IMS 8 - Physical Oceanography

Physical Oceanography

Studies weather, climate, atmosphere, land, seafloor, consequences of water movement

Properties of Seawater

-          Temperature, salinity, density, pressure, water colour, transparency, ice, sound, velocity, specific hear, compressibility, osmotic pressure, eddy viscosity, electrical conductivity, radioactivity, surface tension

Sea temperature depend on

·         Season

·         Atmospheric circulation

·         Ocean current

Temperature range

·         – 2 °C to 30 °C

·         Seawater surrounded by land hotter

·         Open sea  < 30°C

·         Deep and bottom 1 – 4 °C

Surface Temperature

·         Annual range N hemisphere > S hemisphere (Continental winds)

·         S hemisphere high temp consistency – land mass absence & solar radiation

·         Top layer = isothermal layer

Surface Salinity

·         Rise by evaporation, reduce by precipitation

·         Changes by vertical mixing & inflow of adjacent water

·         Reduced near shore by runoff & river (local variation)

·         Rise by ice formation, reduce by ice melting

·         Max. salinity 20 – 23° N & S

·         Minimum salinity @ equator & high latitude

·         High @ surface (evaporation)

Pressure

·         1 meter = 1 decibar sea pressure

·         From 0 – 10k decibars

·         Created by weight of seawater above

·         Weight per unit volume – varies with temp & salinity

·         At constant depth, press rise as temp reduce / sal rise

·         Shrunken plastic foam cups @ 2500 decibars

Density

·         Depend on sal, press & temp

·         Affects buoyancy

·         g / cm³

·         1.022 to 1.030 g / m³

·         Great changes @ surface

·         Reduced by precipitation, runoff, ice melting, heating

·         Less dense float; denser water sinks – little mix tendency & stable

·         Raised by evaporation, ice forming, cooling

·         Convective circulation (vertical motion)

·         Highest density @ polar regions; cold dense polar water sink & spread to low latitude

·         Entire floor covered with dense polar water

Typical density-depth ocean water profile @ 30-40° South

Compressibility

·         Force molecules closer & denser

·         Seawater nearly incompressible; 0.000046 per bar @ std. conditions

·         If compressibility 0, sea level 90 ft higher (total effect) – immense volume involved

Viscosity

·         Flow resistance; seawater > freshwater

·         Controlled by temp & sal

·         Visc high, sal high, temp low (Object sink @ slower rate)

Specific Heat

·         Calories needed to raise temp 1 g seawater to 1 °C

·         Land heat/cool faster than sea

ü  Specific heat of land < seawater

ü  Temp range land > seawater

ü  Causes monsoon & land/sea breeze

Thermal Expansion

·         Seawater expansion coefficient > freshwater

·         Affected by sal, temp, press

·         Thermal expansion high, when sal high & climate hot

Sound velocity

·         Affected by temp, press, sal

·         High temp, sal & press; high sound speed

·         High speed till 200m; minimum speed till 1000m

Sound Fixing & Ranging (SOFAR)

·         Sound channeling through thermocline due to temp & press difference

·         Sound wave refract towards minimum sound speed

·         Up-down bending low-freq sound waves allow distant travel w/o energy lost

·         SOFAR / Velocity Minima channel between 600-1200m below surface (except high latitude)

·         Hydrophone @ right depth – record distant whale calls, earthquakes, man-made noise

Shadow Zone

·         Thin, high-sound velocity layer

·         Depth 80m deflects sound