Anne-Marie Jegou

3D modelling of seasonal evolution of Loire and Gironde plumes on Biscay Bay continental shelf

Abstract A 3D model of the Atlantic shelf has been developed and applied to study shelf dynamics and evolution of hydrology. The model takes the combined effects of tide, wind, river discharges and surface heat flux into account. Time scales vary from 1 day to several years. Following a brief description of the model and forcing variables, the behaviour of the Loire and Gironde plumes is described, first for winter and spring, then over a period of several years, under realistic forcing. The results show that plume evolution depends on the high variability of river runoff and winds. Model simulations performed over 7 years (1990 to 1996) have highlighted several features of these plumes: 1) In early winter and periods of high river runoff, plumes usually spread northwards and along shore. 2) During winter, vertical stratification is weak on the shelf. 3) Near the bottom, low-salinity water spreading hardly varies, reaching about the same extent each year. The low-salinity waters are located in the north of each estuary, often not reaching the 50 m isobath. 4) When river discharges are reduced and prevailing winds are from the north-west, the northward spreading of plumes may be stopped. In that case, plumes may be driven offshore or southwards. This path change usually occurs in spring. Salinity gradients become weaker under mixing and spreading effects. The low-salinity strip along the shore seldom builds up again, and the shelf circulation of water masses becomes mainly wind-driven.

Variabilité du panache turbide de la Gironde par télédétection. Effets des facteurs climatiques

Abstract The Gironde estuary (France) discharges to the ocean an important amount of suspended particulate matter in the form of turbid plumes. The surface plume is more particularly studied from coastal oceanographic surveys and NOAA/AVHRR satellite data collected during a French programme of coastal oceanography (PNOC-Atlantic). The AVHRR reflectances are atmospherically corrected according to an algorithm based on the clear water concept. The comparison with suspended sediment concentrations are realised by direct and indirect calibrations. The correlations obtained show that at a same concentration the reflectance varies, probably because of varying particle (floc) size and composition. The shape and the surface measured from low resolution (4 km × 4 km) and high resolution (1 km × 1 km) AVHRR data are then compared to the forcings introduced by the fluvial output, the tide, neap and spring tide and the wind variations. The latter have an important effect on the orientation of the distal part of the turbid plume.