Measuring ocean currents with sound!

Well we are more than 5 weeks into the cruise and only 8 days to go! We
are currently at 28 degrees South heading into tropical waters and it
definitely feels like it! After more than a month of clouds, rain and
grey we are finally being rewarded with some warmth and sunshine.
Luckily there is still a nice sea breeze blowing and a few clouds so it
isn't too hot. This week I am going to write a bit about a very cool
instrument that I have been running on board the ship, the Lowered
Acoustic Doppler Current Profiler (LADCP; the blue and yellow instrument
in the photo). That's a bit of a mouthful isn't it?

What is an LADCP?
Let's break it down: Lowered = the instrument is lowered in the ocean
with the CTD package; Acoustic= sound; Doppler = refers to the Doppler
shift. Current = ocean current; Profiler = measures a vertical profile.
So the LADCP measures ocean velocities using sound.

How does it work?
The Doppler effect is the change in frequency of a wave when the
receiver is moving relative to the source of the wave. This often
happens when a vehicle with a siren passes by. As it approaches the
sound waves are "bunched up" and the sound appears higher pitched to
someone listening than when the vehicle is moving away and the waves
appear more "spread out". In the case of the LADCP, it emits a ping
sound at a set interval (usually once or twice a second) into the water,
which is reflected by particles in the water about the same size as the
wavelength of the sound wave, usually plankton and other small
organisms. This reflected wave is read by the LADCP instrument and the
difference in frequency between the emitted wave and the reflected wave
indicates the motion of the water relative to the instrument. This is
complicated by the fact that the LADCP instrument is not motionless but
it moving with the ship and up and down with wave motions. To get
absolute ocean velocities this motion needs to be calculated and removed
from the Doppler signal. The LADCP contains a compass and sensors to
measure the pitch and roll of the instrument so its motion can be
calculated. There are several different methods used to get absolute
horizontal velocities and they each have pros and cons. There is even
some recent work on a method using LADCP data to calculate vertical
velocities in the ocean, which are very difficult to measure because
vertical velocities tend to be very small, compared to horizontal
velocities and vertical wave motion.
The Palmer and most other research ships also have an ADCP (not lowered
with the CTD) mounted on the ship bottom, which constantly measures
velocities in the upper ocean as the ship moves. ADCP's can be used in
many other ways, including measuring the discharge from rivers, locating
underwater "tornadoes" that might damage deep water oil drilling
activities, measuring flow through sewer pipes (gross but important!)
and measuring flow of meltwater from icebergs.

What do the results look like?
The resulting LADCP velocity profile can be noisy and have quite large
errors due to difficulties with removing the ships motion. However, you
can see the speed and direction of velocities from the surface to the
seafloor. The shipboard mounted ADCP measures surface currents so we can
see a map of the surface currents along the ship track and see when we
are crossing sharp fronts with strong currents and other features.

Why measure ocean velocities?
Finally, why do we measure ocean velocities in this complicated way?
Well it is actually very difficult to measure instantaneous velocities
in the ocean. Average velocities can be calculated from the trajectories
of drifters and floats. Also we can use a relationship between the
density gradient in the ocean to velocity to determine geostrophic
velocities in the ocean. But these methods are generally only useful
for very large-scale ocean currents. LADCP data is useful to see
detailed structure in velocity profiles and see how the velocity varies
over time and in space. This is important to understanding physical
processes in the ocean on small and larger scales. It can also be used
to calculate transports of water, which are important for determining
the amount of heat, nutrients, carbon and other properties transported
by the ocean.

Stay tuned next week, as I'll be wrapping up when we arrive in Tahiti
and will be sharing a video of the cruise that I have been working on
with some other scientists on the ship!

Happy Easter from the Palmer!

Happy Easter from the Southern Ocean!! Not only are we celebrating
Easter today, but also crossing 40 degrees South latitude heading toward
the tropics!
We had a lot of excitement today- some Easter bunnies came out to
deploy the CTD, there were baskets of eggs, bubbles, and a special hunt
for rainbow wombat droppings filled with candy!

Challenges at Sea

I can't believe that we are already more than 4 weeks into this cruise
already! Once you get into a rhythm and are busy working time really
flies by! Unfortunately though, we are only halfway along the P16S line
to Tahiti and we only have just over 2 weeks to go (remember we did have
11 days of steaming from Hobart before we started the line). This means
that we are quite a bit behind schedule and have had to modify the plan
to sample every 2/3rds of a degree instead of every half degree of
latitude. We have had a series of challenges that have meant a lot of
waiting and troubleshooting that put us behind schedule. This isn't
because the cruise was poorly planned or because of big mistakes, it is
just the nature of working in such a remote and challenging environment.
Our range of options to respond to problems is limited to what materials
and skills we have available on the ship. Luckily, the crew and the
science party have an enormous amount of experience and expertise and so
we have managed to overcome all of the difficulties encountered so far.

Weather has been the biggest challenge so far. The Southern Ocean, no
matter what season, is one of the wildest parts of the global ocean with
intense westerly winds bringing strong storm systems and the world's
largest and strongest current, the Antarctic Circumpolar Current. The
weather also varies across seasons with stronger winds in winter than in
summer. This has meant that historically, most shipboard sampling of
the Southern Ocean has happened in summer (Dec-Jan in the Southern
Hemisphere) meaning that there is a bias in the existing data toward the
calmer (relatively!) summer conditions. This cruise is happening in
Autumn so the weather is probably worse than we might have had in summer
but it is important to get measurements at all times of the year.

We encountered what seemed to be an endless string of low-pressure
systems during the first few weeks of the cruise. Storms like these
happen all the time in this part of the world but it is unusual to have
so many for a continuous period of time. Many people on the ship who
have spend hundreds of days, or even years at sea commented that this
was the worst run of bad weather they had ever had on a cruise.
Unfortunately for us, the weather was too rough to do CTD profiles
during this time so we were stuck with the options of staying put to
wait out the weather, or moving elsewhere in the hope that it might be
better. Another complication is that although we receive weather
predictions from several sources, the predictions are not very accurate
in this region mostly due to the lack of good data included in the
weather forecasting models. So we know what the current weather is but
we really have a very limited idea of what will happen in the next few
hours and next few days. In the end, we spend quite a bit of time going
in circles, fleeing north and then backtracking south rather than just
sitting still to try to get to a spot where we could work and also to
minimize the rocking of the ship.

Another major problem we have encountered on the cruise is damage to
the wire that is used to lower the CTD into the water and transmits data
from the CTD to the ship. The wire has to be able to withstand very high
tensions and large wave motion can cause spikes in the tension, which
puts the wire at risk of damage. We discovered on a very deep cast that
the wire had been damaged at around 4 km along the length of the wire
and after careful examination the crew agreed that the wire was too
damaged to work with as we could risk losing data or losing the whole
rosette at the bottom of the ocean! The ship has a second winch with an
undamaged wire outside so we were left with two options: 1) transfer all
of the 10 km of wire off the spool and roll on the wire from the outside
winch or 2) move the CTD from the inside winch to the outside winch.
Both of these were ambitious tasks and could be very time consuming.
After all of our delays due to weather we couldn't afford to waste any
more time if we want to make it to Tahiti on time! In the end, a
decision was made to attach the CTD to both the inside and outside wires
at the same time and let out the CTD on one wire and bring it back in on
the other (the outside winch is only about 20 feet further down the boat
than the inside winch). No one on the ship had ever done a trick like
this before but it worked perfectly and only took a few minutes! It was
very impressive and I have video footage that I will share at the end of
the cruise. We had a few minor problems with the electronics after that
but once that was resolved nothing has gone wrong for many days! From
now on we will be sampling outside which brings a whole new set of
challenges (getting wet!) but it is great to get outside more,
especially as we move north and the weather improves. Fingers crossed we
don't have any more troubles and it'll be smooth sailing the rest of the
way to Tahiti!

Fun at Sea

We are now in the middle of the Antarctic Circumpolar Current at 55°S,
150°W! We had A LOT of bad weather last week but we have managed to make
up to station 25 on the P16S line northward (with a bit of a detour back
south after a storm) and only skipped two stations along the way. Last
week I wrote about the science goals on our research cruise and how we
go about taking measurements of the ocean. This week I am going to talk
about what we do when we are not doing science: having fun! A research
vessel can be a very isolating place, as we can't go outside for a walk
in the park on the weekend, in fact we don't have weekends at all. We
are stuck for 50 days in a confined space with the same people. We also
have very limited connection to the Internet (15 MB each daily which
will last about 3 checks of your Facebook feed…) and only one satellite
phone shared between everyone on board. So we don't have the normal
forms of entertainment that we are used to having on land. However, the
Palmer is well equipped to keep it's crew and scientists entertained and
happy and we find MANY ways to entertain ourselves while we are working
and in our free time. In the past week especially we have had a lot of
foul weather, which meant we couldn't collect any samples, and so there
were many, many hours of free time. Here is an outline of some (but not
a comprehensive list) of the ways we keep ourselves busy and keep morale
high.

Exercise:
Being in such a confined space for a long time means it is very
important for us to get some exercise in the gym to stay fit and healthy
(and for some of us to stay sane!). The ship has a very nice little gym
(in comparison to other ship gym's that I have seen) with a bike, rowing
machine, step machine and a weight machine and dumbbells. There is also
a treadmill and…. wait for it… a sauna!! The sauna is wonderful and
relaxing, especially in a polar climate!

Motivation to work out can be hard out here, especially when you are
tired from working and used to exercising outside in the sunshine (not
much of that in the Southern Ocean). So along with a few other students
on the cruise, I came up with a plan to have a team competition to see
which team could travel the furthest distance on the bike, treadmill,
rowing machine or step machine. We named the competition the "Meters
Club" and the Chief and Co-chief scientists lead the two teams.
Currently the "P16Stacked" team is in the lead but the "Kill-ometers"
aren't far behind and are catching up! The prize for the winning team is
as yet undecided but may involve a chilled beverage when we get off the
ship in Tahiti.

Movies:
While there are some people on the ship training for a marathon, some
of us are training for ultimate movie marathons! There is a movie lounge
on the ship with extremely comfortable couches (more comfortable than
the beds!), a big screen and an enormous collection of movies on hard
drive, DVD and even VHS!! Also there is a Fussball table and a few
guitars (more about music on board soon). There are movies being played
pretty much 24 hours, although a lot of the time there are more of us
sleeping on the comfy couches in the lounge than actually watching
movies!

Games:
Board games, cards and puzzles are another popular form of
entertainment at sea. One of the crew members prints a large copy of the
New York Times daily crossword for us to work on as a group but it is
usually finished in less than 10 minutes and if you aren't there in time
you'll miss out (sometimes we do multiple a day so everyone gets their
chance)! There are also a few game tournaments going on: the "Cribbage:
Tournament of Champions" (en.wikipedia.org/wiki/Cribbage if you are like
me and don't know a lot about cards) and the "World Series of Cornhole"
which is yet to commence. There have been a lot of intense Cribbage
games going on around the place!

Music:
I have been amazed by the amount of musical talent of the scientists
and crew on board the RV Palmer! Mike (NASA scientist) is lead singer
of a band in D.C., Lynne (Chief Scientist) is a pianist on the side,
Barry (Electrical technician) repairs guitars and plays well and John
Calderwood (CTD deck leader) has the voice of an angel! There are many
more talented musicians amongst us (or at least a lot of us that really
enjoy music, especially singing) and my fellow night shift CTD
watchstander's Tyler and Isa and I do sing-alongs all night long for fun
and to stay awake. I'm not sure how much our talent is appreciated
though as George (Electrical technician) ran into the room and shot us
with a Nerf gun (en.wikipedia.org/wiki/Nerf) one morning after we woke
him with our angelic singing. Mike brought recording equipment on board,
which is totally awesome! I've been recording a lot of video footage
using Go Pro cameras and we are working on some songs to go with the
video so stay tuned for the resulting masterpiece at the end of the
cruise!

Science at Sea

It's now two weeks since we left Hobart on the R/V Palmer. We crossed
the Antarctic Circle at 66.33°S to the start of the CLIVAR P16S line at
67°S, 150°W a few days ago. Now we are moving slowly northward along the
P16S line, sampling at stations up to four times a day, weather
permitting. So what is it that we are measuring in the ocean and how
are we measuring it?

What:
This cruise is part of a large international repeat hydrography program
(http://www.go-ship.org) that collects measurements along lines across
all of the global ocean basins, repeating the same lines every decade to
see how the ocean is changing. The P16S line was completed in 1991 and
again in 2005. We measure the physical and chemical properties in the
ocean from the surface to the bottom, and this global data is essential
for oceanographers to understand ocean circulation and the carbon cycle
and how it is changing over time. It is also very important to have
high quality ocean measurements to help calibrate sensors and to
validate and improve the performance of global climate models.

The second important role of this cruise is to deploy 12 autonomous
Argo (www.argo.net) profiling floats which drift along with ocean
currents below the ocean surface and come up to the surface every 10
days and send temperature, salinity and pressure data to satellites. We
are also deploying surface drifters that send their location to
satellites to measure currents and a team of NASA scientists is making
measurements of chlorophyll (an indication of how productive the water
is) to validate measurements of ocean color made by satellites.

How:
These sound pretty straightforward right? But it takes a whole team of
scientists, plus the crew members on board the ship many hours of
planning and hard work to collect all these measurements. The ship
operates 24 hours a day (ships are very expensive to run, so we use all
the time we can get!!) so everyone is assigned to a 12 hours shift. The
chief and co-chief scientists (Lynne Talley and Brendan Carter) are in
charge of all the science operations, and communicate with the ship's
Captain and the MPC (Main Point of Contact) to decide where and when the
ship will stop to take measurements.

All of the ocean measurements and water samples are collected using a
CTD (Conductivity, temperature and pressure) mounted on a rosette, which
has 36 bottles that can be closed at specified depths. The winch
operator supervised by the MT's (Marine Techs) lower the rosette into
the water, while members of the science team operate the CTD. This is
where I come in. I work as a CTD watchstander with 2 other graduate
students and our job is to prep the rosette before it goes into the
water, control collection of water samples while the CTD is in the water
and manage the sampling after the CTD is back on deck. When the rosette
is back on the ship, it's a race for water samples! There can be more
than a dozen different chemical properties being measured at a station
and so everyone is competing to collect water samples and it can get
very crowded and confused. One of the other CTD watchstander's or myself
acts as the Sample Cop, policing everyone to make sure they get water
from the right bottle in the right order. It can get very wet while
sampling and on many ships this happens outside on the deck where it can
be very cold. However, the Palmer is super fancy and designed for
working in polar conditions so the CTD is deployed and recovered from a
room with an enormous door so we are able to sample comfortably from
inside (although the sample water can still be below 0°C because the
salt in the water prevents it from freezing at that temperature!). All
the samples are taken to different labs on the ship, some inside the
ship and some on portable containers on the decks outside.

Finally, after the rosette is back on deck and we are leaving a station
we can deploy floats or drifters over the side of the ship. These are
(carefully) thrown overboard and are not recovered. The NASA team also
collects measurements with instruments deployed over the side and the
back deck.

BUT a research cruise is not all hard work! When we are waiting to
arrive at the next station, or the weather is too rough to do anything
we find ourselves with a LOT of spare time on our hands and not many
places to go. Next week I'll be writing about the many ways we spend our
time and keep ourselves entertained on the ship!

(Photos courtesy of Lynne Talley)