Prepping for Round Two

We just concluded a 24-hour transit to our new location with a cast from the ship's CTD and a midnight deployment of SWIMS to start surveying the area. During the transit, even though we weren’t actively gathering new data with most of our instruments, we still had plenty to do. As thoroughly as we tested everything before we left, the ocean is a harsh and unpredictable place, and there are always improvements to be made to insure that we make the most of our time out here, whether it’s cleaning an instrument, opening it up to check the circuits, or fixing glitches in our processing programs.  

One of the projects we undertook was stabilizing the ADCP attached to the bottom of the buoy chain. The design of the cage around the ADCP was such that the battery and instrument weren’t being held in place as tightly as they needed to be, threatening the power cable and reliability of the measurements. The new-and-improved design increases the friction between the clamps and the frame, and secures the power cable more tightly to the instrument.

The ADCP (black) and its battery (blue) in the holding cage. It hangs vertically from the buoy chain, which means preparing for strong vertical motions is important for keeping the instrument working. (Photo credit: Rosalind Echols)

Every time we use the big ship-board CTD with the rosette (a set of 24 bottles used to gather water samples from different depths for biological and chemical tests), we have to empty it, reset all the firing pins that close the bottles, and rinse it thoroughly with fresh water. In fact, every time we take an instrument out of the water we perform this fresh water rinse to minimize the corrosive effects of the salt water. One of the primary challenges oceanographers face when trying to make measurements in the ocean is that many of the interesting characteristics of the ocean (it is deep, turbulent, and salty, for example) are really bad for electronics, metals, and so on. Many of the engineering challenges are comparable to launching something into space.

Our biologist, Kate Kouba (from Portland State University) resets firing pins at the top of the CTD rosette after gathering water samples. (Photo credit: Rosalind Echols)

Before we put everything back in the water, we are also looking at the data we’ve already gathered, both to keep an eye out for interesting physics but also to see if there were any bizarre data standouts that might suggest a malfunctioning instrument. Sometimes, weird, unexpected features of the data are interesting phenomena, but they can also mean that a probe isn’t working correctly, so closely monitoring the data as it comes in is an important step in making sure that the data we come home is meaningful.

Looking at Data, Episode 1

After approximately five days of surveying our present area, following the floats and buoy, we’ve been able to gather some really interesting preliminary data. Today we’re going to talk about one particular characteristic of the float data that has caught our attention. As mentioned previously, each of the floats is freely drifting with the currents, profiling from approximately 200 meters depth up to the surface. On the way up, the floats gather temperature, conductivity, pressure, and velocity measurements. The conductivity is then converted into a value for salinity, or how “salty” the water is.  

In much of the world’s ocean, some portion of the top of the ocean gets thoroughly mixed by atmospheric forcing, and so up to some depth, the temperature and salinity remain relatively constant. This portion of the ocean is referred to as the mixed layer and this description is most true for regions or times with regular strong mixing events (such as a storm) without a lot of heat or freshwater input. For example, in the North Atlantic Ocean in winter, where there are incredibly strong storms with relatively weak shortwave radiation from the sun, the mixed layer can be several hundred meters deep. However, in regions where there is a daily cycle of intense solar radiation or extreme rain events under low wind conditions, there is very little mixing and so the ocean can be substantially stratified all the way up to the surface because substantial changes to the temperature or salinity are occurring at the surface that can’t be mixed down.

Depth profile of temperature (red), salinity (blue), and density (green). Notice the twists and turns in each profile between 0m and 50m, suggesting substantial changes in each quantity. 

 Climatological (average) data for the region we are currently in suggests that there should be a winter mixed layer on the order of 100 meters deep. However, the float data we have gathered thus far shows substantial changes in both temperature and salinity beginning from the near-surface. The profile shown here illustrates that (temperature is in red, salinity in blue, and density of the water in green). If there was a well-defined mixed layer, we would see the top section of each of these profiles almost completely vertical and uniform. What we actually see here is that the top 50 meters has variations in all three variables. If you look carefully, you can see that between roughly 50 and 125 meters there is a somewhat uniform section (or at least less variation), which could be the remnants of a previous mixed layer on top of which other effects have been superimposed.

Determining the exact cause of this pattern of surface stratification is difficult, and will certainly require more in depth analysis of our data from multiple sources. Increased temperature at the surface (which appears regularly in different profiles) could be due to strong solar radiation effects (which penetrate some distance into the water column) that are not being fully mixed in, for example. It seems unlikely that this is happening in this case because the near-surface temperature doesn’t show the exponential decay associated with radiation (because the floats typically stop measuring around 10 meters depth, we can’t rule it out yet). Another possibility is that the float is moving between different water masses as it profiles due to differences in horizontal velocity at different depths. A third option is that these variations could also be due to interleaving of pre-existing parcels of water from the type of restratification event we are looking for. At this early stage, it is hard to know for sure.

One step in the float retrieval process. Making sure the instruments at the top don't hit the side of the ship is of paramount importance. (Photo credit: Rosalind Echols)

Today we’re picking up the buoy and floats and heading north. 70 and sunny has been a lovely respite from winter weather, but in order to find the kinds of mixing events that will allow us to observe restratification processes in real time, we need to go in search of a front near some more interesting weather.

Bringing the buoy alongside the ship, while protecting the sensors at the top. (Photo credit: Rosalind Echols)

Floats off the Starboard Bow!

On Tuesday, we deployed 9 EM-APEX profiling floats in a tightly spaced array, all within a 0.4 nautical mile square box. Since then, they’ve been drifting freely as they profile up and down to 200 meters roughly every 90 minutes. These floats have temperature, conductivity, and pressure sensors on them, as well as electrodes that allow us to measure velocity. Roughly half of them also have microstructure instruments, which allow us to measure very small fluctuations in temperature to interpret fine scale motions. Each time they surface, they send back their data and position via satellites so we can monitor them. Since deployment, they’ve drifted apart along a north-south line and are now spaced out across about 4 nautical miles.

Float and ship paths over time. The red triangle that looks like the ship ran it over is probably the float we saw with the spotlight at 3am.

We’ve been following the floats with the ship, moving around and between them as they drift, gathering additional data using our towed instrument. You can see the paths of the floats in the image here (green dots and lines), as well as the ship track (red line). It’s really important to keep track of the most recent float positions relative to the float track to make sure we don’t run them over. Given how tiny the floats are in the vast ocean, this might seem like an unlikely event. Using the spotlights from the bridge at 3 o’clock this morning, however, we spotted two that had surfaced and were reporting to the satellites, so it’s not as improbable as one might think (when preparing the floats pre-cruise, we taped the tops with highly reflective tape, so they show up spectacularly with the spotlight). The good news is that we are gathering extensive data about the region surrounding the floats, and have so far managed to dodge all of the ones that have surfaced near the ship.

Highly-reflective tape on the casing of the microsrtucture instrument. (Photo credit: Rosalind Echols)

 The path the floats are following results from a combination of the mean current of the ocean as they profile, which seems to be generally toward the southeast at the moment, and what are termed “inertial motions”, which is an oscillatory motion that occurs due to the rotation of the earth. We see this show up as periods of speeding up and slowing down in the float positions.

Buoy Away

 One of the first tasks once we selected a starting point for our measurements was to deploy a drifting buoy. This buoy will produce surface measurements of wind, rain, and heat flux using the instruments mounted at the top of the platform as well as subsurface measurements of current, salinity, and temperature using a series of instruments suspended on chains below it. One of the benefits of this type of instrument is that it produces co-located atmospheric and oceanic measurements, which allows us to look at the conditions in both places simultaneously and see how the atmospheric conditions are forcing the ocean. Below the surface, this buoy has two upward-facing Acoustic Doppler Current Profilers (ADCP) and five Conductivity, Temperature, and Depth (CTD) instruments.

Members of the Sikuliaq crew and science team stabilizing the buoy on deck (Photo credit: Rosalind Echols)

Each of these instruments, chains, and attachment mechanisms was packed and shipped individually, so prior to deployment we spent several hours planning out each of the attachment points and organizing the pieces so that the process of getting everything in the water would be as smooth as possible. Deploying the buoy was a complex operation involving close coordination of quite a few people and tools, including the large starboard crane and numerous tie lines and slings. As with all shipboard operations, safety in these situations is of paramount importance. Everyone involved wears a hard hat and life preserver, and communication is clear and unambiguous (getting the buoy in the water is important, but making sure no one falls into the Pacific Ocean or gets hit in the head with the crane always supersedes that). 

Eric Boget from the science team staging the bottom-most instrument, an ADCP, on deck (Photo credit: Rosalind Echols)

Starting with the deepest instrument, we suspended each instrument from its topmost point using a crane, gradually lowered it into the water until the attachment point was at deck level, secured the attachment point to the ship using tie lines, and then prepared the next section of chain and instrument. Thus, one at a time, we built the subsurface instrument chain from the bottom up until we reached the link that would attach to the bottom of the buoy. Getting the 10-foot tall buoy into the water was unquestionably the most exciting part, but both the Sikuliaq team and the science team have a lot of experience with these processes, so it ultimately went quite well.  (As tricky as this was, deploying buoys and moorings is a fairly standard practice in oceanography, as their use is widespread. This one was relatively easy since it isn’t moored and the deepest instrument is only about 80 meters below the surface, not the full depth of the ocean).

Preparing a CTD for deployment, with stabilizing tie-line at the ready (Photo credit: Rosalind Echols)

 This particular buoy is designed to drift along with subsurface currents, unlike the moored buoys that are used to gather long time series at a specific location. In order to accomplish this drifting, the buoy has a series of plastic vanes or “X-wings” suspended roughly 60 meters below it, that create substantial drag on the subsurface chain. (A longstanding engineering challenging in oceanography is achieving a design for drifters that will allow them to follow a parcel of water; this particular design seems to work pretty well). Right now, it has been drifting along following a group of floats we deployed at about the same time, which is the best situation we could hope for.

X-wings hanging from the crane during deployment (Photo credit: Rosalind Echols)

If you want to check out the data the buoy is sending back, see the website hosted at the Woods Hole Oceanographic Institute (one of the collaborators on this project):

http://buoy3.whoi.edu/currentprojects/SMILE/smiledata.html

Successfully deployed buoy drifting away! (Photo credit: Rosalind Echols)

Are we there yet?

After steaming briskly for three days out of Honolulu we have finally reached a potential measurement spot. How do we know when we’ve arrived? We know the phenomenon we want to look at: a sharp temperature front where the surface temperature changes rapidly in the horizontal direction[1]. These fronts can lead to dramatic shifts in density horizontally, which in turn contribute to restratifying[2] a previously well-mixed ocean. In other words, the site of these dramatic temperature changes indicates a location where something exciting might happen dynamically and better understanding these processes is the primary goal of this cruise. The North Pacific Subtropical Front spans a broad longitudinal range, and we know roughly where it will be, but the ocean is always moving and changing, so pinpointing the exact spot where it will be worth making extensive measurements poses a significant challenge at the outset. Deploying an array of 10+ floats is no trivial matter, so it is important to make sure that we put them in the right spot.

Animation of temperature near the front. Yellow/orange indicate warmer temperatures, green/blue are colder.

Fortunately, we have a couple of tools that help us figure out where to look. The first of these are satellite mounted sensors which allow us to capture a regular snapshot of the surface of the ocean. For this mission, we have been looking at data from two types of sensors: a passive microwave sensor that enables us to get a general (low-resolution) picture even when there is cloud coverage, and an infrared sensor that produces much higher resolution images but only works when the region is cloud free. The first picture here shows a false-color image animating the recent movement of the front. As you can see, it is very dynamic, so a feature that was there two days ago may not be there now. (A false-color image is so called because it assigns a color value to a measurement like temperature that does not have an inherent visible color. We use them a lot to visualize measurements because they make patterns stand out in way that a screenful of numbers might not. If you were actually to look at the ocean around us right now, you wouldn’t see this nice assortment of yellows and greens). This type of image allowed us to select a preliminary location to start measuring, but it is unlikely that the front will be in exactly the same position.

Once we arrived “on site”, we deployed the second scouting tool: our SWIMS towed body, which is a torpedo-shaped instrument attached to a cable that cycles up and down between roughly 10 meters and 200 meters depth as the ship moves, gathering salinity, temperature, pressure, oxygen, and fluorescence data at a rate of 24 Hz (that’s 24 measurements per second, which provides outstanding resolution in the data). Once deployed, we started along a cruise-track suggested by the recent state of the front, and have been attempting to drive back and forth across the front to pinpoint a location that will lead to interesting measurements. We will also use SWIMS once we’ve deployed the float array to gather data extensively between the floats and capture a detailed 3D map of the frontal region.

We’ve already noted some interesting features, and the midnight-to-noon shift has been nerding out about salinity plumes and massive temperature shifts, so it promises to be an exciting few days. We’ll be spending the next few blog posts talking about some of our other instruments to paint a more complete picture of the type of data we’re gathering.

---

[1] This is similar to an atmospheric front, which we hear a lot about in relation to weather. When meteorologists talk about a cold front, we know to expect a sudden drop in the temperature in a relatively short span.

[2] From top to bottom, the temperature, salinity, and density of the water can change enormously. This is what we mean when we talk about stratification. In looking at a profile of temperature (for example) we will often see regions of low stratification (temperature remains relatively constant) and regions of high stratification (temperature changes dramatically in only a few meters). A storm might mix the surface waters thoroughly, leading to low stratification. The dramatic density differences across a front can subsequently lead to restratification, with lower density waters sliding over high density waters. This process is incredibly complex.

Secure for Sea!

One of the joys and challenges of working on a ship like the R/V Sikuliaq is that each mission is different. That means each time a new science team gets on the ship, almost 100% of the science equipment has to be unloaded, set up, tested, and then secured so that it can be operated safely and effectively in sea conditions. Nothing is less exciting than watching the instrument you have spent months perfecting slide across the deck and smash into the wall, or less safe for the people on board.

The SWIMS winch bolted to the deck with cables running overhead. (Photo Credit: Rosalind Echols)

 Since unloading the container Tuesday, we’ve been engaged in a mad dash to test all of our equipment: it’s a lot easier to find a replacement part in Honolulu than in the middle of the North Pacific. For our “SWIMS” instrument that we’ll be towing behind the ship, this meant testing out the winch on both manual and remote control, turning on all the sensors and making sure the duplicate sensors were measuring the same thing, and getting cables run from the inside lab out to the deck without creating a massive trip hazard. For our EM-APEX floats, this meant putting together the wooden racks, making custom bungee cords, and continuously modifying the placement and design of the racks until we were sure that they wouldn’t tip over when the ship started rolling. And for our biologist Kate, that meant setting up her filter manifold so she can gather environmental samples of the cyanobacteria Prochlorococcus and prepare them to take back to her lab for DNA analysis.

The ADCP in the stowed position, firmly strapped to the ship (Photo Credit: Rosalind Echols)

 Every piece of equipment needs to be tested before it can be deployed the first time. Often times, this means installing it (like our ADCP current measurement instrument), testing it, realizing it’s not quite working, uninstalling it, taking it apart, trying to figure out what’s wrong, performing delicate operations to fix it, and then reinstalling it. We tested every single float yesterday to make sure it was communicating with the GPS satellites so we know where they are when they’re gathering data and can find them when we need to pick them up. If they aren’t, then we need to figure out how to fix it before we put them in the water. Suffice it to say, we have a lot of tools on board, including but not limited to: a circular saw, a soldering iron, four drills with more bits than most people will see in their lives, and countless straps, zip ties, rolls of electrical tape, etc.

Some instruments need to be set up almost from scratch. For example, Olga and I were assigned the task of building the bow chain. Three days ago, I didn’t even know what a bow chain was, aside from the inference I could draw from the name.  It turns out, it’s a reasonably short (20 meters, or about 60 feet) chain that gets hung from the front of the ship under low-speed conditions so we can look at the temperature and salinity near the surface. In this case, we had a box full of temperature, pressure, and salinity instruments, and we spent two days figuring out how to use the sensors, setting their measurement settings, testing them to make sure they were gathering reasonable data, and then figuring out how to attach them to the chain (turns out, this is an elaborate process of wrapping self-vulcanizing electrical tape around them: awesome to put on, nightmare to get off).

The bow chain coiled up and stowed for transit. Notice the elegant tape work. (Photo Credit: Rosalind Echols)

 Now, everything is secured firmly to the deck. Computers are strapped down, floats are bungee-d to their racks, and all racks and loose object are cinched to the deck. As the captain said, “Secure for Sea!” And with that, we’re off.  

All set up and Ready To Go

After loading all of our equipment, unpacking boxes, and setting up our instruments, we're ready to sail on Friday morning. The core instrumentation (EM-APEX profiling floats and SWIMS towed profiling package) is staged in place for easy access to the sea. This kind of thing is much easier to do when in port and the ship has not started rolling around.

EM-APEX floats in racks in the "Baltic Room." (Photo credit: James Girton)

SWIMS in place under the stern A-frame. (Photo credit: James Girton)