88
Dl.liydrogr.Z-40,1987. H 3. Klein Benthic storms
sous-marines observées entre septembre 1983 et semtembre 1984 sont reliées à des
tourbillons synoptiques atteignant de grandes profondeurs. Ces évènements sont des
mécanismes importants pour la dispersion d éléments — traces libérés sur le fond de
l’océan Ils permettent aux particules de franchir l'interface entre la couche limite de
fond, homogène, et la partie stratifiée de la mer profonde. Les tourbillons qui ont
des durées de \icde plusieurs mois, piègent les particules et facilitent leurs transports
horizontaux et verticaux au sein même du tourbillon Chaque tourbillon du fait du
cisaillement, transforme un volume d’eau contaminée en minces couches, ou plutôt
en minces filets d’eau, si l'on se place dans les deux dimensions Cela génère en
réalité un processus de mélange a petite échelle.
Introduction
The NOAMP Experiment (INordostatlantisches Monitoring-Program in), carried
out from September ’83 until Mat, ’86. was designed to investigate the local transport
in the deep North East Atlantic, also with regard to the fate of matter released at the
seabed During the experiment, 7 current meter moorings were located in an area
between 47' 10' N and 47° 35 N and 18° 45' W and 20° 35' W. All moorings had
current meters with a nominal distance from the bottom of 10, 30, 70, 200, and
250 metres. Most of the deepest instruments, t c 10 m above the bottom (hereafter
a b ), were acoustical Neil-Brown instruments, the others were Aanderaa ACM5
current meters.
The mean velocity near the bottom,!, e. less than 75 m a. b., range from about
2 to 6 cm/s (Tvlitlelslaedt, Bock, Bork, et al. (1986]). These values agree
well with the results of other experiments In the deep North East Atlantic (Dick-
son, Gould, Muller et al. [15985]. V an gries he im [1986]).
Approaching towards the bottom, mean (K m ) and eddy kinetic energy (K e )
increases, showing a local maximum between 10and IDO m a b (see Fig 2) This is
the dynamic signal of the bottom boundary layer (hereafter BBL), which is also
characlcnzcd by homogeneity of potential temperature, salinity, and other proper
ties. and a strong increase of the concentration of suspended mutter due to bottom
friction (see also Fig, I I). In the NOAMP urea, the height of the BBL ranges
between 20 and 150 metres, depending upon the velocity present. Matter, released
at the seabed, is expected to be mixed within the BBL within several hours or a few
clays. At the top of the BBL a front is usually given by a pycnocline which reduces the
exchange with the interior of the deep ocean significantly (Robinson and K Llp-
f i rm a n [ 1985])
During the whole experiment, episodic events with unusually high velocities,
known in the literature as benthic, abyssal or deep-sea storms, can be read from the
deepest current meters (less than 70 m a b ). In the NOAMP experiment, a benthic
storm is defined as aperiod for at least 48 hours with velocities of more than 10 cm/s.
During NOAMP storms with a duration of between 3 and 25 days and velocities
of more than 25 cm/s have been measured at different moorings (see Table I). The
positions of the moorings are marked in Fig 1
