The backbone of Oceanography: The CTD

If there is one instrument that can be considered to be the core of sea-going oceanographic research, it’s the CTD package.  CTD stands for Conductivity, Temperature and Depth.  It’s deployed on a long wire from a winch into the depths of the ocean.  The measurements of temperature, salinity (estimated from conductivity) and depth allow us to study the stratification, or layering, of the ocean.  From this we can derive the currents, estimate mixing, and define how close to the surface the plankton are likely to be confined.

But our CTD, or “package” is special.  In addition to the core measurements, we are measuring optical properties of plankton to estimate their biomass and how happy they are.  We also have a little robot that take pictures of all the little beasties that fly by as the “packaging” goes down to 1,500 m, and then back to the surface.

Deploying the package is quite an ordeal.  The video below shows the process, sped up by 2000%.  Depending on how deep we are doing the cast, or deployment, the whole process take from 1-3 hours and involves a total of 6 people directly, plus the crew on the bridge who make sure we are always facing into the waves and not in danger.  A crewmember mans the winch, a technician on deck orchestrates the whole ordeal, and four scientists tend the lines and trip the bottles from the computer lab.

CTD_small from Peter Gaube on Vimeo.

Oh yeah, the bottle, argh, those bottles.  In total, there are 24 bottles on the CTD that collectively are called the “rosette.”  These bottles are held open until an electrical signal is sent down the cable to the CTD telling them to snap shut, trapping 10-liters of water in each bottle.  The water is brought to the surface and quickly sampled by a swarm of scientist who analyze the samples for a multitude of measurements, including the rate at which viruses attack plankton, how many plankton there are, how fast they are growing, how green they are, and who is around eating them and at what rate.

Overall, on this field program, and most oceanographic cruises, the backbone of our sampling comes from the CTD package.  I’m just the lucky guy who gets to send it overboard and make sure it comes back on deck safely and full of water.

The science and the weather ramp up

We’re heading east, a bit slower than before, but we are making good time.  The waves have come up a bit and the science is in full swing.  There are hundreds of measurements being made each hour, with countless continuous measurements of the ecosystems and physical environment, ranging from ocean properties to aresols and clouds.

The wind has come up a bit and we are on tail end of a storm.  We will be following the storm for the next two days, but luckily we move at 11 knots and the storm is much faster.  Nevertheless, we expect 30 foot (10 meter) seas tomorrow.  I took a short video of the back deck today before the seas came up and they had to limit our access to avoid anyone being washed off the deck.  At the time the video was taken, the seas were no larger than about 15 feet (5 meters).  I have a bet going with some of the crew that the incubators (the plastic water filled boxes seen strapped to the deck on the video) won’t make it until morning.


A normal day on the North Atlantic aboard the R/V Atlantis from Peter Gaube on Vimeo.

For a more “human” perspective on the research and people aboard, follow Nicole’s blog. Nicole is a reporter who decide, last minute, why not spend a month in the middle of the North Atlantic, in November.

On the Shelf

We’ve left port and are steaming east, northeast towards our first station. We will be crushing along the shelf at 11 knots for the next few days.

Our first station will be in an anticyclonic eddy near 50 degrees N (this is eddy A1 in the map below).

Map of our cruise and track and eddies.

Map of our cruise and track and eddies.

It’ going to take us another 6 days to get there. During the transit, samples are being continuously analyzed from the through-flow system which takes sea water from just below the surface and pumps it through a multitude of instruments.

We have also started to collect acoustic data using the ship’s acoustic doppler current profiler (ADCP). Although this is an echosouder that is designed to measure ocean currents, but I’ve also been looking at the amplitude of the acoustic backscattering in the water column. Yesterday around sunset, we observed the vertical migration of zooplankton and micronekton (little beasties that eat smaller beasties and plants) to the surface. It was exciting to watch this unfold in real-time.

At the same time, we started to see these little critteres in the imaging flow cytobot.


Check back in a day or two and I’ll post a short video giving a tour of the ship, our home for the next month.



Preparations are under way for the first cruise of the NAAMES field program.  We will be leaving from Cape Cod, MA, on November 5th to conduct a 30-day cruise into the North Atlantic.  Our mission is to characterize the marine ecosystem, from viruses to phytoplankton and all the way to mesopelac fish, with the goal of quantify the links between ocean variability (seasonality, eddies, and fronts), marine ecosystems and aeresols.  We will be on board the R/V Atlantis, a  142-foot, steel-hulled research vessel operated by the Woods Hole Oceanographic Insitition.  Our cruise will take us from Massachusetts, up to about 48 degrees north, and then down along 41 degrees west to approximately 38 degrees north (see planned cruies track below).

The planned expedition path overlaid on satellite estimate of the April mean climatological chlorophyll-a concentration. The NAAMES program time line is shown along the bottom of the figure with blue triangle indicating planned cruise dates.

My priamry research goals on this cruise will be to help quide the ship into mesoscale eddies using real-time satellite data.  I will also be deploying 20 surface drifters that we will use to study the movement of the eddies and help guide the NASA C-130 aircraft to fly over and sample the eddies.  In addtion, I will be collect multi-band acoustic observations to characterize deep scattering layers, layers of fish and squid below the sun-lit surface layer of the ocean (what is often called the “Twilight Zone”) inside and outside of the eddies.  This research is motivated by recent observations of the use of eddies by white sharks.  We have observed that sharks dive deeper and long in anticyclonic eddies when compared to cyclones (more info here).  Our hypothesis is that his is because deep scattering layers, where the sharks are feeding, are deeper and have higher biomass in anticyclones than cyclones.  I will try to post semi-regular updates during the cruise, so please come back and see what we are doing in the coming weeks.

You can see where the ship is here.


Schematic representation of isopycnal surfaces (black curves), white shark dives (wavy black line) and the DSL (pseudo-color) in (a) anticyclonic and (b) cyclonic eddies. The detection of the DSL by ship-board acoustics is indicated by the cone of solid downward propagating waves with the returned signal indicated by broken upward propagating waves.

Lydia swims 100 miles in less than 2 days into the core of a large anticyclone

Two days ago I reported that Lydia swam towards a large anticyclonic (clockwise rotating) eddy or meanders (see post here).  She approached this anticyclone and swam northeast along its periphery.  In the past 24 hours, Lydia has turned southeast of the previous anticyclone and has entered into the center of an adjacent anticyclonic eddy!  Analysis of her interaction with eddies during her trip offshore in the summer and fall of 2013 (find more info here) suggests that she prefers the cores of anticyclones over cyclones and makes repetitive deep dives (often over 800 meters) while in the cores of anticyclones.  Her activity in the past few days suggest that she might be honing in on some cues, be they potential prey, mates, or other, and is occupying the core of yet another anticyclonic eddy.

Lydia's track overlaid on a map of sea level anomaly. Anticyclonic eddies and meanders are shown as orange and red features, cyclonic eddies and meanders and blue and purple features.

Lydia’s track overlaid on a map of sea level anomaly. Anticyclonic eddies and meanders are shown as orange and red features, cyclonic eddies and meanders are blue and purple features.

This begs the questions; Do white sharks seek out anticyclonic eddies because they are areas that concentrate prey and/or potential mates, or do they prefer the warm water found in anticyclones while swimming elsewhere?  We’ll keep a close eye on where Lydia goes from here and hope to be able to address these questions in the future.

Has Lydia found a new eddy?

There has been lots of excitement recently about Lydia, an approximately 2,000 lbs. white shark, and her recent eastward movement offshore towards a region populated with large Gulf Stream eddies. Until yesterday, Lydia was moving mostly due east. But now, she has taken a sudden turn north. To investigate if her change in course could be cued by the presence of a particular eddy, I overlaid her track on a map of near-real time SSH (data from

Sure enough, her turn to the north puts her in the region of a large anticyclonic (clockwise rotation) eddy or meander, characterized by warm water low in chlorophyll (algae).  We’ll have to see where she goes next to find out of she decides to stay in this eddy, or keeps moving north-northeast.

Lydia's track (starting Nov 4th and ending Nov 24th, 2014) overlaid on a map of sea level anomaly (often referred to as SSH, sea level height).

Lydia’s track (starting Nov 4th and ending Nov 24th, 2014) overlaid on a map of sea level anomaly (often referred to as SSH, sea level height).  Track data courtesy of and SSH data of the Colorado Center for Astrodynamics Research (CCAR).