CLASS NOTES Sept. 5
(01/11:628:200) Fall 2003
 
STRUCTURE OF THE OCEANS

Objectives:

1. To understand the radiation budget of the earth and the role that the oceans play in the heat budget of our planet.

2. To build a picture of the structural (spatial and temporal) consequences of density interactions (controlled by temperature and salinity), that allow us to measure and delineate the internal, layered structure of the world's oceans, and explain much of the deep horizontal and vertical circulation.

Important Concepts:
1. The earth receives solar radiation unequally over its surface with the intensity per unit area greatest at the equator and lowest at the poles.
- If the earth had no atmosphere and the sun was directly overhead, the intensity of radiation arriving at the earth's surface would be two calories per square centimeter per minute ( 2 cal/cm2/min). This value is called the solar constant.
- The earth's atmosphere reduces the amount of solar radiation that can reach the earth's surface.
- The intensity of solar radiation available at the earth's surface varies not only with latitude, but with the time of year.
- To balance the radiation input from the sun, the earth must lose heat back to space. This occurs primarily by the emission of long wavelength (infrared) radiation.
- To understand the heat budget of a portion of the ocean, we need to know the following:
a. the total solar (short wavelength) energy absorbed
b. the net long wavelength radiation from the sea surface
c. the loss of energy due to evaporation
d. warming or cooling of the overlying atmosphere by heat from the sea surface (conduction)
e. the transfer of heat (advectively) through currents (input and outflow of energy)
2. Many atmospheric gases are transparent to incoming short wavelength solar energy, but are very efficient at trapping and absorbing the heat energy re-radiated from the earth's surface in longer wavelengths. This phenomenon is known as the 'Greenhouse effect.'
- Anthropogenic inputs to natural cycles are mostly one-way processes. About six billion tons of carbon are emitted into the atmosphere each year by burning fossil fuels.
- Due to the trend of increasing CO2 concentrations in the atmosphere, a global surface air temperature increase on the order of 2-4 deg C has been predicted if atmospheric CO2 levels double over present values.
- Warming the earth by 2-4 deg C might cause the earth's polar land ice to melt more rapidly, raising global sea level. Thermal expansion of the water will also increase sea level.
- About half of the CO2 produced by fossil fuels remains in the atmosphere; the remainder is absorbed by oceanic and terrestrial chemical pathways.
3. The salinity of mid-ocean surface waters is controlled by heat distribution and atmospheric circulation on the planet--i.e., the salinity is higher where surface waters evaporate the most, or lower where rainfall is abundant.

4. The basic controls on the density of water masses in the world's oceans are temperature and salinity.

- Less dense water masses remain at the ocean's surface, while more dense waters tend to sink to an equilibrium level. These simple physical reactions coupled with wind, tidal and wave generated forces, mean that the density of the ocean waters changes with depth, latitude, and proximity to coastal regions in a fairly predictable (and observable) manner.
- The surface 100 m of the world's oceans is fairly well-mixed, and therefore homogeneous in its density.
- Layers of increasing density occur between 100 m and 1000 m. The plot of density with depth reveals a rapid change until about 1000 m, with deeper waters of very similar or slowly increasing density. The zone where density changes most rapidly with depth is termed the pycnocline.
- Temperature also decreases rather rapidly with depth below the 100 m level to the 1000 m depth. The zone where the temperature changes most rapidly with depth is called the thermocline. Similar in behavior to density, below 1000 m, the temperature of the oceans is relatively uniform, with gradual decreases down to the bottom.
- As with temperature and density, salinity increases rapidly from 100 m to 1000 m, then remains nearly uniform below. The zone where the most rapid changes in salinity with depth occur is called the halocline (halo-for halite, the mineral name for common table salt).
5. Thermohaline circulations are produced by large-scale density variations in the ocean at high latitudes.
- A water column with a density distribution of low to higher with depth into the ocean is called stable. The converse is unstable and leads to overturn and mixing of the water column. A water column with a vertically uniform density, temperature, or salinity is called isopycnal, isothermal, and isohaline, respectively.
- Any process that creates an unstable water column in terms of density (i.e. controlled by temperature and salinity) will generate circulation patterns in the water mass as it attempts to regain stability or equilibrium. These circulation patterns or currents are very important in the mixing and exchange of energy and other properties in the ocean. The thermohaline circulation occurs as a result of these large-scale processes at high latitudes.
6. Dense water sinking in the oceans reaches an equilibrium depth and is displaced horizontally by the continued sinking of dense water behind it. Water at the surface moves into the zone of sinking, while at some other point in the ocean, water must rise to replace it.
- Areas where water masses move downward are called downwelling zones, conversely, areas where water rises are called upwelling zones.
- Downwelling brings oxygenated waters to the depths, while upwelling can bring needed nutrients and decomposition products to surface waters.
- When wind driven surface currents are divergent or convergent, they generate upwelling or downwelling, respectively.
- Rates of upwelling and downwelling (0.1 - 1.5 m/day) are slower than oceanic surface currents which have speeds up to 1.5 m/sec. This is important, as it means that water that sinks to the deep ocean may spend up to 1000 years at depth before it again reaches the surface.
7. Temperature and salinity can be plotted for each water mass via a Temperature versus Salinity graph, thereby defining distinct T-S curves for each geographical region in the world's oceans.
- T-S curves for geographically related areas of the oceans are similar, and fall into groups or families we call 'water masses'.
- A water mass with only one temperature value and one salinity value over its entire extent is called a water 'type'. (For example, Antarctic Bottom Water is a water type.)
- Some organisms have become specifically adapted to very narrow ranges of temperature and salinity--therefore, some can act as biological indicators of water masses or even water types.