Topics Covered in this Lecture:
- The Story of the Clines
- Ways to Measure Ocean Properties
- The Heard Island Experiment
No, Betty Crocker hasn't turned in her apron for oceanography. Rather, the layer-cake model (and sometimes the fudge swirl) aptly describes the arrangement of the different water masses that make up the world ocean. These water masses are formed as a result of differences in density that are usually derived from differences in temperature and/or salinity. In today's lecture, we shall see how these density differences give structure to the oceans, and, in fact, drive the circulation of the deep-sea. Whereas upper ocean processes change over the seasonal cycle, deep-ocean circulation has a longer memory, and may be responsible for the 50-, 100-, and 500 year cycles that we experience in our climate. Finally, we take a look at a controversial experiment using sound waves to determine whether the oceans are warming in response to global warming. This experiment, known as the Heard Island experiment, involves creating an explosive "shot" to be heard around the world. Don't touch that dial!
The Story of the Clines
Before we take a look at Betty Crocker's recipe for the oceans, we need to understand how temperature and salinity affect the density of seawater. As we learned in a previous lecture, density is defined as the mass per unit volume of a substance. Remember the box of cereal?
Temperature affects density by changing the spacing between water molecules in a given volume of water. As temperature increases, water molecules get more energetic (they dance harder) and they tend to space out (i.e. have more space between them, not get dizzy). Thus, increases in temperature decrease the density of seawater. Conversely, as we learned about the formation of ice, decreases in temperature increase the density of seawater. Seawater density and temperature are said to have an inverse relationship; that is, as one goes up, the other goes down. Sometimes it is easier to remember this way.
Salinity, the amount of salts dissolved in seawater, affects density by adding mass to a given volume of seawater. As more and more salt is dissolved in a particular volume of water (as water evaporates or ice forms leaving the salts behind), the density of seawater increases. Thus, increases in salinity cause increases in density. Similarly, decreases in salinity, caused by the addition of freshwater (i.e. rain, snow, ice melt, river outflow) or seawater with a lower salinity, causes decreases in the density of seawater. Salinity and density are said to have a positive relationship; as one goes up the other goes up.
Both temperature and salinity affect the density of seawater and both may change at the same time. As indicated in your book, any given combination of temperature and salinity may produce the same density. The blue lines indicate isolines of density, regions where the density is the same.
This graph is also useful because it constrains the range of temperatures, salinities, and densities we may encounter in the ocean. To refresh your memory, temperature in the oceans ranges typically from -2 to 28 degrees C (except near hydrothermal vents or in inland seas, where temperatures may be higher), and salinity typically ranges between 28 to 41 parts per thousand (ppt), although at the mouths of rivers and in surface waters of the polar oceans in spring, salinity will be much lower. Recall that the highest salinity given here occurs in the uppermost extension of the Red Sea. The average salinity usually quoted by oceanographers is 35 ppt.
While we didn't mention it previously, the density of water is 1 gram per cubic centimeter (g/cm3) at 4 degrees C, its maximum density. At a salinity of 35 ppt, the density of seawater at 4 degrees C is 1.0278 g/cm3. While this may not seem like a big difference, its enough to move considerable volumes of water. Density is usually symbolized by the greek letter ro. Average density values for seawater range from 1.024 to 1.028 g/cm3. Again, while these differences may not seem large, these minute differences mean a lot when you're talking about tons of water.
Now that we have defined a whole bunch of terms and pretty much beat this ocean chemistry horse into shape, let's take a look at what all this means for the world oceans. Recall that temperature and salinity affect the density of seawater. Remember also that evaporation and precipitation, which differ in various regions of the ocean, are the principal factors that influence salinity. Obviously, heat exchange, driven by the seasonal cycle of the Earth's movement about the sun, also influences the density of seawater. As a result of these factors, a world ocean develops with a layered structure made up of water masses of different density. (Why didn't we just say that in the first place and skip all the tedious chemistry?! Torture, my friends, pure torture...).
Lighter, less dense lenses of water float on heavier, denser slabs of water. The end result is an ocean that varies in density both on horizontal scales (across the surface of the ocean or at a particular depth in the ocean) and on vertical scales (from the surface to the bottom of the sea).
Rapid changes in seawater density across horizontal scales form what are known as fronts. Fronts exist at the interface between any two water masses with distinct properties (that give it a unique density). The most common fronts occur where deep water masses are formed, but fronts may be present as a result of wind-driven upwelling and downwelling of ocean water, or where eddies form. We will talk more about upwelling, downwelling, and eddies when we talk about wind-driven processes in the sea.
On the vertical scale, as we examine density as a function of depth in the water column, we notice regions where the changes in density (or temperature or salinity) are quite rapid. As shown in your book, these areas of rapid change in density, temperature, and salinity are quite distinct. And now I must introduce you to the family of the clines. A cline is a region of rapid change. Areas of rapid change in density are called a pycnocline; areas of a rapid change in temperature is called a thermocline, and areas where salinity changes rapidly are known as the halocline.
It is quite likely that all of you are familiar with thermoclines. If you have ever wandered far from shore on a lake, you have probably noticed the cold water at your feet not so distant from the beach. What you experienced was a thermocline, a rapid change in temperature. This thermocline was formed as a result of a lens of warm surface water, heated by the summer sun, lying on top of a pool of deeper, colder water, a permanent year-round "resident" of large lakes and oceans.
This familiar example introduces us to the concept of the seasonal cycle of changes in the vertical structure of the water column. Seasonal changes in heating and cooling, particularly in temperate and polar climates, drive changes in the temperature and, hence, density structure of the water column. As surface waters heat, the thermocline becomes shallower and more distinct, as warm, less dense surface water sits on top of colder water. Alternatively, as surface waters cool, the resulting density change in the surface water causes it to sink and "mix" the water column. This mixing tends to deepen the thermocline or remove it entirely. This layer of water above the thermocline where mixing occurs, usually from the actions of the winds, is known as the mixed layer, and it is one of the most important concepts in determining the productivity of phytoplankton, as we will see in a few lectures.
Let's take a closer look at the seasonal cycle of temperature changes. In summer, shallow, warm water forms on top of deeper cold water. In fact, this figure indicates the presence of two thermoclines: the upper shallow thermocline, also called the mixed layer, and the lower deep thermocline. This shallow thermocline is often called the seasonal thermocline while the deeper thermocline is referred to as the permanent thermocline. In the fall, the surface waters begin to cool, causing the shallow thermocline to deepen. The mixed layer, the layer above the shallow thermocline influenced by the actions of the wind, also deepens. In the winter, the mixed layer may deepen as far as the permanent thermocline, removing the seasonal thermocline altogether. In areas where deep water forms, i.e. regions that form the deep waters of the world ocean, even the permanent thermocline may be absent. Finally, in the spring, as the surface waters warm again, the seasonal thermocline appears, and the mixed layer becomes shallow again.
Don't be fooled by the apparent simplicity of this cycle. Episodic events, such as cold fronts, storms, hurricanes, mesoscale eddies, upwelling, downwelling, coastal jets and filaments, can rapidly change the structure of the water column. Any process which affects a change in the density of the seawater--evaporation, precipitation, heat flux, or wind-driven circulation--will change the depth of the mixed layer. These changes in mixed layer depth, as mentioned above, have a profound effect on the productivity of plankton. Thus, we have a very good example of how physical factors can affect biological factors. In fact, these physical factors also change the chemistry of the seawater and, as phytoplankton grow, they in turn have a profound influence on physical processes, particularly light penetration. The ever-turning wheel of physical, chemical, geological, and biological processes makes an imprint on the water column as well.
Ways to Measure Ocean Properties
All this talk about temperature and salinity and density probably seems a bit esoteric without some idea of how these properties are measured in the a particular region of the ocean. Let's take a moment to see how it is that oceanographers acquire information at sea and what types of instruments are useful for these measurements.
First, let's get a few definitions straight. We have been talking about the water column without any regard for what that means. Simply, the water column is a vertical three-dimensional slice of water from the surface to the bottom. You may think of it as a column of water. In some sense, this word is oceanographer "slang" but it conveys a sense of how oceanographers typically measure the ocean, that is, from the top to the bottom. We've also been talking about vertical profiles. A vertical profile is a set of measurements of a particular property or set of properties as a function of depth in the water column. In other words, a vertical profile describes how a property changes with depth.
Oceanographers wanting to make measurements at sea must have some means of getting to sea. The easiest way is to take a ship, although planes, submarines, and satellites have also been used as a platform for making ocean measurements. Nonetheless, ships are the most efficient way to obtain large amounts of oceanographic data. Ships are not inexpensive, however. Ship time can vary in cost from $5000 per day for a small vessel to $15,000 per day for the largest vessels. A ten-day cruise, not a very long time to spend at sea, would cost $150,000. Imagine a 5-week cruise!
Most large ships can carry a complement of 25-35 scientists and 15-20 crew members to run the ship. Typically, scientists from a wide variety of oceanographic subdisciplines gather to study a particular ocean phenomenon. This multidisciplinary approach to oceanography allows scientists to combine their skills and expertise and share data to gain a more detailed understanding of the sea. Scientific expeditions, or cruises, may last from 5 days to 5 weeks, and scientists work around the clock to obtain data. While crew members aboard ships commonly work "four-on, eight-off", many oceanographers don't have this luxury. Twenty-hour workdays and sleep deprivation are a common occurrence for scientists at sea. As one galley waitress put it to me when I was complaining of lack of sleep, "You can sleep when you're dead!" Such is the sympathy for scientists at sea!
Still, life at sea can be compelling and exhilarating. You never know what you are going to experience. Life at sea has often been described as long periods of endless boredom punctuated by brief moments of utter terror. Storms, 30-foot waves, shipwrecks, sharks, whales, and the green flash are just a few of the things I have experienced at sea. Life is self-sufficient. Cut off from the rest of the world, the ship, its inhabitants, and the sea become the world. If you ever have a chance to spend some time at sea away from the confines of land, you will quickly realize what I mean!
Typical oceanographic cruises may have two components: a station-to-station survey; and an intensive single location study. A station-to-station survey involves steaming from one predetermined location to the next, with distances between stations depending on the phenomenon being observed or the type of study oceanographers wish to conduct. Station locations may be as little as 5 miles apart, or as far as 60 miles apart. Usually, a grid pattern is chosen over a particular region of the sea and this path is followed by the ship with station stops for collecting scientific information. On the other hand, oceanographers may wish to gain more detailed information, or understand how some process changes with time, in which case the ship will remain on station for 1-2 or more days. Intensive, single location studies are common for biological oceanographers; physical and chemical oceanographers typically favor large spatial surveys, although intensive single location studies can be important for understanding some physical and chemical processes. Again, it really depends on what's being studied and the type of information needed to answer a particular scientific question.
Probably the most common instrument in use today and the first instrument to hit the water once a ship is positioned on station is the CTD-rosette. The CTD contains electronic sensors that measure conductivity, temperature, and depth for determining the vertical structure of the water column. The information from these sensors is transmitted to the ship using a water-tight conducting cable. Real-time plots of temperature and conductivity (and any other sensor information, such as dissolved oxygen, light, etc.) are displayed on a computer screen in the on-board lab.
CTDs are typically housed on a metal frame that is surrounded with 10-14 water sampling bottles that can be triggered electronically. This arrangement of water sampling bottles is called a rosette. At any particular depth of interest, oceanographers can hit a button that triggers a water bottle to close. In this way, oceanographers can collect water samples for more detailed shipboard or laboratory analyses of water properties or biological rate processes. This procedure differs considerably from the fixed-depth, bottle-hanging, messenger-sending methods used for decades. Still, this latter procedure is used once the structure of the water column is known, especially since water bottles are much less expensive than CTDs. Putting an instrument over the side always runs the risk of accidental loss, a circumstance that can be expensive financially as well as scientifically in terms of lost information.
Your book has a good picture of a CTD-rosette. Note that the water sampling bottles, called Niskin bottles, are made of plastic. This is much more common than the metal Nansen bottles shown on this page and the previous page. Within the last 20 years, it has become apparent that any metal within a sampling bottle can change the chemistry of the seawater and can be toxic to marine phytoplankton. In fact, it was discovered that the rubber tubing used in many Niskin-type water bottles leached chemicals that were toxic to phytoplankton. Oceanographers who are very careful about their measurements and who wish to obtain the most accurate information on biological processes now use Teflon parts on all the interior portions of water bottles.
Once on station, the CTD, attached to an A-frame or some kind of crane, is raised from the deck of the ship, swung over the side, and lowered to the sea surface. The instrument is held at the surface for a moment to insure that the electrical signals are being transmitted (not always a sure thing!). Once everything checks out, the instrument is lowered at a rate of about 1 meter per second. Most CTDs are rated for depths up to 1000 meters. Some are capable of deeper depths. For the deepest depths of the sea, oceanographers must again resort to water bottles and reversing thermometers (as shown in your book) to obtain measurements. Once the CTD is lowered to its desired depth, a water bottle is usually triggered and the CTD is raised. Stops are made at predetermined depths and water samples are taken by electronically triggering water bottles on the rosette. Once the CTD-rosette reaches the surface, it is quite heavy, and I have heard of CTDs being lost is heavy seas when attempting to bring them to the surface.
Once on board the ship, oceanographers for whom the water samples were taken will gather around the CTD and carefully collect water samples. Water samples are usually taken in plastic bottles, which are rinsed at least three times with sample water before being filled. If oxygen samples are being taken, these will usually be taken first and the water bottles, in this case, glass bottles, will be filled slowly and carefully so as not to introduce air to the sample. One of the biggest limitations of CTD-rosettes is the amount of water that can be sampled at any one depth at a time. Sometimes, oceanographers will trigger more than one bottle at a depth, especially if there is a demand for water samples. A water budget is always compiled at the start of a CTD cast to ensure that everyone has enough water for their particular analysis.
This process of lowering and raising the CTD is known as a hydrocast. The resultant vertical profile of information obtained from the electronic sensors is output as a text file or printout from which T-S diagrams or other types of statistical or graphical analyses can be made. The data obtained from hydrocasts provides the foundation on which many other types of data depend. For example, scientists studying the distribution of zooplankton or fishes in the water column want to know the temperature and salinity of the water in which these organisms are found. Thus, the CTD-hydrocast is one of the most widely used measurements in oceanography.
We will talk more about other types of measurements made at sea as we study other ocean phenomenon. However, you should be aware that many other types of measurement techniques are now being employed to study the temperature and salinity characteristics of the oceans. Rapid and synoptic measurements can now be obtained while underway using towed instrument arrays. Using hydrodynamic "wings", electronic sensors can be pulled through the water and lowered up and down while the ship is underway. A zig-zag type of profile is thus obtained. The advantage of this type of measurement is that large regions of the ocean can be covered in a short amount of time. Not only is valuable ship time saved, but measurements can be obtained in the shortest time possible, providing a "snapshot" of the ocean at a given time. Measurements spaced days apart are subject to variations from the movements of the currents and eddies, thus complicating the interpretation of measurements over a particular region. The disadvantage of these "to-yo" profiles is that water samples cannot be taken. However, the advantages for obtaining rapid, "instantaneous" information are substantial.
Changes in ocean properties at a single location over the course of time are provided through the use of semi-permanent buoys. Instruments are attached to a cable or hydrowire at specific depths in the water column, a railroad wheel is tied to one end and a float to the other, and the whole array is thrown into the ocean. These buoys may remain on location for a few months or longer, depending on the experiment. Data are recorded internally by the instruments or by using attached data loggers into which information is fed. Most modern day buoys of any complexity include a satellite uplink that provides information on cable stress and location. In this way, if the buoy gets loose, oceanographers can track it down and still recover their data (and their expensive instruments). After the desired amount of time has passed, oceanographers return to the buoy, send it an acoustic release code, and the buoy (without the railroad wheel) rises to the surface where it can be retrieved. Just to give you some idea of how tricky this business can be, and how frustrating, a friend of mine who had a buoy in the ocean off Bermuda returned to the location only to find her buoy was gone. It turned out that some other scientist had accidentally sent an acoustic release code in the vicinity, to which her buoy happily responded and drifted on its merry way. The array was eventually recovered, thankfully, by some fishermen. As I said, such is the life of an oceanographer at sea!
Drifters are also a common means for obtaining information about the physical characteristics of the ocean. Nowadays, information from these drifters is transmitted via satellite. The world wide web has some sites where you can download data from ocean drifters and ocean buoys. We will talk more about drifters and buoys when we talk about ocean currents and tides.
Finally, I want to mention that satellites have truly "opened our eyes" with regards to the complexity of ocean phenomena. We will devote an entire lecture to satellite oceanography later in the course.
The Heard Island Experiment
Our discussion of the properties of water and the temperature and salinity characteristics of seawater makes this a useful place to talk about one of the more interesting and innovative experiments being conducted in the oceans today. Discussed in your book, the Heard Island Experiment is using the relationship between ocean temperature and the speed of sound in water to determine whether the oceans are warming and, by implication, whether global warming is occurring.
While we didn't mention it in our earlier discussion of the properties of water, sound travels approximately 5 times faster in water than in air, about 5000 feet per second compared to 1100 feet per second. Variations in the density of seawater alter the speed of sound through seawater. Thus, temperature fluctuations in seawater can be detected as variations in the speed of sound, once density differences due to salinity are taken into account.
Walter Munk of the Scripps Institution of Oceanography cleverly took this information and devised an experiment that would help determine whether the oceans were warming. By determining the travel times for sound from a single source in deep water to various locations across the world, he could measure with reasonable accuracy any variations in the temperature of the deep water. The location he chose was Heard Island, located in the southern Indian Ocean. From this location, a sound source would travel in multiple directions and travel unhindered through all three oceans. By placing underwater receiving stations at various points in the three major oceans, he could determine the speed of sound and, thus, the temperature of the water through which the sound traveled. Note that one of these receiving locations is located in Monterey Bay.
In 1991, the first experiments were conducted off Heard Island using a Navy vessel equipped with powerful underwater loudspeakers. The foghornlike sounds were transmitted at a depth of about 650 feet for an hour and then silenced. This pattern was repeated throughout the day. After 3.5 hours, the sounds were detected in 1000 meters of water by a ship located 11,160 miles away off the coast of Oregon, thus confirming the feasibility of the experiment.
Munk's experiment relies on the ability to measure with great precision subtle changes in the speed of sound and, thus, the temperature of the deep ocean. Computer models of global warming predict that the oceans will warm about 0.005°C per year at a depth of 1000 meters. Such small variations in temperature cannot be detected by conventional monitoring; however, using this acoustic method, such warming would reduce the speed of sound through the water column by a few seconds, which would be detectable. Thus, Munk has devised a very workable and clever means for determining whether the oceans are warming.
The method upon which these experiments rely is called acoustic tomography, a system of transmitters and receivers that provide a three-dimensional picture of water column properties. Many ships now use a variation of acoustic tomography called acoustic Doppler measurements for determining the speed of water currents. By measuring the Doppler shift of small particles of moving seawater, the speed of the current can be determined. This is exactly analogous to the way a police officer using Doppler radar determines the speed of your vehicle on the freeway.
One final note: When the Heard Island Experiments were first proposed, there was great concern that marine mammals would be negatively affected by the sound waves generated at Heard Island. As we will discuss in more detail in a later lecture, many whales make use of "sound channels" in the ocean for communication. The presence of an "explosive" source of sound near animals in the vicinity of Heard Island and the intermittent presence of this sound in the animal's sound channel were causes for concern. As a result, the experiment was delayed until funding was found to study the effects of these sounds on marine mammals in the vicinity of Heard Island. Within the next few years, as the experiments progress, much more will be learned both about the temperature of the deep oceans and the reaction of marine mammals to our measurements!
A Few Final Comments
Our discussion of the layer-cake model of the oceans, the different water masses, T-S diagrams and the different ways that oceanographer use to obtain ocean measurements has covered a lot of concepts. The key message here is that the ocean is a dynamic place with a distinct, but ever-changing, structure based on the simple temperature and salinity characteristics of the water masses that comprise it. As mentioned previously, this structure of the ocean provides it with many other characteristics and provides a framework to which organisms (and oceanographers) must adapt. Our challenge is to define that framework and to understand how it changes in time and space. As new oceanographic techniques are developed, they often improve our ability to determine the fine-scale structure of the oceans, yet they don't necessarily improve our understanding of the processes that underlie the formation of these structures. Hopefully, as a basic description of these processes is formulated, oceanography can enter into a more investigative, experimental stage, such as exemplified by the Heard Island experiment, to begin to understand the global processes at work in creating this dynamic ocean environment.
Now I recommend baking a cake with multiple layers and eating it while reading your notes. It's bound to help your understanding of these concepts!