The physical processes that change the density of seawater differ in very distinct ways across the world ocean. Recall the patterns of evaporation and precipitation we talked about in a previous lecture. As a result of these two processes, different regions of the world ocean give rise to distinctive masses that can be identified based on their temperature and salinity characteristics. In addition, the formation of these waters drive the circulation patterns of deep-sea, or abyssal, waters. Because this circulation depends on the density differences between water masses, driven by changes in the temperature and salinity of seawater, this type of deep-ocean circulation is called thermohaline circulation.
Water masses can be divided into five parts:
The end result is a layered ocean about which we can form some generalities and for which we can derive individual names for the particular water masses that form in a given region.
Let's start with the Atlantic Ocean. A water mass formed off the coast of Iceland, known as the North Atlantic Deep Water, appears to drive the major deep ocean currents of the world ocean. This region of the world is characterized by cool temperatures and lots of precipitation, resulting in a water mass with a salinity of approximately 34.9 ppt and a temperature of 2 to 4 degrees C. North Atlantic Deep Water sinks to the bottom of the ocean and flows south along the east side of the Atlantic Ocean. Eventually, as the story goes, this water loops its way into the Indian Ocean and surfaces in the equatorial upwelling zone in the Pacific Ocean!
This circulation of deep water in the oceans is known as the "global conveyor belt", a concept developed by Wallace Broeker at Lamont-Doherty Earth Observatory, where I did my post-doctoral research. Take a look at the figure in a textbook to get some idea of how this process works. Although North Atlantic deep water is not the most dense water formed in the world ocean, it is by far the most abundant, and, for this reason, it has been suggested that North Atlantic deep water drives the circulation of the deep ocean.
The densest water in the world is produced in the Antarctic Ocean, primarily in the Weddell Sea in the winter, along the edge of the Antarctic continent. The combination of high salinity and cold temperatures leads to water with the highest density of any water formed in the ocean. Antarctic bottom water, as it is called, has a temperature of approximately -0.5 degrees C and a salinity of around 34.8 ppt. Although Antarctic bottom water has the highest density, it is formed in small quantities. For the most part, according to your book, it is limited to the bottom regions along the coast of South America. Here the Mid-Atlantic ridge keeps it from extending into the eastern side of the Atlantic. However, according to other sources, Antarctic Bottom Water has been found in the Pacific Ocean at the equator (a journey of 1,000 years) and as far north as the Aleutian Islands (another 650 years from the equator). In the Atlantic Oceans, Antarctic Bottom Water has been found as far north as 40 degrees N, a journey of 750 yeas. Thus, at times, sufficient quantities may form to creep far from its origins. Its sluggish movement and confinement to the deep sills of the southern oceans means that this water mixes slowly and allows Antarctic bottom water to retain its characteristics for up to 1,600 years!
Intermediate waters are those water masses that form a layer above deep and bottom waters, and generally represent the top of the permanent thermocline, a region of rapid change in temperature, in the oceans of the world. Atlantic Intermediate Water forms as a result of the mixing of cold waters from the north and saline waters from the south. The resultant intermediate water floats as a layer on top of the North Atlantic Deep Water and flows south. Its counterpart, the Antarctic Intermediate Water, forms in a similar way, although its density is not as great as Atlantic Intermediate Water. This water flows north towards the equator at a depth of approximately 500-1000 meters.
In the Pacific Ocean, Intermediate Water masses are again dominated by Antarctic Intermediate Water below the equator and formed as intermediate water, known as North Pacific Intermediate Water, in the North Pacific. In the Indian Ocean, the intermediate waters consist of Antarctic Intermediate Water.
Central waters are those water masses that form directly above the permanent thermocline. Generally, they are confined to regions closer to the equator. In all three oceans, central water masses correspond to the regions where they exist. In the Atlantic Ocean, we have the North Atlantic Central Water and the South Atlantic Central Water. In the Pacific, the North Pacific Central Water and South Pacific Central Waters are present. In the Indian Ocean, central waters consist of Southern Indian Central Water and Equatorial Central Water.
A few "specialized" water masses should be noted. In the Atlantic Ocean, water coming out of the Mediterranean and flowing over the sill at Gibraltar forms water known as Mediterranean Water. Traces of Mediterranean Water have been found as far as 2,500 miles away from its source! This water is very distinct and imparts a greater complexity to deep water circulation in the Atlantic Ocean. In the Indian Ocean, the Red Sea serves a similar role, providing a distinct characteristic water mass to deeper waters in the northern Indian Ocean.
Identification and characterization of these water masses is one of the favorite pastimes of physical oceanographers. As the role of the oceans in controlling the climate of our globe has become better understood, the need to understand the factors which govern the formation and circulation of these water has become increasingly important. Understanding the properties of these water masses and defining their distribution across the globe are one means to better understand how they are formed.
The "take-home" message here is simple, yet profound. The oceans consist of distinct layers or "lenses" (or "blobs") of water that move about the globe and retain their characteristics for hundreds, if not thousands, of years. This "layer cake" model of the world ocean serves to define the "structure" of the oceans. This structure of the sea functions much in the same way that the "structure" of a forest functions; it creates distinct zones and regions on which organisms can depend and to which these organisms can adapt. It creates a type of framework on which the tapestry of ocean ecosystems can be weaved.
Ways to Measure Ocean Properties from a Ship
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.
Typical water sampling bottles, called Niskin bottles, are made of plastic. This is much more common than the metal Nansen bottles used in past decades. 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.
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.
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. 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!
Clearly, the ocean is not a smooth and homogeneous place! With the advent of satellite and space-shuttle oceanography, our view of the oceans has changed dramatically. How can we explain these structures in the context of the simple models of wind-driven circulation of the surface currents, or the density-driven currents of the deep ocean? How can we hope to understand the nature of the interactions of the physical, chemical, and biological processes within these complicated movements of water? These are just a few of the formidable challenges facing oceanographers.