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Introductory Physical Oceanography
(MAR 555) - Fall 2009
Geoff Cowles
Unit 1:
Properties of Seawater
Key Concepts:
1. The Earth
2. Coordinate Systems and Projections
3. Features of the Worlds Oceans
4. Local Setting: Gulf of Maine
5. Properties of Seawater: T & S
6. Equation of State
7. Characteristics of the Worlds Oceans
The Earth: Our Oblate Ellipse
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Not quite round
Probably not flat
Equatorial Radius: 6356.7497 km
Polar Radius: 6378.1349 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
Earth’s Orbit: Principal Axes of Rotation
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To Distant Star
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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
2π/86400s = 7.2722e-5 rad/s
Earth’s rotation around its own axis is
slowing down due to tides
Note occasional increases due to
changes in moment of inertia
Axes have same sign relative to
orbital plane: Prograding planet
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
Difference ~ 1 degree/day, why?
Spherical Coordinate System
Important
Latitudes
• Tropics of Cancer (~23.5N) and Tropics of
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Capricorn (~23.5S) – Within this band Sun
will reach zenith at some point during year.
Polar Circles (~66.5 N/S) – Between these
and poles will experience full 24 hours of
day and night at least once a year.
Climate / Dynamics Zones
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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 one to one)
Meridional: Along a line of longitude
Zonal: Along a line of latitude
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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.
Lengthscales: Real Distances
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At the equator, 1 degree of long or lat is about 111 km.
Moving poleward, 1 degree of latitude varies slightly from 111km
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
Projections: A curved surface in 2D
Mercator Projection of Earth!
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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
Projections: Local Coordinate Systems
Projection Software
• M_Map (Matlab)
• GIS
• Proj (http://trac.oscgeo.org/proj/)
• Pyproj (python wrapper for proj)
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: O (10000 km)
• Vertical Scale: O (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
Ocean Depths: Histogram
Depth / Elevation Statistics
• Average Depth: 3730m
• Maximum Depth: 11,524m
• Maximum Elevation on Land:
8840m
• Average Land Elevation: 840m
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
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
The Geoid
What is it?
Cause
• Even if we shutoff all
• Perturbations in gravity
external forcing (wind,
caused by features in the
sun, tides, etc.) and let the
seafloor warp the sea
ocean come to rest, it
surface.
would not be ‘flat’ (i.e.
• Note a rise in SSH over
distance between surface
an object of large mass!
and satellite not constant)
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)
Canadia
Jordan Basin
Wilkinson Basin Georges Basin
NE Channel- Sill
Depth: 230m
Georges Bank
Great South ChannelSill Depth: 70m
Canyons
Properties of Seawater
Temperature
What is it?
• Measure of the Kinetic
Energy of a Substance
• Fundamental Unit is
Kelvin
• At 0K, no Kinetic Energy
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.
Mercury Thermometers
• Slow
• Accurate to about .001 °C
• Early ocean measurements used
Reversing models
Platinum Resistance
• Mechanism: Electrical
conductivity is temperature
dependent
• Expensive, primarily used for
calibration
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!!
Temperature Distributions
Salinity
What is it?
• Measure of the Kinetic
Energy of a Substance
• Fundamental Unit is
Kelvin
• At 0K, no Kinetic Energy
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.
Mercury Thermometers
• Slow
• Accurate to about .001 °C
• Early ocean measurements used
Reversing models
Platinum Resistance
• Mechanism: Electrical
conductivity is temperature
dependent
• Expensive, primarily used for
calibration
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!!
Pressure
What is it?
• Measure of the Kinetic
Energy of a Substance
• Fundamental Unit is
Kelvin
• At 0K, no Kinetic Energy
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.
Mercury Thermometers
• Slow
• Accurate to about .001 °C
• Early ocean measurements used
Reversing models
Platinum Resistance
• Mechanism: Electrical
conductivity is temperature
dependent
• Expensive, primarily used for
calibration
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!!
Density
In practice, Absolute Density is extremely tedious to
measure
Volume: V (m3)
Mass: m (kg)
Solution: Estimate in situ using an Equation of State
ρ = m/V (kg/m3)
ρ = ρ(T,S,p)
Linearized EOS
    a(T  T0 )  b( S  S0 )  kp
where
kg
o
T

10
C S=35psu
0
3
m
kg
kg
kg
a =.15 3 o
b=.78 3
k = 4.5 10-3 3
(m )( C )
(m )( psu )
(m )(decibar )
  1027
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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