US GO-SHIP is part of the international GO-SHIP network of sustained hydrographic sections, supporting physical oceanography, the carbon cycle, and marine biogeochemistry and ecosystems. The US program is sponsored by US CLIVAR and OCB. Funded by the National Science Foundation and NOAA.
Greetings from the Bay of Bengal. We have now left behind the Equatorial
the Indian Ocean and have finished an intense sampling scheme between 3S and 3N,
with stations spaced just 20 nm apart instead of the average 30nm. We completed
20 stations in just 6 degrees of latitude. Now we are covering the so-called “bow tie”
section of the cruise, where the track makes a funny kind of knot around 10N (see
plot below). This “bow-tie” section covers part of the I01E GO-SHIP cruise which
was last occupied in 1995 but not again. During the I09N 2007 cruise, sections of the
I01E track were also sampled, and we are repeating this track, going as far west as
international waters will allow. The Bay of Bengal is an important source of fresh
water to the global ocean and these stations are useful for estimating freshwater
transports and budgets, among other things. The stations further west also have
interesting carbon and nutrient concentrations, particularly in deep waters.
We are beginning to see some animals out there, finally. Still no sign of a
but some of us have seen the odd turtle, a small shark, and a school of dolphins
splashing in the distance. To compensate for the lack of exciting fauna, the Indian
Ocean is providing us with a non-stop series of amazing sunrises, sunsets and starry
skies, as well as ongoing calm seas.
For those of us who have not worked in the Indian Ocean before, the oxygen
we are seeing have been a surprise. Starting at around 10S, subsurface values began
to drop significantly and we have actually measured O2 values as low as 10 μmol/kg,
compared to values of ~200 μmol/kg at the same depths in our first stations. This
means that subsurface waters in this area are highly hypoxic, and close to the
suboxic range (concentrations lower than 5 μmol/kg). In the suboxic range most
organisms cannot survive. Similarly low O2 concentrations had been measured in
this area in previous cruises so this is not a dramatic change that occurred in the last
On this week’s report we also wanted to talk a little about the LADCP
Acoustic Doppler Current Profiler) measurements. The LADCP is installed on the
rosette and provides zonal as well as meridional current velocities. These currents
are important not only for physical oceanographers, but also for researchers
working with nutrients and carbon, because knowing them helps scientists in
determining nutrient distribution and transports.
Our LADCP scientists on board and back on land, Takaya Uchida and Andreas
Thurnherr, have summarized the main results from the LADCP records so far:
1) There are strong currents along the Diamantina Escarpment (southern flank
Broken Plateau, near 30S) essentially all the way to abyssal depths. From this single
occupation it is not clear whether the northwestward flow along the Diamantina
Escarpment is part of the mean circulation or if it is a transitory feature. The flow
above the topography crosses the plateau in a southwesterly direction; the flow
below the crest depth appears to flow along the topography (probably because of PV
conservation). Based on this observation we hypothesize that the southern limit of
the high-EKE (Eddy Kinetic Energy) wedge seen in the figures below is set by the
2) The strong currents associated with the Diamantina Escarpment are
with significant turbulence and mixing. Based on the VKE (Vertical Kinetic Energy)
parameterization the turbulence levels around the Broken Plateau are similar to the
turbulence levels in the ACC region, although they do not extend above 1000m.
3) The VKE-derived turbulence levels under the entire region of high surface
(roughly 17-30S) are elevated across the entire water depth.
The zonal equatorial undercurrents between the latitudinal bands of 5S~5N
are specific features in the equatorial regions can also be clearly observed.
As always, here are the links to our blogs for those of you who want to
learn more about what’s going on in our cruise:
Carmen and Leticia, chief-scientists I09N
This week started off with an issue around the carousel of the rosette. The
is the mechanism that triggers the closing of bottles (bottle tripping) in the rosette
(see pic below). On a couple of stations, problems with communication in the
rosette prevented the surface bottle from tripping. It was determined that there was
damage to the bulk connector of the carousel, which could have forced us to switch
to our spare 24-bottle carousel (thus losing some of our vertical resolution).
Eventually the connector was repaired and we now have reliable communication
with the rosette during the casts. We were also able to keep using the Y-cable that
allows communication between the carousel and the SBE35 reference temperature
sensor. Special kudos to the ODF team, especially Sergey, Matt, Ted and Courtney,
for committing to diagnosing the problem and fixing the connection with minimal
loss of time.
On the same note, we have been forced to take Niskin 12 out of the rosette
the magnet operating the trigger mechanism is damaged and the bottle was not
tripping. Given that we are now moving towards shallower depths (all the stations
deeper than 4500m are now behind us), we think that the science objectives will not
be significantly impacted.
On this week’s report we want to highlight some of the work being done as an
ancillary project of our regular GO-SHIP work. Scientists from UC Irvine and the
Bigelow Laboratory for Ocean Sciences are conducting biological experiments using
the ship’s underway water line as well as CTD samples. The samples will be
analyzed back on land to determine Particulate Organic Carbon, Nitrogen and
Phosphorous (POC, PON and POP, respectively). POP in particular had never been
measured in these waters prior to our cruise. They are looking at differences in
C:N:P ratios and anticipate that because the South Indian Ocean Subtropical Gyre is
a nutrient poor region, the C:P ratio will be higher here than in regions such as the
equatorial upwelling region, for example. Other samples are collected from the
underway waterline to determine the surface phytoplankton community, and the
presence of genes indicating key nutrient sources. The group is also conducting
incubation experiments to determine nutrient uptake rates. This will allow them to
know whether microbes are using different nutrient sources (nitrate, ammonia or
phosphate, for example). These studies are carried out in combination with results
from trace metal analyses (we are doing roughly one trace metals cast per day) and
data from our CTD nutrient samples.
In the plot below, there is a representation of the preliminary nitrate
values measured so far on the cruise and the location of 2 “regional stations,” where
more intense biological sampling has taken place. One more “regional station” is
planned close to 5 degrees North.
Amanda Fay, one of our CTD watch-standers modified a matlab program previously
coded by Seth Travis and Natalie Freeman (CTD watch-standers on I08S), that
allows us to have a look at the weather conditions predicted along our track. We
continue to enjoy low waves and great weather, and from the privilege of our calm
waters, we can appreciate the weather maps and watch as a huge storm with
predicted wave heights of up to 10 meters forms off the coast of Madagascar.
Week 2 of our I09N cruise seems to have flown by as we have settled into our
routine of 3-4 stations per day. The weather has been fantastic and we have been
working under calm seas. We can definitely tell that we have entered tropical waters
now. The temperatures at the sea surface are around 30C (that’s 86 Fahrenheit) and
we can feel the high humidity in the air. No one is complaining if they get splashed
with the cool, refreshing water from the deep bottles anymore.
This past week we have sampled what will be our deepest stations of the
reaching up to 6122 m depth. That is 3.8 miles deep! The sensors on our Rosie
(nickname for our rosette), however, can only withstand depths of 6000 m, so we
had to stop her short. I am sure she was wondering why we didn’t lower her to the
near-bottom, like we normally do.
While on one of these deep stations, the secondary conductimeter sensor of the
stopped working (not to worry, this is why we have 2 installed). The conductimeter
provides a continuous profile of salinity, so it is an essential piece of equipment in
the package. We replaced the faulty conductimeter while in transit to our following
station, without any waste of ship time. A couple of days later we had to replace the
primary conductimeter as well - so we are now working with two brand new ones.
At the same time, the oxygen sensor started to show increased scatter. Normally,
only one oxygen sensor is installed, so it needs to provide a clean profile. We did
some minor install modifications, tested all our spares, and are now back with a
reliably functioning oxygen sensor.
We have just sailed past Cocos Islands. These small coral atolls belong to
and are located southwest of Christmas Island (see picture below). Unfortunately
we were just a little too far to be able to see the atolls, but that didn’t stop us from
learning about them. Apparently, two British captains (one Scottish, one English)
rediscovered them in the 19th century and planned to settle there. One came with a
harem of 40 women. The second came shortly after with his family for a more
traditional settlement. As you can imagine, these 2 groups didn’t get along,
especially after some women from the harem started deserting to live with the
sailors of the second group. Another interesting fact? Charles Darwin stopped there
during his journey of discovery aboard the Beagle. Participating in a GO-SHIP cruise
always brings with it the chance to learn interesting little facts about remote parts
of our planet!