Chem 1103: Lecture 2: Wind and Ocean Circulation (atmosphere and surface
ocean): Chap. 6
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Physics – more developed than the descriptive, have mathematical equations
o Equations to describe the motion of the fluid
o Coordinate system x,y,z and u, v, w
o Conservation of Mass
 Equation
 What goes in, what comes out, or increase in density or elevation or
compression
o Conservation of Momentum
 Equation
 acceleration, advection, Coriolis, pressure, surface stress (Wind),
bottom and internal friction, gravity (in vertical)
o Most all fluid ocean motion can be explained by these equations, forces
o What can’t is usually due to instabilities
o Not all of them are important in all process
Atmosphere and Ocean linked at surface
o Mainly we are interested in the oceans, but some of the atmosphere influences
the ocean and many of the processes are similar
o How does atmosphere affect the ocean
 Wind blowing on surface creates waves and currents
 Heat transfer from the ocean to the atmosphere, if air warmer, air heats
ocean; if ocean warmer, ocean heats air
 Moister transfer from the ocean to the atmosphere – evaporation and
precipitation
o Only acts at the surface of the ocean where the atmosphere and ocean meet
Atmospheric Processes
o Density of air is controlled by temperature, pressure, and moisture content
 Warm air is less dense than cold air and moist air is less dense than dry
air because water vapor is lighter than air
o Pressure
 Air pressure is the weight of the air from Earth’s surface to the top of
the atmosphere and equals 1.04 kg/ cm2 (standard air pressure, one
atmosphere) at sea level
 P=gh
 High pressure where pressure is more than 1 atm
 In a low pressure zone, air density is lower than in surrounding areas
because the air is warmer or has a higher moisture content
 Fluids (liquids and gases: air and water) flow from areas of high
pressure to areas of low pressure
 Water that flows down the hill
 Change in pressure across a horizontal distance is a pressure gradient
 The greater the difference in pressure and the shorter the
distance between them, the steeper the pressure gradient and
the stronger the wind
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o Wind response to pressure
 Global winds blow in response to variation in pressure related to
uneven solar heating (insolation) of Earth’s surface
 Equatorial low pressure occurs because intense heating and
high moisture content makes the air less dense. Rising air
produces a convergence at the surface into which the air flows
and rises
 Polar high pressure occurs as cold, dry dense air sinks to the
surface and flows outward forming an area of divergence
 Convergence and divergence - explain
 Winds from North are northerly, currents from South are northerly
 Difference between meteorology and oceanography, but need
to understand to understand weather forecasts and currents –
know that they are different
 But the winds here don’t flow from the North, mostly they flow from
the West – so this hypothesis fails to describe winds
o Rotation of the Earth strongly influences winds
 Coriolis deflection is the apparent deflection of objects moving across
Earth’s surface to the right of the direction of travel in the northern
hemisphere and to the left of the direction of travel in the southern
hemisphere
 Applies to objects moving relative to the Earth – air or water
 Use merry-go-round and airplane examples
 The deflection is caused by the differential speed of rotation
between the Earth’s equator and polar regions, relative to an
object moving across the Earth’s surface
 Coriolis deflection increases poleward and as speed increases
 f = 2 sin   = rotation of earth = 7.292E-5 s-1  =
latitude
 Falkland War – British missed by a mile
 Flow goes around the hill
 Example of weather
o Three major convection cells are present in each hemisphere
 Zonal wind flow is wind moving nearly parallel to latitude as a result
of Coriolis deflection
 The Hadley cell extends from the Equator to about 30o latitude
 At the equator, warm moist air expands, becomes less dense,
and rises forming a zone of low pressure called the doldrums
 As the air continues to rise, it cools and most of its moisture
condenses and falls as tropical rainfall
 At high altitude the air flows poleward, becoming cooler and
denser, until it sinks to Earth’s surface at about 30o latitude,
forming a high pressure zone called the horse latitudes
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Most of this air flows southward, as the Coriolis-deflected
trade winds, gradually warming and absorbing moisture. As
the air approaches the equator it again rises and is recycled.
 The Ferrel Cell extends from 30o to about 50o latitude.
 Some of the air descending at 30o latitude flows poleward as
the Coriolis-deflected westerlies to about 50o where they
encounter cold, dense air moving equatorward from the poles.
 The less dense warmer air is forced up over the denser cold air
at the polar front and much of it flows equatorward at high
altitude to about 30o where it enters the horse latitudes.
 The Polar Cell extends from 90o to about 50o latitude
 Some air from the polar front travels poleward becoming cold
and dry
 At the pole the air descends, forming a polar high pressure
zone, and flows outward as the Coriolis-deflected polar
easterlies which eventually collide with the westerlies at the
polar front
 Explain the wind figure of the Earth and the weather chart
o Other forces
 Advection – movement of water – carrying things along
 Diffusion  Friction – slowing down of flow
 Convection – heat rising, cold sinking
BREAK
 Surface Ocean Currents – Oceanography now
o Remember ocean is 3 layers
o For currents, we are looking at average over years, not day to day,
occasionally some seasonal
o Wind-driven currents are produced by the interaction between the wind and
the water
 As wind moves across the water, collision of air molecules with water
molecules inefficiently transfer energy from the air to the water causes
waves and currents
 Westerly-driven ocean currents in the trade winds, easterly-driven
ocean currents in the Westerlies and deflection of the ocean currents
by the continents results in a circular current, called a gyre, which
occupies most of the ocean basin in each hemisphere
o Pressure gradients develop in the ocean because the sea surface is warped into
broad mounds and depressions with a relief of about one meter
 Mounds are caused by convergences, places where water flows
together and sinks
 Depressions are caused by divergences, places from where water rises
to the surface and flows outward
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Water flowing down pressure gradients on the ocean’s irregular
surface are deflected by Coriolis and the amount of deflection is a
function of location and speed
 So this is pressure and Coriolis that are important
o With time, wind-driven surface water motion extends downward into the
water column, but speed decreases and direction changes because of Coriolis
deflection
 Eckman spiral is the spiraling pattern described by changes in water
direction and speed with depth
 Eckman transport is the net transport of water by wind-induced motion
 Net transport of the water in an Eckman spiral has a Coriolis
deflection of 90o to the direction of the wind
 Along the coastal areas Eckman transport can induce downwelling or
upwelling by driving water towards or away from the coast,
respectively
 Upwelling is when near shore surface water is driven seaward
resulting in deeper water being pulled to the surface
o Upwelling also occurs between gyres
 Downwelling is when near shore surface water is landwand
and its accumulation depresses the deep water
o Downwelling also occurs in the center of the gyre
o Langmuir circulation is a complex horizontal helical (spiral) motion that
extends parallel to the wind –
 Adjacent helices rotate in opposite directions creating alternating
zones of convergence and divergence
 Material floating on the surface becomes concentrated in the zones of
convergence and forms sea stripes which parallel the wind
o Geostrophic flow allows currents to flow long distances with no apparent
Coriolis deflection - Coriolis and Pressure
 Coriolis deflects water into the center of the gyres, forming a low
mound
 As the height of the mound increases, the pressure gradient steepens,
pushing the water outward in an attempt to level the mound
 When the pressure gradient equals Coriolis deflections, the current
flows parallel to the wind around the mound as a geostrophic current
and this is called geostrophic flow
 Gyres in the northern hemisphere rotate clockwise and in the southern
hemisphere counterclockwise
 The current flow pattern in gyres is asymmetrical with narrow, deep
and swift currents along the basin’s western edge and broad, shallow
slower currents along the basin’s eastern edge
 Western boundary current intensification is the squeezing of
the western boundary current between a continent and the
mound of water in the gyre, forcing the current to flow faster,
deeper, and narrower
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o The geostrophic mound is deflected to the western part of the ocean basin
because of the eastward rotation of the Earth on its axis
 This creates a steeper slope on the western side of the mound because
more water is being forced into the mound by the greater deflection
from the faster flowing current
o The Sargasso Sea is a large lens of warm water enriched by the North Atlantic
gyre and separated from cold waters below and laterally by a strong
thermocline
o Western boundary currents, such as the Gulf Stream, form an meandering
boundary separating coastal waters from warmer waters in the gyre’s center
 Western intensification – due to variation in Coriolis with latitude
 Meanders of the current can break free and form rings, large swirling
migrating eddies, hundreds of kilometers in diameter and kilometers
thick. They can last for years until they are reabsorbed into the gyres.
 Warm-core rings are those, which enclose a mass of warmer water
compared to the body of water in which they are located.
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END
Lab Lecture
Labs 1 & 9, but with 1 do 2, 3 and 9 do 1, 3, 6, 7 –
Explain how to contour
BRING
1) round tank
2) food color
3) big spoon
4) fiberfill, books
5) tennis or raquet ball
6) spinning chair
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