Lecture Notes-Seawater Physics

Lots and lots of info on this subject! For this class, we will divide the pertinent information into three catagories: Physical Properties of Seawater; Heat Budget of the Ocean-Earth; and Light and Sound Propagation in Seawater.

Physical Properties of Seawater

1. Specific heat of seawater-Although it is vague, TEMPERATURE and HEAT are two different things. Temperature is a measure of how hot or cold a substance is and we measure the temperature in units of degrees of Celsius or Fahrenheit (written as °C or °F). Heat is the energy put into a substance and is measured in Calories. A Calorie is defined as "the amount of heat needed to raise the temperature of 1 gram of pure water by 1°C." Usually this heat causes the substance to raise in temperature. Sometimes the input of heat does not cause a raise in temperature, but causes a change in the state of the material (solid to liquid to gas). The opposite happens as well. Removing heat can cause the temperature to drop or cause a change in state (gas to liquid to solid). How that substance reacts to the input of heat is called its "specific heat." In other words, does it soak up heat readily or not readily? As defined, the specific heat of pure water is one calorie. Nearly all other substances (except liquid ammonia) have a much lower specific heat.

2. Density of seawater-The density of seawater is the measure of the weight of a unit of seawater divided by the volume of that unit. The density of seawater is given in grams/centimeters3 or g/cm3. The density varies greatly through the ocean because of changes in the salinity and temperature of seawater (see below). Because of these differences, the ocean is divided into discreet water masses of varying densities. This is similar to the makeup of the Earth (density stratification), with the most dense layers at the bottom of the ocean and the least dense layers at the top.

2a. Salinity of seawater-Adding dissolved salts to pure water increases its density. Adding salts also lowers the specific heat of seawater, thus the seawater is more affected by the input or export of heat. Salinity also lowers the temperature at which water freezes. Pure water freezes at 0°C, but typical seawater, at a salinity of 35‰, freezes at -1.33°C. Salinity also causes seawater to evaporate more slowly than pure water.

2b. Temperature of seawater-Increasing the temperature of seawater (and most substances) decreases its density, and visa versa. Therefore, warm water will float above cold water (every jumped into a pool??). However, there are some unique qualities with how temperature affects the density of water. If you start with a cup of water at room temperature and take the heat away, thus lowering the temperature, the density increases. No shocker. BUT once the temperature drops below about 4°C, the density actually begins to decrease, even though the temperature is also decreasing! Even more bizarre, when the water freezes the density drops way, way down. For pure water, the density at freezing drops from 0.999 g/cm3 for liquid at 0°C to 0.917 g/cm3 for solid ice at 0°°C. That is why ice floats! One more point, adding heat to seawater causes both the temperature to rise and, because some of the water evaporates, increases the salinity. This leads to the big question, is the density more controlled by changes in temperature or changes in salinity??

3. Stratification of the oceans-The physical properties listed above cause the ocean to be divided into discreet layers. That is, one can measure unique layers in the ocean defined on salinity, temperature, or density. As you now know, the three are tied in together.

3a. Temperature stratification-At the surface, the ocean water is warmest due to heating of the sun. This is known as the "surface zone" or "mized zone." This layer extends down to about 150 meters but can go as low as 1,000 meters if the conditions are right. Deep, deep water is very cold, hovering just above freezing. This water mass, called the "deep zone," lies below about 1,000 meters and accounts for most of the ocean water. Between the "mized zone" and the "deep zone" lies a mass of water that is transitional. This zone is called the "thermocline." Note that at the poles, there isn't really a thermocline to speak of.

3b. Salinity stratification-Salinity is lowest in the upper waters in most of the world's ocean due to input of fresh water from rain and rivers. The zone between this upper low salinity waters and the deep, salty waters is called the "halocline."

3c. Density stratification-The thermocline and halocline are often found in the same zone of the ocean and form a real boundary between low density waters at the surface and higher density waters at depth. The boundary zone between the two is the "pycnocline."

Heat Budget of the Ocean-Earth

Heat is added to the Earth primarily by solar input and from the decay of radioactive elements within the Earth. Because of the high specific heat of water, the oceans serve as the heat "buffer" for the Earth, holding the heat when needed and letting it go when it isn't. Because of the constant trade off of heat, we say the heat budget of the Earth is more or less in balance, or that it is in a "steady state." Note that the steady state applies to very long time scales, but the Earth fluctuates in its heat budget daily, seasonally, yearly, and on scales of thousands of years. Most of the incoming solar heat is taken in at the equator and reflected back at the poles. The ocean water near the equator (called the "low latitudes") absorbs this incoming heat and then travels toward the poles ("high latitudes") and releases the heat. Also, air masses warmed in the low latitudes absorb the solar heat through evaporation and then also move to the high latitudes and release the heat, causing the water molecules in the air to condense and form rain and snow. In addition, the large amount of ice on the surface also absorbs and releases heat throughout the day.

Light and Sound Propagation in Seawater

Both light and sound move through seawater as waves. Differences in the properties between air and between various water masses cause the light and sound waves to bend. This is called refraction. Simply put, when a wave moves through a more slow medium, it slows down. If the boundary between the two mediums is perpendicular to the path of the waves there is no change in direction. But if the wave approaches the boundary at an angle, it will be bend toward the more slow medium. The measure of how much light will bend from one medium to the next is known as the "refractive index." The refractive index between air and water is high enough that fish in an aquarium look distorted.

In addition to being refracted, light will also be scattered and absorbed as it passes through the water. The longer waves of light (infrared and visible "hot" colors) will be absorbed first. That is why the ocean looks blue-because we see the blue light reflecting back after travelling through the water. The importance of this absorption is that the photic zone, that layer in which plants can photosynthesize, is relatively thin.

Sound waves travel by vibrating one molecule which causes an adjacent molecule to vibrate which causes an adjacent…and so on. Because of the closer packing of molecules in a liquid than in air, sound travels faster and much greater distances in water. In fact, sound travels at about 1.500 meters per second in the ocean-five times the speed in the air! Sound waves, like light waves, also become refracted as they pass through water. This has some interesting implications when we think back on the various density layers in the ocean. Sound travels very fast near the surface of the ocean in warm water. The sound decreases with depth as the water is colder (and more saline) to a minimum speed at about 1,000 meters below sea level. Below this, even though the water is colder, the high pressures cause an increase in the speed of sound. In fact, sound travels fastest in the deepest water! The zone of slowest speed is called the "sofar layer" and is very important in ocean studies. Because sound waves want to stay in the slow layer due to refraction, if you set off a sound charge in the sofar layer, you can hear it virtually everywhere on the planet as long as your listening devise is also in the sofar layer.

Key Points to Know:

1. Water has a high specific heat.

2. The density of seawater varies due to changes in salinity and temperature. This leads to "density stratification." Increasing salinity and decreasing temperature raises the density.

3. Density of seawater increases with decreasing temperature to a maximum at about 4°C. Below this, the density decreases. Once ice is formed, the density again increases with decreasing temperature.

4. The oceans are stratified by temperature, salinity and density. There is a mixed or surface zone at top, a transition zone in the middle, and a deep zone at bottom. The transition zone is called the thermocline (temperature), halocline (salinity), or pycnocline (density).

5. Heat is added to the Earth primarily by solar input and from the decay of radioactive elements within the Earth

6. The high specific heat of water enables the oceans to serve as the heat "buffer" for the Earth.

7. Most of the incoming solar heat is taken in at the equator and reflected back at the poles.

8. Water and air masses transfer the heat from the low latitudes to the high latitudes.

9. Polar ice also stores and releases solar heat.

10. Light and sound waves refract as they pass from air into water and between various water masses. The waves bend toward the more slow medium

11. Light is scattered and absorbed as it passes through the water

12. Sound travels faster and much greater distances in water than air.

13. The rate of sound through the ocean is not constant and is affected by temperature and pressure. The fastest zones are at the surface and deep ocean.

14. The slowest zone is at about 1,000 meters below sea level and is called the sofar layer.

WEB SITES:

http://www.nodc.noaa.gov/index.html

Site for the U.S. National Oceanographic Data Center (NODC); the largest public resource for data on ocean water.

http://www.nodc.noaa.gov/GTSPP/gtspp-home.html

Home page for the Global Temperature-Salinity Profile Program (GTSPP) Database. Useful current data on a variety of chemical parameters. One of the many programs accessible through the above link.

http://topex-www.jpl.nasa.gov/

Official site for the TOPEX/Poseidon project-a joint U.S.-French study that is using satellites to monitor ocean chemistry/physics.

http://www.cape.com/~profiles/general-info.html

Program that is measuring water temperatures across the Gulf of Mexico. This site allows you to see the actual data and where it comes from. The site is easy to use and well organized. See how oceanographers work!

http://www.jamstec.go.jp/jamstec/MOKEI/T62/model_T62.html

Group that is making models of global ocean dynamics.

http://www.aoml.noaa.gov/phod/uot/faq_what.html

Simple figure of temperature through the upper 700 m of the ocean, showing the mixed zone, thermocline, and deep zone.

http://seawifs.gsfc.nasa.gov/SEAWIFS.html

NASA program (Sea-viewing Wide Field-of-View Sensor (SeaWiFS)) to monitor the oceans by recording the color. Aren't these scientists smart! Excellent site for satellite images of sea surface temperatures and lots of other variables.

http://nic.fb4.noaa.gov/products/analysis_monitoring/enso_update/index.html

El Nino update site! Shows measurements of outgoing radiation and lots of temperature data. Updated weekly.

http://www.pmel.noaa.gov/toga-tao/vis5d/anim/sst-wind-cur-eqt-20c.html

Another neat El Nino site that shows a movie of how the warm water mass developed. Way cool (or way warm…)!

http://atocdb.ucsd.edu/index.html

The ATOC program homepage. This is the study that is setting off sound charges in the sofar layer. Inside is a link for the scientific study of how the ATOC program and other ocean noises affect marine mammals (see alternative view below).

http://www.angelfire.com/ca/fishattorney/lfaslinks.html

Alternative site maintained by a group that believes the low frequency noises being set off by scientists in the sofar layer affect marine mammals.