Physical Oceanography
Physical Characteristics of Water
• Water displays the bizarre trait of increasing,
then decreasing and later increasing its density
with steadily lowered temperature. (the range
oceanographers use however is a simple inverse
relationship.)
• Rho or TS diagram- a simple device to get water
density (Rho) by factoring in both temperature &
salinity.
• Rho diagram of the main 3 oceans-considerable
variation of water masses, but stunningly
uniform, extremely cold signature for AABW
– AABW is largest single ocean water package
Thermoclines, Haloclines,
Pycnoclines
• Represent major change in Temp., Salinity
and density with depth
• Divide oceans between surface water and
deep water
• Can rise and fall on seasonal basis
• Pycnocline (water density change) is byproduct of both T&S as they affect density
Physical Oceanography
(Currents)
• Thermohaline circulation is water cooling at
the poles, gaining density and sinking to run
near the ocean floor towards the equator.
• At the equator, it rises as it warms, then wind
directs surface currents into gyres as the warm
water makes its way back to boreal latitudes,
cooling again
• In the thermohaline cycle water collects nutrients
as it passes through polar areas
• The rich nutrient package carried by boundary
currents makes them the most important water
masses on Earth!
Ocean Currents
• Ocean currents are driven by water
density and wind
• Coriolis effect does not drive water
currents, it directs them
• For most currents, density variations by
temperature differences count the most
– An exception is the Mediterranean, where
currents are primarily salinity driven
• General rule- deep subsurface currents
are density driven, surface currents are
wind driven
Surface Ocean Climate Zones
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Tropics – O °(equator) ~30 ° N/S
Subtropics - 30 ° N/S to ~45 ° N/S
Temperate zone - ~45 ° N/S to ~60 °N/S
Boreal (subpolar & polar) - ~60 °N/S to Poles
Zones are separated by “isotherms” that spread
in the west and converge in the east due to
surface wind driven currents
Surface Ocean Water Circulation
• Driven by thermohaline engine but
diverted and redirected into gyres by wind
• To understand, we must look at wind
circulation patterns created by rotating
atmospheric cells (Hadley, Ferrell, and
Polar cells)
• In oceanography, only the bottom portion
of atmospheric cells are important
Wind Across the Sea
• Bottom portion of Hadley, Ferrell and Polar
Cells
• Directed by Coriolis Effect into wind
patterns
• Predominant patterns - Trade Winds,
Westerlies, Easterlies, but calm at
Doldrums & Horse latitudes
• Merry-go-round effect generates gyres
• Fetch = distance that wind blows over
ocean creating waves
Gyres
• 5 major gyres (largest is SPSTG)
• Of the major east-west current systems,
four of the nine flow to the east- PECC,
AECC, IECC, ACPC (West Wind Drift or
WWD)
The Gulf Stream
• First mapped by Ben Franklin
• Western portion of NASTG (North Atlantic
SubTropical Gyre)
• Major meanders & eddys
• Warm, relatively nutrient poor but huge potential
for power
• Shaped like a high meandering wall reaching
deep into western Atlantic
• Easily mapped by infrared sensors
• Basis of Matthew Maury‘s recognition of “Rivers
in the Sea”
Waves and Wave Energy
• Crest= top portion of each wave
• Trough = lowest portion of each wave
• Wavelength = distance between
successive crests or troughs
• Water moves in circles as energy passes
through water-circles get successively
smaller with depth
• Wave base = one half wavelength–
defines lower limit of water motion
Ekman Spiral
Effect of Coriolis on Surface Currents
• Successively deeper levels of water-each
directed to right or left in N&S hemispheres
• Strong surface winds drive the system by
spiraling vectors-net result is 90° angle
• Working along a coast line - Coriolis can cause
upwelling or downwelling
• Working in mid-ocean – it can mound up water
causing downwelling & poor bioproductivity
• Working in tropics – N&S Equatorial currents pull
water away from equator causing upwelling at
EEC- very high bioproductivity
Langmuir Circulation
• Wind in excess of 8 mph for longer than a
fortnight will set up huge, barrel-like
horizontal circulation cells
• Langmuir rotating cells windrow the
surface plankton into linear oases.
• Sargasso Sea in mid-Atlantic gyre displays
Langmuir flow with lines of rich flora/fauna
El Niño and La Niña
• Normally, the NE and SE Trade winds
push a mound of warm water to western
equatorial Pacific - “loading the gun”
• During El Niño, trade winds slow abruptly
& the mass of warm water comes racing
back to eastern margin of ocean
• The E.N. water spreads N&S along west
coasts of Americas, bringing severe,
abnormal weather to affected regions
Effects of El Niño
• Excess precipitation (torrential rains,
flooding & landslides) to S.&N. America
• Devastation of Peruvian fisheries
• Drought (wild fires and insect attack) in
Australia, New Zealand
• Large die-off of fish, birds and marine
mammals
– May generate local eutrophication (loss of
oxygen and complete crash of food chain)
In short, a reverse of normal weather
patterns for all regions
Plate Tectonics and Ocean
Currents
• Thermohaline circulation established by ~ 38
mya when Australia moved far enough north of
Antarctica to allow WWD to develop
• Position of coast lines globally causes tidal
friction – slowing Earth’s rotation (450 days/year
in Devonian ~350 mybp) “spinning egg stunt”
• Today, oceans are N-S oriented- during
Mesozoic (200 -66 mya) they were E-W (Tethys
seaway allowed free movement of tropical forms
and more equal heat flow)