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Unit 1: Properties of Seawater

Unit 1: Properties of Seawater. Introductory Physical Oceanography (MAR 555) - Fall 2009 Prof. G. Cowles. Key Concepts: The Earth Coordinate Systems and Projections Features of the World’s Oceans Local Setting: Gulf of Maine Properties of Seawater: T & S Equation of State

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Unit 1: Properties of Seawater

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  1. Unit 1:Properties of Seawater Introductory Physical Oceanography (MAR 555) - Fall 2009 Prof. G. Cowles

  2. Key Concepts: • The Earth • Coordinate Systems and Projections • Features of the World’s Oceans • Local Setting: Gulf of Maine • Properties of Seawater: T & S • Equation of State • Potential Temperature and Density • Stability and Stratification • Characteristics of the Worlds Oceans

  3. The Earth: Our Oblate Spheroid • Not quite round (due to rotation) • Probably not flat • Equatorial Radius: 6378.1370 km • Polar Radius: 6356.7523 km • Spins CCW from a point of view of astronaut above North Pole • Spins CCW around sun from point of view of astronaut on North Star • 71% covered with water

  4. Earth’s Orbit: Principal Axes of Rotation • Earth’s rotation around its own axis is slowing down due to tides • Note occasional increases due to abrupt changes in moment of inertia • Spin direction of principal axes has same direction relative to orbital plane: Prograding planet To Distant Star S • Solar Day: 86400 seconds • Time between success zeniths of the sun for a fixed observer • Day is 86400 seconds (24*3600) • Angular Speed of Solar Day • Inertial “Day”: ~86164 seconds • Time between success zeniths of a distance star for a fixed observer • Inertial Ang. Speed 7.2921150e-5 rad/s • This is the Angular Velocity we will use for dynamics 2π/86400s = 7.2722e-5 rad/s Difference ~ 1 degree/day, why?

  5. Geographic Coordinate System Important Latitudes • Tropics of Cancer (~23.5N) and Tropics of Capricorn (~23.5S) – Within this band Sun will reach zenith at some point during year. • Polar Circles (~66.5 N/S) – Area between these and poles will experience full 24 hours of day and night at least once a year. Climate / Dynamics Zones • Latitude: E-W (90°S-90°N) or (-90° < Lat < 90°), huge influence on ocean dynamics • Longitude: N-S (-180° < Long < 180°) or (180°W to 180E°) • Lat/Lon pairs uniquely specify a point on the Earth (reverse mapping not injective) • Meridional: Along a line of longitude • Zonal: Along a line of latitude • Tropics (a.k.a. low-latitude), between Tropics of C’s • Temperate (a.k.a. mid-latitudes), between Tropics and Polar Circles • Polar (a.k.a. high-latitude, frigid zone), between Polar Circles and Poles.

  6. Real Distances • At the equator, 1 degree of long or lat is about 111 km. • Moving poleward, 1 degree of latitude varies slightly from 111 km due to ellipticity of the Earth • Longitude varies greatly, 1 degree of longitude at the Poles is 0 km. • Rough calculation of km/degree longitude is 111*sin(latitude) • At SMAST (41.60N, 70.91W), degree of longitude is about 75km • Very rough rule of thumb in general: 100km /degree • 0.1° resolution ocean model ~10 km resolution

  7. Projections: A curved surface in 2D Mercator Projection of Earth! Artificial stretching: Circles actually all equivalent in area • Popular Conical Projection: Mercator • Useful Properties for Navigation pertaining to Rhumb Lines • Key issue: Greatly Exaggerated Landmass near Poles • Greenland appears as big as Africa but actually is 15x smaller • Conicals centered on the equator have trouble at poles: Singularity

  8. Projections: Local Coordinate Systems • Localized regions O(100K) can work in Earth-Attached Euclidean (Cartesian) coordinate systems • Governing equations are simplified in Cartesian coordinates • Coordinates are more intuitive as they are real distances • Common projection: Lambert Conformal • Geographic Coords are standardized, (x,y) Euclidean pairs depend on details of projection! • Projection Software • M_Map (Matlab) • GIS • Proj (http://trac.oscgeo.org/proj/) • Pyproj (python wrapper for proj)

  9. Oceanic Dimensions • Ocean covers 71% of the Earth’s Surface • Pacific: 181e6 km2 • Atlantic: 106e6 km2 • Indian: 74e6 km2 • Ocean (and Atmosphere) are extremely thin layers of fluid • Horizontal Scale (L): O (10000 km) • Vertical Scale (H): O (1 km) • Pacific: Similar ratio of dimensions to a sheet of paper • Ratio of length scales: Aspect Ratio • H/L very small: Plays a Major Role in the Dynamics

  10. Ocean Depths: Histogram • Depth / Elevation Statistics • Average Depth: 3730m • Maximum Depth: 11,524m • Maximum Elevation on Land: 8840m • Average Land Elevation: 840m

  11. Typical Cross-Basin Profile (Exaggerated Vertical Scale!!) Shelf Break • Shore • Land-Water Interface • Continual Reworking • Adjustment to Glacial, Seasonal, Tidal Time Scales and Storm Events • Continental Shelf • Majority of Worlds Fisheries • Gradual Slope • Shelf Width Varies • Storm Events • Shelf Slope • Steep (Relatively) Gradient • Gravity-driven mud flows

  12. Features: Canyons and Sills • Sills • Shallow Regions separating Two Deeper Regions. Control the Exchange of Water (both Volume and Type) between them. Example: Fjords • Canyons • Sharp features in the relatively gentle cont. shelf • Generated by runoff from previous retreated glaciers • Notable in our region: Hudson Canyon

  13. The Geoid • What is it? • Even if we shutoff all external forcing (wind, sun, tides, etc.) and let the ocean come to rest, it would not be ‘flat’ (i.e. distance between surface and satellite not constant) • Cause • Perturbations in gravity caused by features in the seafloor warp the sea surface. • Note a rise in SSH over an object of large mass! • Why Care • We can use this to detect seafloor features using measured sea surface height (SSH) from satellites • We need to know position of geoid to subtract it out and obtain real SSH anomalies (tides and such)

  14. Canadia Jordan Basin Wilkinson Basin Georges Basin NE Channel- Sill Depth: 230m Georges Bank Great South Channel- Sill Depth: 70m Canyons

  15. Properties of Seawater => Scalar (state) variables that we can measure, observe, and/or estimate • Examples • Temperature (Kinetic Energy) • Salinity (Dissolved Salts) • Density (‘Heaviness’) • Dissolved Oxygen Content • Optical Absorbance • Estimating Density • Salinity/Temp can be measured in situ using relatively cheap instruments • Using an equation of state, these properties can then be used to estimated density quite accurately • Tracing Water Masses • S/T provide cheap markers for water masses Utility • Light Profiles • Absorbance helps estimate how light penetrates the water column, influencing heating and photosynthesis

  16. Temperature • Mercury Thermometers • Slow • Accurate to about .001 °C • Early ocean measurements used Reversing models • What is it? • Measure of the Internal Kinetic Energy of a Substance • Fundamental Unit is Kelvin • At 0K, no Internal Kinetic Energy • Platinum Resistance • Mechanism: Electrical conductivity is temperature dependent • Expensive, primarily used for calibration • How is it Measured • Absolute Temp is very difficult to measure • Solution: use an interpolating device, calibrated to absolute scale at two known points, e.g. a thermometer. • Temperature (T) typically reported using the temperature anomaly scale °C, where T(°C) = T(K) – 273.15 where T=0 °C is the freezing point of water at 1 atm. • Semiconductor Resistance (Thermistor) • Fast • Accurate to about .001 °C • Commonly Used • Remote Sensing: Radiometers • AVHRR instruments on satellites • Convert sensed infrared into electric signals • Incredible Temporal and Spatial Coverage • Surface Only!!

  17. Avg. Sea Surface Temperature Distributions

  18. Salinity • What is it? • Measure of total amount of dissolved salts in g / kg • Chlorine (55%), Sodium (30.6%), Sulfate (7.7%) • These ratios are nearly constant through the ocean – mixing? • If units are dimensionless, should I specify o/oo, or PSU (see below)? Background • 1800s: Chemical Methods: Measure Chl, uses constant ratios of salts to determine total salts and S • 1900s: Electrical Conductivity: Measure K, link to Chl through complex relation, derive S from constant ratio assumption. • 1970: Cox et al- Ratios aren’t constant enough for good accuracy. However, some good news: Conductivity correlates better with density than Chl measurements. What we really want is a measure of salt that can accurately be used to determine density. • 1978: Development of the Practical Salinity Standard based on conductivity: Unit: Practical Salinity Unit (PSU). • Today: Accurate to +/- .005

  19. Average SSS Distributions Tropics, Evaporation > Precipitation Med and Red Seas, Hot Dry Winds Lead Drive Massive Evaporation Poles, Precipitation > Evaporation

  20. T-S Stats Mean Values in the Ocean: T: 3.52°C (75% between 0° and 4°) S: 34.72 (75% between 34.5 and 35) Pacific is Fresher (S = 34.62) than the Atlantic (S = 34.90) Ocean is cold! – Warm water is confined to shallow depths High Salinity Zones: Red Sea (> 40 ) and Dead Sea (293) Seabird CTD

  21. Pressure (Static) and Depth • What is it? • Force per unit area • Units: N/m^2 (ak.a. Pascals, Pa) (S.I.) • Units: bar (100 kPa) • Decibar: 10 kPa (oceanographer) How is it Calculated Constant density approx. Note: • How is it measured • Diaphragm: Membrane with Strain Gauge • Quartz Resonator: Frequency depends on applied pressure • Accuracy +/- 0.5 dbar

  22. Density In practice, Absolute Density is extremely tedious to measure Volume: V (m3) Solution: Estimate in situ using an Equation of State Mass: m (kg) ρ = m/V (kg/m3) ρ = ρ(T,S,p) Linearized EOS (For Estimates Only!) • Coastal applications, influence of p may be ignored • Areas of high suspended sediment load must include mass of dry material • Standards Maintained by UNESCO • Matlab/Python/Ruby Functions on the Web • Online EOS Calculators

  23. Density Anomaly Most seawater density is typically 1020-1030 kg/m3 It is common then to use sigma density defined as: Working within oceanic layers, the influence of pressure (i.e compressibility) may be ignored giving the “sigma-t” density anomaly.

  24. Partial Phase Diagram at 1 ATM • S < 24.7, maximum density occurs at higher temp than freezing • Ice (solid phase) floats on liquid • As surface is cooled, colder, denser water sinks until temperature of max density reached. Further cooling produces relatively lighter water which eventually freezes • At typical ocean salinities (34-35), seawater remains liquid until nearly -2C

  25. Issue: In Situ Temperature and Salinity Unstable water Column?

  26. Depth (m) Parcel 1 P: water pressure T: water temperature 100 T1, P1 Parcel 2 1000 T2, P2 Potential Temperature and Density If T2>T1, does it means that the water parcel 2 is warmer ? Answer: NO! The water is slightly compressible and these two water parcels have different pressures

  27. Potential Temperature and Density How could we compared two water parcels with different pressures? T(Po) = : potential temperature Reference pressure level Adiabatically (no thermal contact with the surrounding water) T(P): in-situ water temperature Replacing T by the potential density, we can define the potential density (sigma-) as

  28. Static Stability: Two Layer Stratification Work (specific) Required to Move Parcel Up a Layer • Low Density on High = Stable • Increasing Density with Depth = Stable • Work requires source of energy, either mechanical (Mixing) or thermal (Heating/Cooling) • ρ1 = ρ2 No work required to move water parcel => no change in potential energy (neutral stability) • In reality, density of the ocean increases with depth. z ρ1 ρ2

  29. Typical Profiles Stratified Water Thermocline: Temperature Gradient Halocline: Salinity Gradient Pycnocline: Density Gradient

  30. Static Stability: Continuously Stratified Stability Measure E > 0: Stable E2 > E1: E2 more stable E = 0: Neutrally Stable E < 0: Unstable (Convection will occur) z Buoyancy Frequency N (s-1): ρ • Natural frequency of oscillation of a fluid parcel at z • ρ is true density (not anomaly) • We will revisit this when we explore dynamical stability in the context of mixing in Unit8.

  31. Light – Beers Law c: attenuation coefficient (depends on level of suspended material) I: incident light at the surface (W/m2) Light profile assuming attenuation does not depend on depth Note: Pay attention to the orientation of the z-coordinate

  32. Data Sources – Examples: • Realtime Data: Ocean Observing Systems (OOS) • GoMOOS • NERACOOS • MACOORA • NDBC • other COOS’s • Archival Data • NODC (point + field data) • NDBC (point data) • Future Data – Model Predictions • NeCOFS: FVCOM model, Gulf of Maine • SCCOOS: ROMS model, Southern Cali • GoMOOS: POM model, Gulf of Maine

  33. Review: • The Earth • Coordinate Systems and Projections • Features of the Worlds Oceans • Local Setting: Gulf of Maine • Properties of Seawater: T & S • Equation of State • Potential Temperature and Density • Stability and Stratification • Characteristics of the Worlds Oceans

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