Dissertation - HQ

Results 77
Figure 4.3 Mean of instantaneous wind recordings (every 30 s) during the
sampling period (ca. 1 hour) at each station, for the four rotations. Wind was
weakly blowing from the East in rotation 1. In rotations 2 and 3 the direction
regularly shifted in time. Wind speeds reached storm levels (instantaneous peaks
at 70 km h -1 ) with high variability during rotation 4.
winds, which occurs once a year, started earlier than usual in 2006 and
lasted from the end of the first rotation to the end of the cruise. Therefore,
for most of the cruise, the wind was highly variable in both direction
and speed (Figure 4.3). During rotation 2 for example, a 180º reversal
in wind direction was noticeable: the origin of the wind gradually
shifted from the Southeast in station 1 to the Northwest in station 36,
in counter-clockwise direction.
As a consequence of the wind regime shift, the current was also very
variable. While the objective analysis smoothed small scale variations, by
considering geostrophic flow and using a large correlation distance, the
variations between rotations are still conspicuous (Figure 4.4). Globally,
water flowed from the South-East to the North-West in rotations 1, 2,
and 4, but with considerable local variations. In rotation 3, even the
large scale picture was different. The flow was globally oriented from
the West to the East with a returning current in the North-Western
corner of our sampling grid.
Because wind conditions changed so radically even during a single
rotation, great caution is in order when interpreting the results of the
Which induces variable
surface flow . . .