Case Studies - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 376

available inside the lagoon. Water salinity and temperature at the surface (solid line)
and near the bottom (dotted line) do not show considerable differences. Hence, waters
entering the lagoon are vertically mixed. Inside the lagoon, waters normally also remain
well mixed 16 under the influence of omnipresent winds. Winds during the May 5-20
period were oriented along the lagoon axis only occasionally (arrows on top,
Figure 9.2.3) . This study focuses on the May 12-14 reversing wind event because
current measurements inside the lagoon were available then. Winds on May 12 were
southwesterly, along the lagoon axis, and they reversed on May 13 to become north-
easterly. Low-frequency sea levels (dotted lines superimposed on sea levels in Figure
9.2.3) indicate that sea levels at the mouth (L1) and inside the lagoon (L2) were almost
identical, as expected in a “leaky” lagoon. 11 Northeasterly (southwesterly) winds led
sea-level rises (falls) at L1 and L2. It is not clear if this response was a local set-up
or a gulf-wide response to large-scale winds. The application of a 3D hydrodynamic
model to the Gulf of St. Lawrence 17 showed that, under prevailing southwesterly winds,
sea levels rise in the northern gulf and decrease in the southern gulf and in the Magdalen
Islands region. So it is possible that sea levels at the mouth (L1) and inside the lagoon
(L2) respond to such nonlocal forcing as well as to local winds.
Winds, sea levels at L2, and wind-driven currents in the deep basin at C6, C10,
C11, and C12 ( see Figure 9.2.2) were then examined during the reversing wind event
of May 12-14 to detect upwind return flows in the lower layers of the deep basin. The
hourly time series were first low-pass filtered (cut-off frequency set at 34 h) in order
to remove tidal oscillations and high-frequency noise. 18 These series are shown in
Figure 9.2.4. The numerical simulations (Section 9.2.4) will focus on the same wind
event. Several points are worth noting about the wind-driven current observations.
Measured at 1 m above the bottom, the currents shown are located in the deeper layers
of the basin. Their principal axes of variability ( Table 9.2.1) indicate that they are
oriented along axes that differ slightly (between 8 and 27 ° ) from the lagoon longitudinal

). They also exhibit an out-of-phase relation with the wind direction. This
out-of-phase relation can be objectively established from empirical orthogonal function
(EOF) analysis 19 of the wind component resolved along the lagoon axis and the current
vectors resolved along their principal axis of variability (Table 9.2.1). The correlation
matrix for the resulting series at C6, C10, C12, and C11 and the longitudinal wind
series is presented in Table 9.2.2a. Results from the EOF analysis of this matrix are
presented in Table 9.2.2b in terms of the percentage of the total variance in the series
explained by each empirical mode and the percentage of the variance in each series
explained by each mode. Results suggest that all near-bottom currents resolved along
their principal axis are negatively correlated with the longitudinal wind and that 75%
of the total variance is explained by the first empirical mode. This mode explains 96%
of the wind variance and the largest portion of the variance in each current series,
except for the variance in currents at C6, which is partly explained by mode 2. In
summary, these observations tend to support the hypothesis that the wind-driven
circulation in a lagoon can be three-dimensional in space and that the return flow can
occur at depth in an upwind direction.
Simple analytical reasoning provides insight into these findings. 20 Consider the
steady-state motion in a lagoon of variable depth H ( x, y ) resulting from horizontal
pressure gradients and vertical shear friction, in the absence of Coriolis accelerations

axis (40 °