KEY PHYSICAL CONCEPTS The Maury Project

Definition of Oceanography Oceanography – scientific study of the oceans, Poor name for our science. What would be a better name? “Ography” implies map making only, but we do so much more! As scientists, we emphasize the environmental characteristics of the ocean.

Why even study the oceans at all? Governments spend a considerable amount of $$$$ to do so. What are some important reasons for investigating at the oceans? Do the Oceans affect your “every day” life?

Goal of the Oceanographer Basic goal of the oceanographer – to obtain a quantitative description of the oceans so that we can make predictions.

Branches of Oceanography Engineering Geological History of ocean basins Coastal dynamics Chemical Ocean rxns Air/sea gas fluxes Biological Prediction of HABs Carbon budgets Marine Archaeology Physical Marine pharmaceuticals Marine fisheries

Physical Oceanography Two basic approaches are taken by physical oceanographers. Descriptive or synoptic Dynamic or theoretical Which is more important?

Physical properties of seawater The water molecule is unique, if we didn’t have it we would have to invent it! (In other words, we wouldn’t be here without it!) Dipolar structure gives rise to many unique characteristics -–actually mostly due to hydrogen bonds.

Water Phases To view this animation, click “View” and then “Slide Show” on the top navigation bar.

Physical Properties Hydrogen bond allows water to make a polymer like chains of up to eight molecules, known as the clatheritic structure.

Physical Properties Interesting effect – as water heats up molecules increase in activity causing water to expand, but energy is available to create chains, causing water to shrink. Effect is that pure water has its maximum density at 4 degrees C not at its freezing point. This is important to marine life!

Fundamental Properties Temperature- Very important characteristic of seawater. Easily measured by thermometer or thermistors. One of the first parameters measured. Vertical profiles of temperature are very illuminating of the oceans structure. Not usually measured in SI units, degrees C. temperature differences used in calculations (thermodynamics) must use Kelvins.

Salinity Seawater is a complicated solution. All known naturally occurring elements have been found in the oceans. Important effect on boiling point and freezing point of water.

Salinity Principle of constant proportions states that the absolute amount of salt in sea water varies, but the relative proportions of the ions is constant. How can we can use this to measure salinity?

Salinity There are several ways we can measure the salinity of the oceans. Can we list a few?

Chemical Composition Rivers Ocean Bicarbonate 58.4% Chloride 55% Calcium 15% Sodium 30.6% Silicate 13.1% Sulfate 7.68% Sulfate 11.2 Magn. 3.69% Chloride 7.8% Calcium 1.16% Sodium 6.3% Potassium 1.10% 0.1 ppt vs. 34.7 ppt

Traditional Definition of Salinity Knudsen (1902) - Salinity defined as the weight in grams of all dissolved constituents (inorganic) in one kilogram of seawater. (ppt) But some complications: Must replace all the Br and I by the equivalent amount of Cl, convert all carbonate to oxide; and all organic material is burned off at 480 degrees C.

The many Definitions of Salinity Chlorinity: a measure of the total mass of halogen ions in seawater (F, Cl, Br, I) By international agreement, salinity can be defined as: Salinity (ppt) = 1.80655 * Chlorinity (ppt) The average chlorinity is 19.2 ppt, so what is the average salinity of the oceans?

Salinity Today we use electrical conductivity It is fast, easy, fairly cheap (now it is) very accurate and the most precise method. K15 = Conductivity of sample ---------------------------- Conductivity of KCl standard

Practical Salinity At 15 degrees C and one atmosphere of pressure, the concentration of the KCl standard is exactly 32.4356 g/kg Actually, in Open ocean, ppt and psu are closely related ppt = 1.00510 * S (psu)

Method Accuracy Precision Refractometer 1.0 ppt 1 part in 70 Evaporation 1.0 ppt 1 part in 30 Hydrometer 0.02 1 part in 100 Titration In laboratory 0.01 1 part in 350 Titration At Sea 0.001 1 part in 3000 Cond. Meter 0.0003 1 part in 40,000

Ocean buffering Ocean pH = 8.1 (slightly basic) Buffering protects the ocean from experiencing large pH changes

Pressure Pressure is the force per unit area. A function of gravity, density and depth, If we hold Density constant, then we can use the hydrostatic equation: P = -r g z

Pressure The SI unit for pressure is the K Pa = 10 3 pascals. Previous unit was the Bar. 1 bar = 1 standard atmospheric pressure (sea-level) Standard Atmospheric pressure = 101.32 Kpa = 1.01325 bars = 1013.25 mb = 760 mm Hg. Important to note: 1 decibar = 10 Kpa = pressure due to 1 metre depth of seawater. Why is this useful?

Density For the physical oceanographer, we are particularly interested in temperature and salinity because they determine density, and identify water masses. Density = Mass/volume, typically expressed in Kg/m3

Density Difficult to measure in situ Varies from about 1021.00 to 1070.00 Kg/m3 So as a short hand method we only use the last four digits as a convenience. s stp is defined as density – 1000 Kg/m3 - this is the in situ value.

Density Often times, one can ignore the effects of pressure or compressibility because we are usually comparing water masses at the same depth and compressibility is only important at very deep depths. So we often express density in terms of st st = ssto

Density and Pressure At large depths compressibility becomes important, and pressure will effect temperature and density. Adiabatic changes – changes that occur independently of any transfer of heat. Consequence of the compressibility of fluids. Liquid not as compressible as gas.

Density and Pressure For seawater the adiabatic rate is about 0.2 degrees C per Km Leads us to an important concept: Potential Temperature – temperature that the fluid would obtain if brought adiabatically to the surface. Important for stability of the oceans. Can also define a sq (sigma theta)

Density and Pressure International Equation of State: Equations of state relate important environmental variables to density. For us this is temperature, salinity and pressure.

Equation of State for Seawater

Basic Forces Conservation of mass - continuity Conservation of energy Basic Laws Conservation of mass - continuity Conservation of energy Newton’s three laws of motion Conservation of angular momentum - vorticity law of gravitation

Forces Primary vs. secondary forces Primary – forces that cause motion Gravity, wind stress, pressure gradients, seismic Secondary – result from motion Coriolis, friction

Fundamental Concepts Summary 4 Areas of Oceanography Water Molecule = Unique Structure Density = F(salinty,temperature,pressure) Salinity – Not just NaCl pH (ocean acidity) Forces at work: Primary (there all the time), Secondary (results from motion)