Resuits
The year 1995 was characterized by significant freshwater discharges.
Mean river flows over the beginning of the year (from January to the
end of April) exceeded the seasonal mean with respectively 2400 na/s
for the Loire, 1700 m
3 /s for the Gironde and 490 m
3 /s for the Adour.
The seasonal diagram of wind stress is close to the average year pattern.
However, it shows periods of very strong winds during the first four
months. During the winter of 1995, south-westerly and westerlv winds
prevailed. In April, a wind shift occurred and prevailing winds became
north-westerly from spring to the end of summer along the coast of
Landes but rather westerly-north-westerly near the Adour estuarw
Figure 2 shows tidal ftltered sea surface salinity fields in different seasons as well as vertical transect aktng the coast between the Adour in the
south and Oleron island in the north. These results highlight the seasonal evolution of the river plumes.
ln February (fig. 2A), low salinity waters were advected polewards (Adour)
or north-westvvards (Gironde), close to the coast, due to favourable vfinds
which increased the geostrophic densitv-driven circulation. The vertical
section shows strong stratifications next to the mouth of the estuary.
In spring (fig. 2B), river plumes spread over the continental shelf with
strong horizontal gradients. The combined effects of north-westerly
winds and the decrease in river flows led to a reversed circulation of the
Gironde plume. Gironde freshwater then extended towards the south
and mixed with those from Adour.
In summer (fig. 2C), the low river flows induced a smoothing in the
horizontal and vertical gradients. The lower salinitv of the Adour plume
was no longer visible. However, the prevailing north-westerly winds increased the offshore and southward advection of freshwater discharges
towards the Spanish coasts.
In autumn (fig. 2D), the continental shelf was nearly covered by marine
waters. Nevertheless, a low-salinity residual tongue remained near the
coast under southerly winds. The water masses between the Adour and
the Gironde were still slightly stratified. Thev originated from the
mixing of two freshwater river plumes and the sea over time. Thus,
since a few years, a sign of residual river discharges remained along the
coast until the start of a new run-off cycle in the following winter.
Conclusion
The first simulations carried out showed the ability of this new, extended
model to correctly describe the behaviour of a river plume over great
depths. The description of salinity trends in the southeastern part of the
bay of Biscay in 1995 could not be validated, but tbis preliminary
7 experimentation allows us to simulate several years and compare the model results
with existing measurements.
52
The year 1995 was characterized by significant freshwater discharges.
Mean river flows over the beginning of the year (from January to the
end of April) exceeded the seasonal mean with respectively 2400 na/s
for the Loire, 1700 m
3 /s for the Gironde and 490 m
3 /s for the Adour.
The seasonal diagram of wind stress is close to the average year pattern.
However, it shows periods of very strong winds during the first four
months. During the winter of 1995, south-westerly and westerlv winds
prevailed. In April, a wind shift occurred and prevailing winds became
north-westerly from spring to the end of summer along the coast of
Landes but rather westerly-north-westerly near the Adour estuarw
Figure 2 shows tidal ftltered sea surface salinity fields in different seasons as well as vertical transect aktng the coast between the Adour in the
south and Oleron island in the north. These results highlight the seasonal evolution of the river plumes.
ln February (fig. 2A), low salinity waters were advected polewards (Adour)
or north-westvvards (Gironde), close to the coast, due to favourable vfinds
which increased the geostrophic densitv-driven circulation. The vertical
section shows strong stratifications next to the mouth of the estuary.
In spring (fig. 2B), river plumes spread over the continental shelf with
strong horizontal gradients. The combined effects of north-westerly
winds and the decrease in river flows led to a reversed circulation of the
Gironde plume. Gironde freshwater then extended towards the south
and mixed with those from Adour.
In summer (fig. 2C), the low river flows induced a smoothing in the
horizontal and vertical gradients. The lower salinitv of the Adour plume
was no longer visible. However, the prevailing north-westerly winds increased the offshore and southward advection of freshwater discharges
towards the Spanish coasts.
In autumn (fig. 2D), the continental shelf was nearly covered by marine
waters. Nevertheless, a low-salinity residual tongue remained near the
coast under southerly winds. The water masses between the Adour and
the Gironde were still slightly stratified. Thev originated from the
mixing of two freshwater river plumes and the sea over time. Thus,
since a few years, a sign of residual river discharges remained along the
coast until the start of a new run-off cycle in the following winter.
Conclusion
The first simulations carried out showed the ability of this new, extended
model to correctly describe the behaviour of a river plume over great
depths. The description of salinity trends in the southeastern part of the
bay of Biscay in 1995 could not be validated, but tbis preliminary
7 experimentation allows us to simulate several years and compare the model results
with existing measurements.
52