MSM60–CapeTown/SouthAfricatoMontevideo/Uruguay
2. Weekly Report - Maria S Merian MSM60
take a virtuell tour through the ship:
https://www.ldf.unihamburg.de/en/merian/merian-virtuell.html
In the meantime, we have traveled about 2500
km from Cape Town and have penetrated
westwards along 34 ° 30'S into the South
Atlantic. Overall, everyone has been well on
board, but the work in shifts by day and night,
and by 7 days a week, is always a challenge.
In the night of Friday we approached the
southern end of the Walvis-Ridge, a Sunset on RV Maria S Merian
submarine mountain range separating the expedition MSM60 in the eastern
Cape Basin to the south from Angola Basin. South Atlantic (Photo: L. Branlard)
Impressive Guyotes can be found here on the
seabed. These submarine table mountains rise steeply from the depths of the plain.
On a distance of just 15 km, we saw an increase in the depth of the sea floor in the
echosounder by 3 km - corresponding to a slope of 20%. Interesting is the interaction
of these submarine mountains with the currents - during our 24-hour crossing over
two neighboring Guyote, the acoustic current profiler showed strong east / west
currents changes occuring exactly in tidal cycle.
The measurements in the Cape Basin were particularly interesting for us because
here the dynamics are dominated by oceanic mesoscale vortices. We were fortunate
that one of the vortices lay directly on our course and could thus be surveyed with the
instruments and we were able to determine its vertical structure well. The acoustic
current profilers record the flow conditions under Merians keel down to about 1200 m
depth. Every 20 km we launch an eXpandable BathyThermograph (XBT) probe from
the stern of the ship and that shows us the temperature structurebetween the surface
and down to about 800 m depth. So far we launched about 130 of these probes and
also in the ocean vortex (see figure).
The vortice rotated clockwise, in the southern hemisphere this chracterizes a cyclon,
and it had a diameter of about 140 km. The rotation of the water causes the surface
of the ocean to lower across the vortex, an effect which can be seen very well in the
sea surface topography as can be measured by the satellite (see figure). The flow
speed we measure is fast, up to 1 m/s - it takes about 5 days until the water masses
made a complete turn.
The permanent rotation not only leads to a drop in the sea surface but surfaces of the
same density (called ‘isopycnals’) or of same temperature raise towards the surface
within the vortex. Indeed our XBT temperature measurements show this rise very
nicely.
MSM60–CapeTown/SouthAfricatoMontevideo/Uruguay
Having a couple of
CTD
measurements in
the vortex we also
have images from
the
"Underwater
Vision
Profiler"
(UVP) avaiable, an
instrument
supervised
by
Andreas
Rogge
from the AWI. The
UVP makes HD
images in high
frequency while it is
lowered into the
abyss. A first result
ifrom
this
measurements
is
the
vertical
distribution
of
particles. In the
vortex
Andreas
found a marked
decrease in the
frequency of large Measurement of a cyclonic rotating vortex in the Cape
particles
with Basin. The north/south speed is shown in the upper left,
the white lines indicate the temperature distribution.
simultaneous
increase of small Below a map of the surface deflection, derived from
particles. From the satellite data for January 6 is shown with the vortex
(yellow circle). At the top right is the distribution of
HD images he will
"large" particles from the UVP casts across the vortex.
later analyze which
plankton species occur in the vortex. The UVP produces many, many pictures and
help is needed - so if you want to help with the identification you can do it the
following the link https://planktonid.geomar.de
For recreation the table tennis turnament has started and the first round is almost
completed. At the moment we have relatively quiet sea and Maria does not interfere
much the game. We are glad if this remains so.
With greetings from the South Atlantic,
Johannes Karstensen for the participants MSM60