Ocean Dimensions and Shapes

1. Ocean Dimensions and Shapes

2. Geography The oceans are basins in the surface of the solid earth containing salt water • Major ocean areas The 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 Sea The Caribbean Sea The 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.)

3. 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

4. Basic Ocean-Land Comparison • Percentage of earth covered by sea (71%) and by land (29%). Land-sea ratio: Southern hemisphere (4:1) Northern hemisphere (1.5:1) • The oceans are much deeper than the land is high The 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

5. 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 paper Bottom topography (bathymetry) plays an important role in the distribution of water masses and the location of currents

6. Nomenclature of Topography • Continental margins: 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.

7. • Deep-sea bottom: 3000-6000 m (74%, deeper 1%) 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.

8. Mid-ocean ridges

9. Schematic reconstruction of continents prior to sea-floor spreading

10. Schematic map of ridge with series of magnetic reversals

11. 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.

12. 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.

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

14. 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.

15. Sea Floor Charts and Data Sets ETOPO-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.

16. Physical Properties of Sea Water • Property of pure water • Pressure • Temperature • Salinity • Density • Equation of the state • Potential temperature • Static Stability

17. 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.

18. 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".

19. 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 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.

20. 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.

21. Pressure and Depth 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.

22. 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

23. Temperature Measurement Thermometer (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)

24. Satellite SST measurement • Thermal infrared (IR) sensors Scanning Radiometer (SR), 8km resolution (early 1970s) Advanced Very High Resolution Radiometer (AVHRR), 1km Cloud blocks IR totally and water vapor attenuates it Atmospheric moisture correction using measurements from different channel A “blended SST analysis” combining AVHRR and in situ SST with 100km resolution is in wide use (Reynolds 1988; Reynolds and smith 1994, 1995) • Passive microwave sensors (6-12GHz) Tropical Rainfall Mapping Mission (TRMM) Microwave Imager (TMI) through cloud layers lower 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

25. 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 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 • dropped from moving ship of opportunity and circling aircraft • Graph of temperature against depth • Range of measurement: 400, 800, 1500 m • depth is estimated from lapsed time and known falling rate (error 20%)

26. Bottle measurement: An Example From Knauss: Introduction to Physical Oceanography

27. Expendable bathythermograph (XBT)