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Properties of sea-water and their distribution in the oceans • What are the physical properties of seawater? (temperature, salinity, density, etc.) • How we observed these properties? • How one property relate to another? • What is the typical distribution of these properties in the world oceans? • How much they vary in space? (horizontally & vertically) • How much they vary in time? (daily, seasonally, annually) • How are they affected by different forcing? (wind, sun, etc.) Special characteristics of water molecules: •One oxygen and two hydrogen atoms (H2O)- strong interaction: very stable •Polar molecule- dipole (positive & negative sides) - withstand electric field (allow conductivity measurements) •High solvent ability- able to dissolve more substance than any other fluid •High surface tension- capillary waves, friction between wind and water •Anomaly- max density of pure water at 4°C, above freezing (0°C)… but different for salt water… H: atomic No.=1 O: atomic No.=8 High Surface Tension: Hydrogen bonding creates “skin” allows bugs to “ walk on water” allows surface “Capillary Waves” Solvent ability: seawaters are salty NaCl Effects of Salinity on the Properties of Seawater: •Lowers freezing point •Lowers temperature of maximum density •Lowers evaporation rate Liquid water NOTE: •water with S<24.7 max density > freezing point •water with S>24.7 max density < freezing point lakes turn over as they freeze, while oceans remain stratified as they freeze Without this property, no marine biology as we know it (or life on earth!) Seawater freezes before reaching max density -1.33 Ice 24.7 (from Pinet, 1998) Temperature • One of the first ocean parameters observed and studied (easy to measure). • Affects many other properties in the ocean (e.g., density, speed of sound, etc.) as well as weather and climate. • Many oceanic features and circulation patterns can be inferred from temperature measurements. • Scientific units: Celsius or Kelvin (absolute temperature) °C = °K – 273.15 (not Fahrenheit!, °F = °C*5/9 + 32) 1000 km depth 1 km 1 km Latitude Antarctica Alaska North-south temperature across the Pacific Ocean: Isotherms: lines of constant temperature How oceanographers obtain such sections? Horizontal-vertical oceanic scales Density distribution is affected by heating and cooling at the surface and water mass transports: Cold-dense waters are formed in high latitudes and sink to deeper layers in lower latitudes Isopycnals: lines of constant density Typical temperature profiles Mixed Layer Seasonal Thermocline Permanent Thermocline Thermocline: region of rapidly changing temperature with depth Surface Mixed-Layer: region of nearly constant temperature (mixed by winds) The seasonal thermocline Balance between: solar heating, surface cooling and wind mixing more solar heating less wind mixing less solar heating more wind mixing July large seasonal changes in mid-latitudes small seasonal changes in tropics and in high-latitudes January Annual mean surface temperature distribution: • Almost Zonal- small change with longitude, large change with latitude, warmer near equator, colder in high latitudes •Non-zonal pattern is affected by ocean circulation: Lat>40N warmer in east side of basins; 40S<Lat<40N warmer in west side of basins In situ and potential temperature • in situ temperature: the observed temperature, T(x,y,z,t) • potential temperature: the temperature of a parcel of water after it has been raised adiabatically from some depth to the surface, θ=func(S,T,P). – adiabatic: without heat exchange with environment – water sinks pressure increases water compresses energy increases water warms (example: T(5000m)=1°C; θ=0.57°C) – removing compressibility (pressure) effects helps to trace water masses and mixing processes Note: compressibility of the atmosphere is much larger than water (up to 10°C/1km, thus its cold at mountain tops!) Note: the relation between water properties (temperature, salinity, pressure & density) are based on empirical formulas – best fit of lots of seawater samples. Note: in deep trenches potential temperature is isothermal and water is neutrally stable, even though in situ temperature increases with depth. Salinity • Units: ‰, ppt (parts per thousand), pss (practical salinity scale) or psu (practical salinity units- controversial units…) Almost all the elements found on earth are present in seawater: • the amounts may change from place to place (river flows, biological activities, etc.) • However conservative constituents are those that maintain the same proportions in most of the oceans Salinity • Simple definition: “total amount of dissolved material in grams in one kilogram of sea water” – Almost impossible to measure (what about gasses?, material lost when drying?, etc.) • Complex definition: Salinity - how the definition of salinity evolved over the years: • Definition based on Chlorinity: S = 1.80655 x Cl Why? 1. Salinity proportional to amount of Chlorine in sea water 2. Chlorine is accurately measured by chemical analysis Accuracy: ±0.02‰ • Salinity based on Electrical Conductivity: C=func(T,S) Why? Most practical and quick way to measure However, need to measure temperature very accurately (±0.01°C error in T ±0.01‰ error in S) Accuracy: ±0.001‰ to ±0.003‰ Salinity “salinity based on conductivity is not really a new definition of salinity, but a relation between Chlorinity and Conductivity relative to standard seawater” (Millero, 1996) • Practical Salinity Scale (PSS-1978): Salinity defined using only conductivity (not Chlorinity) Define standard Potassium Chloride (KCl) sample Why is it important to measure salinity very accurately? The variations of salinity are much smaller than variations in temperature (34.6-34.8ppt) (in the deep ocean- coastal oceans are different story…) Histogram of water masses colder than 4°C Typical distribution of salinity in the oceans: - mostly zonal (but not as much as temperature) - minimum north of equator - maximum at 25°N & 25°S - North Atlantic saltiest ocean, North Pacific least saline Mean salinity at 1000 m shows why N. Atlantic saltier than N. Pacific: Mediterranean outflow Latitudinal distribution of salinity Salinity in mid & low latitudes is controlled by evaporation & precipitation In high latitudes fresh water runoff may be important Vertical profiles of salinity in the open ocean halocline thermocline Light in the sea • Oceans are much more effective absorber of light than the atmosphere (50% can penetrate the entire atmosphere) • Most of the sun’s short wave radiation that reaches the surface of the ocean is absorbed by the top few meters heating the ocean sun controls variation of ocean temperatures • How much is absorbed depends on the clarity of the water, wave-length, and even biology • Solar energy penetration decays exponentially: E(z)=E0e-kz k=vertical attenuation coefficient Turbid coastal water Clearest ocean water The penetration of radiation into the ocean depends on: • clarity of water (including biological activity) • wavelength • depth • scattering • surface reflection Clear ocean water Turbid coastal water The absorption and reflection of the sun’s light spectrum in the ocean cause clear ocean to look blue and turbid coastal water to look green-yellow or brown Ocean color is important for remote sensing of chlorophyll, which indicates biological activity, but also traces oceanic features Temperature Chlorophyll California coast Red Tide: • Excessive bloom of specific phytoplankton (algae) that have a reddish color. • The toxin released by the algae causes severe disturbance for biological activity and fisheries Next Class: • Density and the Equation of State (EOS) • Hydrostatic Stability • Sound Waves in the Ocean