Ocean Dimensions and Shapes

Geography

The oceans are basins in the surface of the solid earth containing salt water

• Major ocean areasThe Southern Ocean (south of 30o-40oS) The Atlantic Ocean (Younger, expanding, a few centimeters/year) The Pacific Ocean (Older, ringed with trenches and volcanoes)The Indian Ocean The Arctic Sea

These regions are distinguished in terms of land masses (last four) and oceanographic characteristics (circulations, the Southern Ocean)

• Other smaller water basin - Marginal seas (fairly large basins of salt water that are bounded by land or island chains and connected to the open ocean by one or more fairly narrow channels, sometimes called Mediterranean seas)

Mediterranean SeaThe Caribbean SeaThe Sea of Japan (The East Sea)The Bering Sea(The Arctic Sea)etc………

• Areas of open oceans are sometimes also referred to as "seas", mainly for historical reasons and geographical convenience, or local distinguishing oceanographic characteristics (examples: Greenland, Norwegian, Iceland, Labrador, Weddell, Ross, Arabian Seas, Bay of Bengal etc.)

The scales of the major oceans• Percentage to total ocean area and zonal scales

*neighboring sectors of the Southern Ocean included

• Pacific is as large as the Atlantic and Indian Ocean combined

Oceans Percentage* zonal scale

Pacific 46% 15,000 km

Atlantic 23% 5,000 km

Indian 20% 5,000 km

Basic Ocean-Land Comparison• Percentage of earth covered by sea (71%) and by land (29%).

Land-sea ratio: Southern hemisphere (1:4) Northern hemisphere (1:1.5)

• The oceans are much deeper than the land is highThe average ocean depth: ~4000 meters (3730 m)

Marginal seas: 1200 meters or less(the ratio of the horizontal and vertical scales are very large

generally scaled by 1/1000 in vertical sections)The average land elevation: 840 m (The smaller seas are generally about 1200 m deep or less)84% sea bottom is more than 2000 m deep (11% land surface is more than 2000 m)

• Maximum depth in the oceans: Mariana Trench (11,034 m)

• Maximum height on land: Mt. Everest: 8840 m

Histogram of elevations of land and depth of the sea floor as percentage of area of the Earth, in 50 m intervals showing the clear distinction between continents and sea floor. Right: Cumulative frequency curve of height, the hypsographic curve. The curves are calculated from the ETOPO 30’ data set (Stuart 2007).

Sea floor dimensions

From M. Tomczak, 1996: Introduction to Physical Oceanography(http://gaea.es.flinders.edu.au/~mattom/IntroOc/lecture01.html)

Note that the vertical scale is exaggerated by about 1000 times. In true scale, the Ocean is as thin as a sheet of paperBottom topography (bathymetry) plays an important role in the distribution of water masses and the location of currents

Cross-Section of the South Atlantic along 25S showing the continental shelf offshore of South America, a seamount near 35W, the mid-Atlantic Ridge near 14W, the Walvis Ridge near 6E, and the narrow continental shelf off South America. Upper: Vertical exaggeration of 180:1. Lower: Vertical exaggeration of 30:1. If shown with true aspect ratio, the plot would be the thickness of the line at the sea surface in the lower plot (Stuart 2007).

Nomenclature of Topography• Continental margins:

Topography Width (km) Depth (m) Gradient

shelf 65 (up to 300) 130 (up to 200) 1/500~1/20

slope 20-100 200-4000 ~1/20

rise up to 300 4000-5000 1/700~1/1000

Continental rise is the lower part of the continental slope.

In general, continental slope is considerably steeper than the slope from lowland to highland on land.

A typical feature of the shelf and slope is the submarine canyons, carved by rivers usually in hard granite rocks or by the turbidity currents in softer sedimentary rocks.

• Deep-sea bottom: 3000-6000 m (74%, deeper 1%)Topography Horizontal scale Vertical scale Character

Abyssal plain very flat 2m/100km

Seamount ~1000m

Trench arch of a circle

narrow width

up to 11,000m deep

land or islands on one side

Mid-ocean ridge up to 400km wide

rise to1000-3000 m

global with fractured zones

Central rift valley 20-50 km wide cuts 1000-3000 m deep into ridge

Bottom topography is mapped by echo sounders, which measures the time taken for a pulse of sound to travel from the surface to the bottom, and, more recently, inferred from satellite measurements of the earth gravity fields.

Movement of the earth’s tectonic plates shapes the sea floor.

Mid-ocean ridge is a tectonic spreading center. There are narrow gaps in the ridge, called fracture zones.

Seamounts are virtually all volcanic in origin.

Trenches are the active locations where oceanic plates are sinking beneath other plates.

Sills refers to ridges beneath the sea level that separates one basin from another.

An example of a seamount, the Wilde guyot. A guyot is a seamount with a flat top created by wave action when the seamount extended above sea level. As the seamount is carried by plate motion, it gradually sinks deeper below sea level. The depth was contoured from echo sounder data collected along the ship track (thin straight lines) supplemented with side-scan sonar data. Depths are in units of 100 m. From William Sager, Texas A&M University.

An example of a trench, the Aleutian Trench; an island arc, the Aleutian Islands; and a continental shelf, the Bering Sea. The island arc is composed of volcanos produced when oceanic crust carried deep into a trench melts and rises to the surface. Top: Map of the Aleutian region of the North Pacific. Bottom: Cross-section through the region (Stuart 2007).

Mid-ocean ridges

Schematic reconstruction of continents prior to sea-floor spreading

Schematic map of ridge with series of magnetic reversals

Left: Echo sounders measure depth of the ocean by transmitting pulses of sound and observing the time required to receive the echo from the bottom. Right: The time is recorded by a spark burning a mark on a slowly moving roll of paper. From Dietrich, et al. (1980).The accuracy of depth also depends on the sound speed (c≈1480m/s), which is also a function primarily of temperature, less of pressure, and, to a much lesser extent, salinity. Errors also tend to be larger in regions of rapid depth change.

Figure 3.11 Locations of echo-sounder data used for mapping the ocean near Australia. Note the large areas where depths have not been measured from ships. From Sandwell.

Echo sounders make the most accurate measurements of ocean depth. Their accuracy is ±1%. The tracks, however, are not evenly distributed. In some areas, the sampling error can be large.

Figure 3.13 A satellite altimeter measures the height of the satellite above the sea surface. When this is subtracted from the height r of the satellite's orbit, the difference is sea level relative to the center of the Earth. The shape of the surface is due to variations in gravity, which produce the geoid undulations, and to ocean currents which produce the oceanic topography, the departure of the sea surface from the geoid. The reference ellipsoid is the best smooth approximation to the geoid. From Stewart, 1985.

Gaps between ship tracks are now being filled by satellite-altimeter data

Seamounts are more dense than sea water,and they increase local gravity causing a plumb line at the sea surface (arrows) to be deflected toward the seamount. Because the surface of an ocean at rest must be perpendicular to gravity, the sea surface and the local geoid must have a slight bulge as shown. Such bulges are easily measured by satellite altimeters. As a result, satellite altimeter data can be used to map the sea floor. Note, the bulge at the sea surface is greatly exaggerated, a two-kilometer high seamount would produce a bulge of approximately 10m.Typical seamounts produce a bulge that is 1-20 m high over distances of 100-200 kilometers. By combining data from echo sounders with data from GEOSAT and ERS–1 altimeter systems, Smith and Sandwell (1997) produced maps of the sea floor with horizontal resolution of 3 km and depth accuracy of ±100 m.

Sea Floor Charts and Data SetsETOPO-2: The sea-floor topography of the ocean with 3km resolution produced from satellite altimeter observations of the shape of the sea surface. From Smith and Sandwell.

• Property of pure water• Pressure• Temperature• Salinity• Density• Equation of State• Potential temperature• Static Stability

Physical Properties of Sea Water

Property of pure water

• Sea water is a mixture of 96.5% pure water and 3.5% other material, such as salts, dissolved gases, organic substances, and undissolved particles.

• Many physical properties of sea waters are determined by the 96.5% pure water.

Left diagram: Arrangement of the oxygen atom (O) and the two hydrogen atoms (H) in the water molecule. The angle between the positively charged hydrogen atoms is 105°, which is very close to the angles in a regular tetrahedron (109° 28'). Right diagram: Interaction of two water molecules in the tetrahedral arrangement of the hydrogen bond. The hydrogen atoms of the blue water molecule attach to the red water molecule in such a way that the four hydrogen atoms form a tetrahedron.

Consequences:1. The water molecule is an electric dipole. 2. Water has an unusually strong dissolving power, i.e. it splits dissolved material into electrically charged ions.3. Oxygen atoms in water try to have four hydrogen atoms attached to them to form a "hydrogen bond".

Water molecules form aggregates of single, two, four and eight molecules. At high temperatures the one and two molecule aggregates dominate; as the temperature falls the larger clusters begin to dominate. The larger clusters occupy less space than the same number of molecules in smaller clusters. As a result, the density of water shows a maximum at 4°C.

About 90% of all molecules form chains in normal temperatures.

Due to the energy that goes into chain formation, water has quite high heat capacity

Density decreases as the freezing point is approached. (Ice floats). In the sea water, salt tends to inhibit chain formation, the max density is at the freezing point, below 0oC.

Freezing point decreases under pressure. (Melting occurs at the base of glaciers)

Water has high heat of vaporization and high surface tension

Definition of TermsDetermination: the actual direct measurement of a variable

Estimation: a value for one variable derived from the determination of one or more others

Accuracy: the difference between a result obtained and the true value

Precision: the difference between one result and the mean of several obtained by the same method, i.e., reproducibility

Systematic error: one which results from a basic (but unrealized) fault in the method and which causes values to be consistently different from the true value (cannot be detected by statistical analysis of values obtained and affects Accuracy)

Random error: one which results from basic limitations in the method. It is possible to determine a value for this type of error by statistical analysis of a sufficient number of measurements. It affects precision

Pressure (p)• Pressure is the normal force per unit area exerted by water

(or air in the atmosphere) on either side of the unit area.• Unit: 1 Pascal = 1 Newton/m2 = 10 dynes/cm2, or 1 bar = 105

Pascal = 106 dynes/cm2.• Force due to pressure difference is down the pressure

gradient.• Vertically, upward pressure gradient force is largely balanced

by gravity. p~0-1000 bar (A pressure change of 1 decibar (0.1bar) occurs over a depth change of slightly less than 1 meter).

• Horizontal variation of pressure is in the order of 0.1 bar (1 dbar) over 102-103 kilometers, much smaller than pressure change with depth.

• Accuracy of pressure measurement is 3 dbar.

Hydrostatic pressure:

where d is depth (instead of height)If we choose:=1000 kg/m3 (2-4% lower than of sea-water)g=10 m/s2 (2% higher than gravity)then p=1 decibar (db) is equivalent to 1 m of depth(p=1 db = 0.1 bar = 105 dyn/cm2 = 104 Pa (N/m2))• True d is 1-2% less than the equivalent decibar depth.• A pressure change of 1 dbar occurs over a depth change

of slightly less than 1 meter.

∫ ≅−=−d

gdgdzp0

ρρ

Pressure and Depth

Temperature: (T or t)

• Unit: Celsius scale (oC) in oceanography (sometimes Kelvin, K)

• Ocean range: -1.7oC to 30~31oC• Primary parameter determining density. especially in

mid- and lower latitude upper ocean• Major factor in influencing the atmosphere at the surface • Temperature profile provides information on circulation

features and sound speed distribution• easier to measure than other oceanic properties

Temperature MeasurementThermometer (accuracy 0.04oC, precision 0.02oC)Thermistor (0.02oC, 0.0005-0.001oC)

In Situ Sea Surface temperature (SST) Measurements• Bucket-sample (prior to 1950s, thermometer)

wooden to canvas (1880-1890) bucket sampling causes cold bias of

SST in 1900s-1940s from the level between 1850s-1880s (accuracy

~0.1oC) • “Injection temperature” (1950s, thermistor)

measurements in the engine cooling intake water from 2m to 5m,

partially causes a warm bias after 1940 (accuracy 0.5-1oC)

• Bottles, XBTs, and CTDs

• In situ observations measure “bulk” temperature (0.5-5m)

Satellite SST measurement• Thermal infrared (IR) sensors

Scanning Radiometer (SR), 8km resolution (early 1970s)Advanced Very High Resolution Radiometer (AVHRR), 1km, local precision 0.1oCCloud blocks IR totally and water vapor attenuates it Atmospheric moisture correction using measurements from different channelA “blended SST analysis” combining AVHRR and in situ SST with 100km resolution is in wide use (Reynolds 1988; Reynolds and Smith 1994, 1995), Estimated Accuracy, ±0.3oC in the tropics, ±0.5oC near the western boundary (Reynolds, 2002)

• Passive microwave sensors (6-12GHz)Tropical Rainfall Mapping Mission (TRMM) Microwave Imager (TMI)through cloud layerslower resolution (25-50 km)

• Satellite measures the “skin layer” temperature “skin layer” is a very thin (< 1mm)“skin” temperature is usually more than 0.3o colder than bulk temperature

SubsurfaceTemperature Measurement

Nansen bottle• Protected reversing mercury thermometer (±0.02K in routine use) • in situ pressure with unprotected reversing thermometer (±0.5% or ±5

m)• only a finite number (<25) of vertical points once• Deployment and retrieval typically take several hours Mechanical Bathythermograph (MBT, 1951-1975) • Continuous temperature against depth (range, 60, 140 or 270 m) • Need calibration, T less accurate than thermometer (±0.2K, ±2 m)Expendable bathythermograph (XBT, since 1966) • Expendeble thermister casing • Accuracy, T, ±0.1oC, Z, ±2%• dropped from moving ship of opportunity and circling aircraft• Graph of temperature against depth (resolution 65cm)• Range of measurement: 200, 400, 800, 1500 1830m• depth is estimated from lapsed time and known falling rate (error 20%)

Bottle measurement: An ExampleFrom Knauss: Introduction to Physical Oceanography

Expendable bathythermograph (XBT)