TOPIC 3: SURFACE OCEAN CIRCULATION
Part A: WINDS
Reading: “
Ocean Circulation
” Chaps 1, 2 (pgs.11-21) and 3 (pgs. 37-68), 4 (pgs. 107-113).
Reading: “ Introduction to Physical Oceanography”
(on-line text) Chap 9 (9.2, 9.4, 9.5) and
Chap 10 (10.2-10.10)
I. INTRODUCTION
Surface Ocean Circulation
1.There are some clear circulation features in the surface ocean ( Fig. 1 ) a.
ocean gyres are the large circular clockwise or counterclockwise features a.
they are found in all the ocean basins b.
most apparent in the subtropical latitudes (~10º to 40º) and are referred to as subtropical gyres c.
also appear in subpolar latitudes in the N Pacific and N Atlantic
2. Subtropical gyres cover a large region of the ocean (~50%) and most of the warm water portion of the ocean.
3. Zonal (E-W) circulation in the tropical latitudes near the equator
-especially apparent in the Pacific Ocean
4. Zonal (E-W) circulation in the Southern Ocean associated with the Antarctic
Circumpolar Current
-unique example of where surface current extends into the Deep Sea
A.
Importance of Surface Circulation
1.
Heat Transport ( Figs. 2, 3 ) a.
Pattern of strong poleward flowing warm currents in the western portion of the subtropical gyres (e.g. Gulf Stream) and cooler equatorward return flow in eastern portion of the basin (e.g. Canary Current) b.
As a result, the gyre currents are a mechanism to transfer heat poleward on earth and cause an asymmetry in the position of SST contours especially prominent in the N. Atlantic Ocean
2.
Anthropogenic CO
2
Uptake ( Fig. 4 ) a.
Much of the CO
2
produced by fossil fuel combustion that has been adsorbed by the ocean lies within the subtropical gyres.
3.
Photosynthesis rates (Biological Productivity ) in the surface ocean ( Figs. 5, 6 )
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II. WINDS a.
Subtropical gyres are sites with low nutrient concentrations in the surface layer, whereas the subpolar (and equatorial) regions have high nutrient concentrations b.
Subtropical gyres are regions of lower productivity rates in the ocean
Importance : The pattern and strength of winds on the earth have a major impact on the pathway and speed of currents (circulation) in the surface ocean.
A. Global Wind Patterns
1. There are three major surface wind regimes on earth ( Fig. 7 )
Trades Winds (0-30º), Westerlies (30-60º) and Polar Easterlies (60-90º)
-these three wind regimes occur in both hemispheres
2. Note: Wind directions are usually referred to using the direction from which winds blow
-Westerlies are winds blowing from the west towards the east and easterlies are winds blowing from the east towards the west
- unfortunately, this convention is opposite of the convention used for currents e.g., eastward currents are flowing towards the east
3. What causes the winds to blow in the observed patterns on earth?
4.The direction and speed of winds on earth results from the interaction of two forces .
-the horizontal pressure gradient force and Coriolis force
-horizontal pressure gradients in the atmosphere cause air to move (winds) and
Coriolis Force acts on the direction of flow (wind direction) once the air starts to move
5. Fluids, like air and seawater, move from regions of high pressure to regions of low pressure
-Analogy: If you sit down on an air mattress, the air moves out from under you because you’ve increased the pressure at that spot
-if there is a difference in air pressure between two locations on the earth’s surface
(that is, a pressure gradient ) air will move from the site of high pressure to the site of low pressure
6. Differences in air pressure at the earth’s surface are the driving force for air motion
-at all spatial scales…global, regional, local
7. Air pressure exerted at the earth’s surface depends on the mass of air above that location, which in turn depends on the air density and column height of air.
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-thus Surface Air Pressure (P) = gravitational acceleration (g) * air density (ρ)* height of column of air (Z), that is, P = g*ρ*Z
-the denser the air and the higher the column height, the greater the air pressure at the surface of the earth
8. The density of air depends mostly on temperature and, to a lesser degree, on the water content.
-colder air is denser than warm air (like for seawater)
-wetter air (higher humidity) is denser than drier air at the same temperature
-adding water vapor to air increases the density of air
9. Warmest air in the tropics and coldest at poles ( Fig 7 )
-high solar insolation rates at the equator warms the air and causes it to rise
-(warmer air is less dense than colder air)
-in contrast, the low solar insolation at the poles results in colder air temperatures and denser air
-cooling air makes it denser and causes it to sink at the poles
10. If the distribution of solar heating (insolation) on earth was the only process causing winds then the expected global wind patterns would be very different from the observed pattern on earth
11. On a non-rotating earth, surface winds would blow from the poles (cold dense air) toward the equator (warm less dense air) ( Fig. 8 )
-that is, from a region of high pressure to low pressure
-there would have to be return air circulation towards to pole to balance the surface air flow to the equator, to maintain a steady-state distribution of air mass
-this return air flow occurs at higher altitudes in the atmosphere, and results from warm air rising at the equator, moving poleward, cooling and, ultimately, sinking at the poles.
12. What would be the direction of heat transport with this simple system (on a nonrotating earth)?
-hot air transported poleward and cooler air transported equatorward
-there would be a net transport of heat poleward in both hemispheres (as observed on (rotating) earth)
B. Coriolis Force
1. The fact that the earth is a rotating sphere has a major impact on the observed wind patterns
2. There is an ‘ apparent
’ force needed to deflect moving fluids, including winds and ocean currents, because the earth is a rotating sphere
-this “apparent” force is called the Coriolis Force
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-it is a force that appears to act because our reference frame, the earth, is a rotating sphere
3. The effect of the Coriolis Force is often demonstrated using a merry-go-round ( Fig. 9 )
-a ball thrown towards someone on the opposite side of a spinning merry-goround will wind up missing the person because the target has moved while the ball has traveled in a straight line
-if the merry-go-round is rotating counterclockwise, the ball appears to be deflected to the right of the target by the person throwing the ball
-if the merry-go-round is rotating clockwise, the ball appears to be deflected to the left of the target by the person throwing the ball
-the Coriolis Force is the force needed to cause this observed deflection
4. Look at the following link to see a video of the merry-go-round explanation of the
Coriolis Force
-link: http://www.wiley.com/college/strahler/0471480533/animations/ch07_animations/animation2.html
-Can you explain why the ball is deflected when the target is in the middle of the merry-go-round and not changing position?
5. On earth, the Coriolis Force accounts for the deflection of currents in the ocean and winds in the atmosphere observed on earth due to the earth’s rotation
-since the ocean and atmosphere are not attached to the solid earth, these fluids act like the ball (unattached) that was thrown on the merry-go-round
6. A key observation is the rotational velocity of the earth surface decreases as one moves from the equator to the poles ( Fig. 10 )
-the circumference of the earth (along a line of constant latitude) decreases poleward but the rotational period is 24 hours at all latitudes
-thus the rotational speed or velocity of the earth decreases poleward
7.Because of the poleward decrease in earth’s rotational velocity, when winds or ocean currents move poleward on earth, their starting rotational velocity would exceed the rotational velocity of the “target” they are moving toward
-in contrast, if winds or currents are moving equatorward , their starting rotational velocity would be less than the rotational velocity of the “target” they are moving toward
-this difference in rotational velocity causes an observed deflection in currents and winds
8. The effect of the Coriolis Force on winds (and ocean currents) on earth can be visualized by the following example ( Fig. 10 )
-if one launched a rocket directly northward from the equator (Galapagos Islands) towards a target at 30ºN (New Orleans) along the same longitude line, the rocket would land east of the target (to the right of the target looking in the direction of the target)
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-this deflection occurs because the eastward rotational velocity of the rocket is
1600 km/hr at the launch site (equator) and greater than the eastward rotational velocity of 1400 km/hr at the target site (30ºN) because of the spherical shape of the rotating earth
-in contrast, if the rocket was launched from the north pole to 30 ºN, the rocket would land west of the target
-the rocket’s path would appear as a deflection to the right of the target in both situations (looking at the target)
9. Class Problem : In the above example, if it took 1 hour for the rocket to travel between the Galapagos Islands and New Orleans, by how many kilometers would the rocket miss the target?
10. Demonstrate to yourself that a rocket would be defected to the left of the target
(facing the target) if it were launched either poleward or equatorward in the southern hemisphere
11. The Coriolis Force deflects winds and ocean currents moving eastward or westward , to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, just like the situation when they are launched equatorward or poleward
-although this is harder to demonstrate simply
12. Only for winds (and currents) flowing along the equator , either eastward or westward, is there no deflection on earth and, thus, no Coriolis Force.
-this absence of deflection occurs since a fluid moving along the equator moves along the path of constant and maximum rotational velocity
C . Influence of Coriolis Force on Winds on Earth
1.On earth, the Coriolis Force deflects air mass trajectories from meridional (north-south along meridians) towards zonal (east-west along latitudes)
-Coriolis Force prevents direct pole to equator air flow as there would be on a non-rotating earth
2. Warm air rising at the equator moves poleward at higher altitudes in the atmosphere
-as this air moves poleward the Coriolis Force causes it to be defected towards the east (in the Northern Hemisphere) and west in the Southern Hemisphere
-this air cools at these high altitudes and begins to sink at around 30° ( Fig. 7 )
-when this sinking air reaches the earth’s surface some of it moves towards the equator and some moves poleward
3. The equatorward moving air at the earth’s surface is deflected by the Coriolis Force
-deflected to the right in the Northern Hemisphere and to the left in the Southern
Hemisphere
-these surface winds are known as the Trade Winds and are located between 30º and 0º in each hemisphere
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-these winds are called the Northeast Trades in the Northern Hemisphere and
Southeast Trades in the Southern Hemisphere
-(Remember: wind names reflect the direction from which they blow)
4. The poleward moving surface air at 30º is also deflected by the Coriolis Force (
Fig. 7 )
- deflected to the right in the Northern Hemisphere and to the left in the Southern
Hemisphere
-these surface winds are known as the Westerlies and are located between 30º and
60º in each hemisphere
5. As the Westerlies move poleward along the earth’s surface they warm which causes the air to rise at around 60º (
Fig. 7 )
6. The rising air at 60º moves poleward at higher altitudes in the atmosphere (
Fig. 7 )
-it is deflected to the right (in Northern Hemisphere) and left in Southern
Hemisphere at height
-It cools as it moves poleward at these higher altitudes and ultimately sinks at the poles
7. When the sinking air at the poles reaches the earth’s surface, it moves equatorward.
-the Coriolis Force deflects these surface winds to the right in the Northern
Hemisphere and to the left in the Southern Hemisphere
8. The surface winds at these high latitudes are known as the Polar Easterlies ( Fig. 7 )
-the Polar Easterlies lie between the poles (at 90º) and 60° in both hemispheres
9. Thus the three global scale wind regimes on the earth’s surface ( Trades , Westerlies and
Polar Easterlies ) are the result of two processes: atmospheric pressure gradients driving air flow and Coriolis Force deflecting air flow
10. The strongest wind regimes are the Southeast Trades and the Westerlies in the
Southern Hemisphere ( Fig. 11 )
11. The global pattern of wind flow on earth depicted in Fig. 7 ( or Fig. 11 ) would only occur if there were no continents (that is, a water-covered earth)
12. The actual observed winds at the earth's surface deviates from this ideal picture of
Trades , Westerlies and Polar Easterlies water covered earth ( Fig. 12 )
-the observed deviation occurs because air masses located over the ocean heats up and cools down at a different rate than air masses located over continents
-air masses over continents is hotter during the summer and cooler during the winter than air masses over the ocean
-there is typically lower air pressure over continents during the summer
(warm air) and higher air pressure over the continents in winter (cold air) than over the ocean at similar latitudes
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-the location of regions of high and low pressure centered over the ocean and continents causes rotational air flow (like ocean gyres) on regional scales
-also there are topographic barriers on continents (mountain chains) that block or deflect the flow of winds from the idealistic flow patterns show in Fig. 7
13. The influence of the Coriolis Force on regional winds can be seen by the clockwise circulation pattern of winds in regions of high pressure and counterclockwise pattern of winds in regions of low pressure in the Northern Hemisphere ( Fig. 12 )
-Explain why the opposite flow directions around regions of high and low air pressure are observed in the Southern Hemisphere.
14. Evidence in the observed winds of the Trades, Westerlies and Polar Easterlies is present, especially over the ocean, but these wind regimes are neither consistent nor continuous across the continents and oceans ( Fig. 12 ), as they appear on the water covered earth ( Figs. 7, 11 )
15. The Intertropical Convergence Zone (ITCZ) is defined as the contour (isotherm) that connects the locations of highest surface air temperature on the earth’s surface (
Fig. 12 )
-the ITCZ is usually located near, but not necessarily on, the equator
-the ITCZ is a region where warm, moist air rises and thus precipitation is high
-the ITCZ generally corresponds to the contour of lowest surface air pressure
-this is important because it means that the Trade Winds will blow towards the ITCZ from the north and south
-the ITCZ acts as a partial barrier to the direct flow of air masses between the hemispheres
16. The large scale atmospheric circulation patterns affect the global distribution of precipitation on earth ( Fig. 13 )
-air cools as it rises and colder air holds less water vapor than warm air
-precipitation rates are highest where air is rising and cooling (at the equator)
-precipitation rates are lowest where air is sinking and warming (30º and poles)
-these meridional precipitation trends have a major effect on the global distribution of tropical rain forests (equator) and deserts (~30°)
III. Heat Transport
1.The amount of solar radiation received at the top of atmosphere varies during the year
(seasonally) and at different latitudes ( Fig 14 )
-if the seasonal insolation is integrated over an entire year, there is a net surplus of heat in the tropics (equatorward of 40º) and a net deficiency of heat in the polar regions
(poleward of 40º)
2. Thus there must be net heat transport poleward in order to offset or balance the solar heating excess in the tropics and deficiency in polar latitudes
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-otherwise the temperature in the tropics would be increasing and temperature in the polar regions would be decreasing over time
-such long term tropical temperature increase and polar decrease are not observed
-(although increasing levels of greenhouse gases have caused the average temperatures on earth to increase during the last century)
3. Atmospheric winds and ocean currents are the mechanisms that cause a poleward redistribute heat on the earth’s surface (
Fig. 15)
-thus ocean currents and winds play a major role in climate regulation on earth.
-slower winds and currents speeds would likely cause higher surface air temperature gradients between the equator and poles (cooler poles and warmer tropics)
IV. LATITUDES AND LONGITUDES
1. Latitude and longitude define a right angled grid for specifying locations on the earth
( Fig 16 )
2. Latitude indicate position north and south of the Equator (0º)
3. Longitude indicate position east and west of the Prime Meridian which goes through
Greenwich, England at 0°
4. The 180° longitude line is located half way around the earth from the Prime Meridian and lies in the western Pacific Ocean near New Zealand
-What is latitude and longitude of Seattle?
-Generally you should know which longitudes fall in the Atlantic, Indian and
Pacific Oceans
5. Oceanographers often use the terms "meridional" to mean in the direction of a meridian
(North-South) and "zonal" to mean along a latitude line (East-West)
-e.g., a meridional cross section is in the north-south direction and a zonal cross section is in the east-west direction
6. The latitudinal circles (latitude lines) decrease in circumference as one goes poleward
-at equator, 1° of zonal (east-west) change equals 110 kilometers (kms) or 70 miles
-at any other latitude the east-west distance equals 110 km*cos(latitude in °)
-e.g., at 50°, the distance between 1° of
(50)=110*0.64=71km longitude is 71 kms, i.e., 110km*cos
7. The north-south distance (between 1 º of latitude) is always 110 km. Why?
8. Question: What are the current speeds (cm/s) of the current moves 1° zonally in 100 hours at the equator and at 50ºN?
-at equator:
Vel= 1° *110km/deg *1000m/km *1/100hrs *1hr/3600sec =0.3 m/s=30cm/s
-at 50°N (or S):
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Vel = 1° 110km/deg
* cos(50) *1000m/km *1/100hrs *1hr/3600sec = 0.2 m/s =
20cm/s
9. Longitude lines also determine time intervals because during one rotational period (24 hrs) the earth rotates through 360° of longitude
-thus the earth’s rotation speed = 15° longitude per hour
-since Seattle's longitude is 123°W, it’s local time is 8 hours earlier than time at the prime meridian site at Greenwich, England (0º)
-that is, 123º / (15º/hour) = 8.2 hours
-the nomenclature is Seattle's time is +8, that is the time we add to local time to get Greenwich time
-when you are on a research cruise most records are kept in both local and
Greenwich time so that there isn't any confusion about time zones and daylight saving etc.
10. The 180° longitude line is known as the
International Date Line and is 12 hours different than the time at Greenwich
-e.g. if it is 12:01 AM in the morning on Monday at Greenwich, GB it will be
12:01 PM in the afternoon on Monday in New Zealand and 4:01PM Sunday in Seattle
(126°W), and 1:01 PM Sunday in Honolulu (160°W).
- traveling from Honolulu to New Zealand you go back 1 hour (from 1:01 PM to
12:01 PM) but ahead 1 day (from Sunday to Monday)
V. KEY POINTS
1.The pattern of surface winds on earth has a critical role in determining the pattern of surface currents in the ocean.
2. The direction and speed of winds is determined by patterns of horizontal gradients
(changes) in air pressure and the Coriolis Force.
3.The Coriolis Force is an apparent force that acts on winds and ocean currents pushing them to the right in the northern hemisphere and left in the southern hemisphere
-it accounts for the rotation of our spherical reference frame (earth)
4. Winds blow from regions of high air pressure to regions of low air pressure
-on a global scale, this causes surface winds to blow from 30º towards the equator and towards 60° and from the poles at 90º towards 60 º
5. There are three generalized global wind regimes on earth
-the Trade Winds (0-30°), Westerlies (30-60°) and Polar Easterlies (60-90°)
-these regimes occur in both the Northern Hemisphere and Southern Hemisphere
-they are the results of meridional trends in surface air pressure and Coriolis Force
6. Differential solar heating rates over continents and ocean and topographic features
(mountain chains) distort the observed global patterns of wind circulation from the generalized three wind regime pattern expected for a water-covered earth
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7. The InterTropical Convergence Zone (ITCZ) connects the locations of highest surface air temperatures, and is a boundary between the NE and SE Trade Wind regimes of southern hemisphere and northern hemisphere
-the ITCZ is a region of high precipitation due to rising moist air and low wind speeds (doldrums)
8. On a local or regional scale, regions of low pressure have counterclockwise ( cyclonic ) circulation in northern hemisphere and clockwise in southern hemisphere
-local regions of high pressure have clockwise ( anticyclonic ) circulation in northern hemisphere and counterclockwise in southern hemisphere
9. There is a poleward transport of heat in both the atmosphere (via winds) and in the ocean (via ocean currents), which has an important role in controlling the observed air and ocean temperatures on earth.
VI. QUESTIONS/PROBLEMS
1. Show how the circulation pattern of winds between 0-30º, 30-60º and 60-90º would change if the earth’s direction of rotation was reversed (clockwise looking down at the north pole)
(Hint: what happens to the Coriolis Force?)
2. What would be Coriolis Force effect on a cylindrical shaped rotating earth?
3. Explain why the earth isn’t heating up in the tropics and cooling at the poles given the surface heat flux distribution shown in Fig. 14
4. Can you explain why the Coriolis effect increases with increasing latitude (poleward)?
(Hint: picture how the circumference of the latitude lines changes with latitude. Where is the change rate the greatest and least?)
5. What two effects cause the observed wind directions on earth (Fig. 12) to deviate from the idealized (water covered earth) wind patterns (Fig. 7)?
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