the 90s could be related to the observed shift of the QBO toward slightly higher frequencies in the NAO time series. On the other hand, a modulation of the QBO by a %6-year interannual fluctuation would suggest the existence of a %6-year climate pattern that could affect precipitation and, to a lesser extent, the NAO. Cross-wavelet analysis between the NAO and precipitation revealed a loss in correlation across the 1970-2005 period, which seemed to be the fact of a QBO-like fluctuation. This loss of correlation was related to the above-mentioned shift of the QBO and 6-year rainfall interannual band since the 90s.