February 7, 2003

How did we find the gyre?
by Frank Hernandez
One of the most utilized instruments in oceanographic studies is the CTD. The acronym stands for "conductivity-temperature-depth", which are the primary physical variables the instrument can measure and record. A CTD, therefore, allows us to characterize and describe the water under the ship. The instrument can simply be placed over the side of a boat and lowered to a specified depth (vertical data recording), or it can be configured for towing behind a vessel (horizontal and vertical data recording). Our CTD is a SBE19 (Seabird Electronics, Inc.) and is designed for vertical data recording, or profiling. With measures of temperature, depth and conductivity, we are able to use equations to calculate other important parameters, particularly salinity and density.

(above: the SBE19, photo courtesy of Sea Bird Electronics)

The main components of the CTD are housed in a cylindrical, water-tight housing. Inside of this compartment are the electronics needed to acquire the data, a microprocessor that controls data acquisition, memory to store the data and batteries to power the instrument. Attached to the outside of the main housing are the primary sensors: pressure, temperature and conductivity. Since pressure changes with depth (higher pressure at deeper depths), data collected with the pressure sensor are used to calculate the depth at which the CTD is recording. A thermistor (thermometer) is used to measure temperature. The specific conductivity sensor is composed of a glass tube, or cell, with three electrodes inside. As water flows through the cell, the sensor measures the electrical resistance in the fluid between the center electrode and the end pair electrodes. This value is the specific conductivity, which in and of itself is not of great interest. However, these data are used by the CTD to calculate salinity, which is usually one of the major parameters of interest. A pump is attached to the specific conductivity cell to make sure adequate water passes over the sensor. The main housing unit, along with the sensors, are surrounded by a steel frame with a metal eye ring on the top for attaching a cable. Other instruments, such as oxygen sensors and fluorometers (which measure chlorophyl concentrations,) can be added to this basic setup. The CTD used on this cruise, for example, has a light sensor attached to it, in order to measure photosenthetically active radiation, or PAR. PAR is that part of the energy spectrum that is important for photosynthesis, and is composed primarily of the same wavelengths as the visible spectrum.

Deployment of the CTD actually begins inside the ship's dry lab. A communication cable on the CTD is plugged into a computer with the CTD software. We are then able to program the CTD to measure what we want and at what rate we want. On our cruise, we want readings for all of our sensors (temperature, pressure, conductivity and light) at 2 Hz. That means the sensors scan for data every 0.5 seconds, or two times per second. Once these commands are entered, the CTD is initialized and ready to record data. Now the deployment is mainly in the hands of the ship's crew. A cable is attached to the eye ring on the CTD frame. Since the instrument is likely to encounter different water masses and currents as it is being lowered, a heavy weight (75 pounds) is attached to the bottom of the CTD frame. This will help keep the CTD wire as straight as possible as it descends. The CTD is turned on and the instrument is boomed over the side of the boat and lowered into the water. The instrument is lowered by winch to the desired depth at about 10 meters per minute. At this rate, it takes approximately 20 minutes to reach a depth of 200 meters. Once it reaches the desired depth, the CTD is retrieved and returned to the ship's dry lab. There, the communication cable is connected to the CTD once again and the data is downloaded and saved onto the computer's hard drive.

The file is now in a data storage format (hexadecimal), which is not a very good format for calculations and data manipulation. The data, therefore, must be processed with the CTD's software to make any sense. There are several processing steps that help put the data in a useable format. One step, for example, allows us to average all of the scans into depth bins. At a sampling rate of two scans per second, we collect a lot of data points during one CTD cast. We are not interested in changes that may occur on small scales (centimeters). Rather, we are using the computer's software to average data into one meter depth bins. All scans taken between 0 and 1 meters depth, for example, are averaged together and one value is reported for that depth range. Another processing step is used to convert the data from hexadecimal format to a more user-friendly format. Yet another step removes data points that the software deems "bad scans", perhaps due to a sensor glitch of some kind. Using these and other steps, we are able to "clean up" the data for further analyses.

So how does the CTD data help us get an idea of what's going on beneath us? First of all, we need more than one CTD cast to give us any useful information. One CTD cast gives us vertical information for one point in the ocean. We therefore take multiple CTD casts along a transect, or a line, that we've mapped out. If we have information for five points along a line, we can use statistical and graphic software to interpolate (or estimate) the data that is between these points. The attached figure is a contour plot of one of our transects across the sampling area. The only "real" data points are those along the sampling stations (22-26). These stations are roughly five miles apart, so the information in between is interpolated. The data are then color-coded to represent different values under the sea surface.

Let's use the temperature contour plot (top panel) as an example. We can see that temperature values along our transect range from 10 °C (dark blue) to about 21 °C (dark red). We can see that changes in temperature structure in our transect happen vertically more so than horizontally. That is, water temperature does not change very much as you move horizontally along a certain depth, but it does change dramatically as you move vertically at a single point in the ocean. Specifically in this image, we can see several features important to our research efforts. In the top left hand corner of the plot (offshore edge of transect), we can see a warm (dark red) water mass. That is the edge of the Gulf Stream. The warm water mass on the top right hand corner (inshore edge of transect) is a filament from the gyre that we are following. A phenomenon known to occur with gyre systems is the upwelling of cooler water from below. We can see that clearly in the center of the plot. As the gyre spins, cooler water rises, as indicated by the peaks in contour lines, and changes in temperature occur over a much shorter depth range, as indicated by the closeness of the contour lines at the peaks. This "snapshot" of the water masses below, generated by CTD data, allow us to closely follow the gyre as it moves along the shelf. Using these plots, we are also able to confirm that we are indeed sampling the desired water mass.

Career of the day

Mike Green, Biological Science Technician – Fisheries, Beaufort lab.

Mike has 2 associates degrees. One is in science, the other in fisheries management. Mike was brought up in the outdoors and has always enjoyed it. He decided one day that he needed a career change from being a drummer in a band, so he went back to school. While working on his science degree he enjoyed the ichthyoplankton and zoology classes and decided to go on and get his fisheries management degree as well.

While in school, Mike volunteered with numerous agencies. He worked for State Wildlife Management with endangered species, Camp Lejeune on a deer harvest project, Ft. Bragg and a turtle rescue project to name a few. When he graduated he sent out resumes and applications and was hired by NOAA’s National Marine Fisheries Service. He still loves it.

His duties include supporting the principal investigators on cruises such as this one. This entails operating the MOCNESS, operating boats, repairing and maintaining equipment and compiling and analyzing data. He likes working with different equipment, likes the people he works with and enjoys seeing the final product when all the data has been analyzed and the results are published. Mike also sketches many of the fish species that he works with. There’s nothing he doesn’t like about his job but his advice would be to make sure you don’t get seasick, are able to live in small quarters, and are able to be away from friends and family for weeks at a time.

Mike said he would be a sunfish (mola mola) if he had a choice. He said they’re unique, they range around the world, and no one could dislike the sunfish because it’s so pretty.