Physical and Geological Processes in the Oceans · 2019-04-08 · important tracers of ocean physical processes • Many elements are present in variable concentration in seawater;

AQ 233

Unique Properties of Water

Physical and Geological Processes in the Oceans

Lecture Two

• High specific heat: provides stable temperature

conditions, prevents extreme ranges in

temperatures. Water can store large amounts of

heat

• High latent heat of evaporation/condensation.

Moderate climates, as important in heat and water

transfer with the atmosphere

• Immense capacity to dissolve other substances.

Important in erosion and many biological

processes

• Maximum density at 4 ⁰C, so lakes do not easily

freeze solid

How is saltwater different from freshwater?

• Heat capacity decreases with increasing

salinity. The boiling point of seawater increases

with increasing density

• Density increases almost linearly with

increasing salinity

• The freezing point is lowered with the addition

of salt

Water’s Thermal Properties

▪ Water exists on Earth as a solid, liquid and

a gas

▪ Has the capacity to store and release great

amounts of heat

▪ Water’s thermal properties influences the

world’s heat budget, moderate coastal

temperatures and are in part responsible

for the development of tropical cyclones,

worldwide wind belts and ocean surface

currents

General characteristics of the seawater• The three most important are salinity, temperature and density.

Others are sound and light

Why measure temperature, salinity and density

• Distribution of temperature and salinity at the ocean's surface influenced by heat fluxes, evaporation, rain, river in flow, and freezing and melting of sea ice

• Changes in temperature and salinity can increase or decrease the density of water at the surface, leading to convection.

• If water from the surface sinks into the deeper ocean, it retains a distinctive relationship between temperature and salinity which helps to track the movement of deep water.

• Temperature, salinity, and pressure are used to calculate density

• The distribution of density inside the ocean is directly related to the distribution of horizontal pressure gradients and ocean currents

Salinity• Salinity is defined as the total amount in

grams of dissolved salts in 1 kg of seawater

Chlorine ion (Cl-) 55.04%

SO4-- 7.7%

Na+ 30.6%

Mg2+ 3.7%

Ca2+ 1.2%

K+ 1.1%

Salinity• The Principle of constant proportions – the major

dissolved constituents responsible for the salinity of seawater occur nearly everywhere in the ocean in the exact same proportions, independent of salinity

• A measurement of only one concentration of major constituents is enough to give ocean salinity. Cl-seems to be an easy ion to measure

• Salinity varies within the oceans. Open ocean – 33 to 38 parts per thousands. In coastal areas, variations can be extreme and varies seasonally. Baltic Sea –salinity average 10 parts per thousands while the Red Sea, salinity average 42 parts per thousands

• Salinity tends to be lower in the areas where large rivers enters estuaries and then the ocean

• Salinity is higher where evaporation is higher such as the Red Sea and Persian Gulf

Salinity distribution• Salinity variations are caused by the addition or removal of the

dissolved substances within the water or of water molecules from seawater. Processes that remove water molecules are: evaporation and the formation of ice. Precipitation (rain, snow, sleet), river runoff and ice melting add water molecules

• Changing the salinity does not affect the amount or the composition of the dissolved components, which remain in constant proportions

• Evaporation and precipitation are the principal factors influencing salinity.

• The distribution of sea-surface salinity tends to be zonal.

• Salinity tends to be greater off desert regions at about 25o N and 25o S, where evaporation is high. Minimum values near the equator where rain freshens the surface water, and the polar regions where melted sea ice freshens the surface waters

• The zonal (east-west) average of salinity shows a close correlation between salinity and evaporation minus precipitation plus river input

Salinity layers in the ocean

• The surface zone, a well-mixed layer of

generally uniform salinity, 50 to 100 m thick

• The halocline, a zone with a rapid change in

salinity

• A thick zone of relatively uniform salinity,

extending to the ocean bottom

• An occasional zone of minimum salinity in

some areas, at a depth of 600 and 1, 000 m

Question

• Why salinity varies widely at the surface, but

very little in the deep ocean?

Temperature• Temperature is a thermodynamic property of a fluid, and is due

to the activity of molecules and atoms in the fluid. The more the activity (energy), the higher the temperature.

• Temperature is a measure of the heat content. Heat and temperature are related through the specific heat

• The heat is input at the ocean both directly and indirectly through: direct radiation from the sun and sky; conduction from the atmosphere & condensation of water vapor

• The surface of the ocean is cooled by: radiation from the surface back to the atmosphere; conduction of heat back to the atmosphere & evaporation, which removes heat from the ocean to power the evaporation process

• Ocean currents can transfer heat from one area to another by bringing into contact bodies of water having different temperature

Temperature

• The decrease with depth is more rapid at the

surface than at depth

• Warm water is found at shallow depths because

the ocean is an effective absorber of the sun’s

radiation

• Since the surface water is warmer than the

water below, it is less dense and does not mix

downward easily.

• In the atmosphere, it is different as when it is

heated by the land below, it is subject to

convection and active mixing.

Ocean temperatures (contd)• The ocean is constantly transferring heat from

equatorial regions towards the colder poles

• A large portion of the seawater in the ocean is

relatively cold, about 75% has a temperature below

4o C.

• Below the main thermocline, temperature and salinity

values are usually closely related.

• This relationship can be used to define different

water types or water masses. Temperature-salinity

relationships can also indicate the source and mixing

of water masses and are especially important in

studies of deep-ocean circulation

Statistics on ocean water temperature and salinity

• 75% of total ocean water volume has temperature

ranging from 0-6oC and 34 – 35 (psu) salinity

• 50% of water has temperature ranging from 1.3-

3.8oC and 34.6 – 34.8 salinity

• mean temperature of world ocean is 3.5oC and

34.7 salinity

• 75% of the ocean's water have a temperature and

salinity within the green region, 99% have a

temperature and salinity within the region coloured

in cyan. The warm water outside the 75% region is

confined to the upper 1000 m of the ocean.

Ocean temperatures• The surface temperature of the ocean is closely

related to latitude and time of year

• Temperature pattern with depth in the ocean:

➢A warm, well-mixed surface layer, from 10 to about

500 m thick

➢A transition layer, below the surface layer, called the

main thermocline, where the temperature decreases

rapidly with depth. The transition layer can be 500 to

1,000 m

➢A layer up to several kilometers thick that is cold and

relatively homogeneous

• The character of the main thermocline varies with

latitudes.

Temperature profiles

Density• Density of seawater is its mass per unit volume ( grams/cubic

centimeter).

• Seawater contains dissolved substances that increase its density. The density of seawater is 2 to 3% greater than pure water

• Below 4oC, density of freshwater decreases. Seawater increases in density until it freezes at 1.9oC. At it freezing point, it behaves as freshwater, ie its density decreases – sea ice floats

• Density in the ocean is determined by temperature, salinity, and pressure.

• Density changes in the ocean are very small, and studies of water masses and currents require density with an accuracy of 10 parts per million.

• Density is not measured, it is calculated from measurements of temperature, salinity, and pressure using the equation of state of sea water.

• Density differences determine vertical position of ocean water and cause water masses to float or sink – creating deep ocean currents

• What are the factors that influence density

variations and how

Density • Seawater density is controlled by three variables

➢Density increases with increasing salinity (due to addition of more dissolved material)

➢Density increases with increasing pressure (due to the compressive effects of pressure), which is to say that density increases with increasing depth

➢Density increases with decreasing temperature (due to thermal expansion)

➢Only temperature and salinity influence the density of surface water

➢Temperature has more influence on density than salinity because of its greater range

➢Only in the extreme polar areas of the ocean, where temperature is low and constant, does salinity affect density

Density layers • The upper 100 m or so of the ocean is strongly

influenced by wind and waves therefore is well

mixed and relatively uniform

• Pycnocline, the zone where water density

changes rapidly, usually due to changes in

temperature and salinity

• The deep denser waters of the ocean occur

below the pycnocline

Questions

• Conservative and non-conservative

constituents – give examples

Conservative and non-conservative constituents

• Conservative constituents – occur in constant proportions or change only very slowly through time. For example: the major dissolved components of seawater, Argon and the other noble gases (Helium, neon, Krypton and Radon, which are chemically unreactive ). They have long residence times in the oceans. Though they are present in very small quantities, they serve as important tracers of ocean physical processes

• Many elements are present in variable concentration in seawater; these are the non-conservative constituents. The variability of their concentrations is caused primarily by biological and chemical processes. They can be used to trace biological and chemical processesThey have short residence time. Examples: gases oxygen, carbon dioxide and nutrients

Conservative and non-conservative properties

conservative properties, are not involved in biological process, are changed by:

• evaporation and precipitation (rainfall, snow)

• formation of insoluble precipitates whereby formerly dissolved elements or compounds settle out of seawater

• mixing of water masses having different salinities

• diffusion of dissolved materials from one water mass to another

• movement of water masses within ocean

Characteristics of Tracers for water

masses

• An ideal tracer is easy to measure even when

its concentration is very small;

• it is conserved, which means that only mixing

changes its concentration;

• it exists in the water mass we wish to trace, but

not in other adjacent water masses; and

• it does not influence marine organisms

Some of widely used tracers• Salinity is conserved, and it influences density much less than temperature.

• Oxygen is only partly conserved. Its concentration is reduced by the respiration by marine plants and animals and by oxidation of organic carbon.

• Silicates are used by some marine organisms. They are conserved at depths below the sunlit zone.

• Phosphates are used by all organisms, but they can provide additional information.

• 3He is conserved, but there are few sources, mostly at deep-sea volcanic

• areas and hot springs.

• 3H (tritium) was produced by atomic bomb tests in the atmosphere in the 1950s. It enters the ocean through the mixed layer, and it is useful for tracing the formation of deep water. It decays with a half life of 12.3 y and it is slowly disappearing from the ocean.

• Fluorocarbons (Freon used in air conditioning) have been recently injected into atmosphere. They can be measured with very great sensitivity; and they are being used for tracing the sources of deep water.

• Sulphur hexafluoride SF6 can be injected into sea water, and the concentration can be measured with great sensitivity for many months.

Each tracer has its usefulness, and each provides additional information about

the flow.

Light in the Sea• Sunlight in the ocean is important for many reasons: It

heats sea water, warming the surface layers; it provides energy required by phytoplankton; it is used for navigation by animals near the surface; and reflected subsurface light is used for mapping chlorophyll concentration from space.

• Light measurement provide data on biological activity, suspended load or water quality

• The light from the Sun affects the oceans:

i) The major wind belts of the world, which produce ocean currents and wind-driven ocean waves, derive their energy from solar radiation. Wind belts and ocean currents strongly influences world climates

ii) A thin layer of warm water at the ocean surface, created by solar heating. This is the “life layer” where most sea life exists

iii) Photosynthesis can occur only where sunlight penetrates the ocean water

• The Sun radiates a wide range of wavelengths of electromagnetic radiation – Electromagnetic spectrum from Ultraviolet to Visible light to Infrared

• Light is a part of electromagnetic radiation that reaches Earth from the sun. The important wavelength is 0.4 – 0.8 μm i.e Violet to Red in the Visible Spectrum. Longer wavelength –infrared radiation and TV and radio waves. Shorter wavelength – X-rays and gamma rays

• Most sunlight reaching the sea surface is transmitted into the sea, little is reflected (2%). This means that sunlight incident on the ocean in the tropics is mostly absorbed below the sea surface, little sunlight is reflected back to the atmosphere.

• Since the Radiation Intensity decreases with depth, the surface waters are warmer than deep waters

• The rate at which sunlight is attenuated determines the depth which is lighted and heated by the sun.

• Attenuation is due to:

i) absorption by pigments. The light energy is converted into heat or chemical energy. Absorption caused phytoplankton (use light for photosynthesis), particulate matter, dissolved material and seawater

ii) scattering by molecules and particles. The more particles present, the greater the scattering

• Attenuation depends on wavelength. Blue light is absorbed least, red light is absorbed most strongly.

Colour of the seawater• Seawater colour ranges from deep blue

• Deep or Indigo blue colour is typical for tropical and Equatorial waters especially where biological productivity is small

• At higher Latitudes the colour changes to green-blue and is green in Polar regions

• Coastal waters are generally greenish

• In the subtropics and mid-latitudes closer to the coast, sea water contains more phytoplankton than the very clear central-ocean waters. Chlorophyll pigments in phytoplankton absorb light, and the plants themselves scatter light.

Factors contributing for open ocean waters to

be BLUE:

• Light scattered by water molecules accounts for

most underwater lights. Molecules scatter the short-

wave (BLUE) light much more than long-wave

(RED) light, so colour seen in BLUE

• Because the red and yellow components are

rapidly absorbed in upper few meters, the only light

remaining to be scattered by water is mainly BLUE

light

• Very productive waters, those with high

concentrations of phytoplankton, appear blue-

green or green. On clear days the color can

be observed from space. This allows ocean-

color scanners, to map the distribution of

phytoplankton over large areas.

• Phytoplankton change the color of sea water,

and the change in color can be observed

from space. Water color is used to measure

phytoplankton concentration from space

• Because light intensity diminishes with

depth, the water column could be divided

into vertical domains

• Photic zone: plants receive adequate

levels of sunlight and can photosythesize

• Aphotic zone – where plants can not

survive

Sound in the Sea• Underwater sound is an important tool for

oceanographers.

• Sound is transmitted faster in water than in air. The average velocity in water is 1 445 m/s in the sea and 334 m/s in air

• It is used to measure ocean depth and to examine the character and thickness of Earth’s crust. Biological oceanographers can use sound to detect and study organisms.

• Military use of sound for submarine detection and locating objects on the seafloor has also encouraged study of underwater sound.

• In clear ocean light may be detected down to 1000m, however human eye can see details in the sea to about 50m only. Further than that it is only by use of sound wave that information in the ocean can be obtained

Properties of Sound

• The velocity of sound in the ocean directly depends on temperature, salinity and pressure (depth)

• The velocity of sound in water increases with increasing salinity, temperature and pressure (depth).

The changes of sound velocity in the ocean can be divided into three zones:

• The surface zone (about 50-100m), where the waters are well mixed. The sound velocity increases with depth in this zone mainly due to the pressure effect (because temperature and salinity stay about the same)

• A zone where the sound velocity decreases because of rapid temperature decreases (thermocline)

• A zone where the sound velocity increases with increasing pressure and the temperature is relatively constant

Use of sound

• Echo sounder to determine the depth to the sea bottom

• SONAR (Sound Navigation and Ranging) to determine the sea-bed images and direction and distance to objects such as submarines.

• Because low frequency sound can be heard at

great distances - to track submarines, used to

listen to and track whales up to 1,700 km away,

and to find the location of sub-sea volcanic

eruptions.

Presentation of observation dataLarge sets of observational data of S, T and ρ as

functions of horizontal space, depth and time

are not easy to assimilate. A basic concept is

to look at a data set every possible way.

Possibilities:

i) Profiles – F = F(z) or F(t)

ii) Successive profiles, taken in the same place

at different times

iii) Sections. The same information can also be

plotted as contours

iv) Horizontal maps

v) Characteristic diagrams, particularly T-S

diagram

Profiles

Contours

Sections

• We tend to draw cross sections of oceanic

properties with considerable vertical

exaggeration, but it is well to remember

that a typical ocean width is measured in

thousands of kilometers and depths are in

thousands of meters: thus vertical

exaggerations of 1000:1 or more are the

rule rather the exception.