OCEAN CIRCULATION

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OCEAN CIRCULATION

• As the world turns.

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Ocean currents

• Three kinds
• 1. surface currents - are driven primarily by
  winds Fig 7.6 P 211 (drawing)

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Ocean currents cont.

• Three kinds
• 2. subsurface currents - Fig 7.27 p 239 are
  driven by the sinking of chilled waters from the
  polar oceans that spread out through all the
  oceans and eventually return to the surface to be
  warmed (account for the mixing of the ocean
  waters) temperature-salinity (thermohaline)
  differences

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Ocean currents cont.

• Three kinds
• 3. boundary currents - follow parallel to the
  continental margins
• East-west currents in open ocean
• North-south currents are results of deflection
  by land
• Mapped from observations averaged over last
  century
• Driven primarily by prevailing winds so they
  resemble those surface winds - average
  atmospheric circulation (Remember Fig. 6.10)

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Ocean currents cont.

• Nearly closed current patterns in ocean basins -
• subtropical gyres - centered in the subtropical
• region north and south of the equator and polar
• gyres best developed in the Atlantic waters
• between Greenland and Europe and in the
• Weddell sea off Antarctica
• Monsoons in Indian Ocean cause seasonally
• variable currents Fig 7.18 p 226
• (remember monsoons from last time)

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• A body in motion tends to remain in motion unless
  acted upon by another force.
• Think continents and ocean waters.

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CORIOLIS EFFECT

• Quick review
• What held for the atmosphere, holds for currents
  in the oceans.
• At the north pole
• rocket goes straight up
• then comes straight down one hour later
• Rockets speed to east or west is what ever the
  land was doing at the time of launch

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CORIOLIS EFFECT cont

• Rocket from pole shoot for 30 degrees N (Canary
  Islands)
• Earth rotates 1400 Km/hr at Canary Islands
• The Rocket lands 1400 Km west of Islands one hour
  later

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CORIOLIS EFFECT cont

• Rocket starts at the Equator
• Equator is moving faster than the Canary Islands
  at 1600 Km/hr
• The rocket lands 200 Km east of Canary Islands

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Ekman spiral Fig 7.7 P 211

• Caused by steady wind blowing across water
  surface, therefore wind driven
• Currents diminish in strength and rotate to right
  with increasing depth because of Coriolis effect
• USE (L) INDEX FINGER TO INDICATE WIND
  DIRECTIONAND CUPPED HAND TO INDICATE EARTH
  ROTATION

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Ekman spiral cont.

• Because of Coriolis effect, the surface current
  moves in a direction 45 to the right of the wind
  in the Northern Hemisphere and left in Southern
  Hemisphere

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Ekman transport

• is perpendicular to wind direction SA 54 WIND
  DRIVEN
• direction and speed of flow change with
• depth
• surface waters converge in center of basins
• waters are transported away from equator and land
  in certain areas, causing up welling

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• Take out time to carefully read the section
• on ocean gyres Table 7.1 p 209

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Geostrophic currents

• currents where water flow due to gravity is
  balanced by deflection of Coriolis effect
  (difficult concept) Fig 7.8 P 214, SA 55
• 1. Remember Ekamn transport always turns water
  currents to the right in Northern Hemisphere, and
  this clock wise rotation tends to produce a
  convergence of water in the middle of the gyre.

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Geostrophic currents cont.

• 2. Water piles up in the center of the gyres -
  can be gt 1 meter above the water level at the
  margins of the gyres T 85
• 3. As Ekman transport continually pushes water
  into the hill, gravity also acts to counter this
  effect moving water down the surface of the slope
  Fig 7.8 p 214. Coriolis effect deflects the
  water flowing down the slope to the R in Northern
  Hemisphere

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Geostrophic currents cont.
Using the subsurface water-density distribution
to describe the extent of the depression of the
deeper water, oceanographers are able to
calculate the elevation and slope of the sea
surface and so calculate the velocity, volume
transport, and depth of the currents present in
the geostrophic flow around the mound. T 85
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Geostrophic currents cont.

• 4. Sargasso Sea is the classic example of gyre
  in geostrophic balance (home work print off a
  page showing the location of the Sargasso Sea and
  its biota for next time)
• 5. Ocean surface topography caused by winds
  (Ekman transport) mapped using data on
  temperature and salinity remember what a
  topographic map is.
• 6. Ocean current model SA 55

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geostrophic flow

• refers to cyclonic fluid motions that are
• maintained as a result of a near balance
• between a gravity-induced horizontal
• pressure gradient and the Coriolis effect

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BOUNDARY CURRENTS

• more changeable than the major
• currents Fig 7.5 p209 currents
• Table 7.2 p 215

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Western boundary currents

• strongest in oceans well developed in the
  Northern Hemisphere
• Gulf Stream in the Atlantic - Kuroshio in the
  Pacific
• Gulf Stream - 20o C salinity around 36o
• Deep, narrow, swift - can not come up on
  continental shelf (western intensification)
• Intensified by Earth's rotation (clock wise)

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Western boundary currents cont.

• Often meander and spin off rings that move
  separately (we will see these later)
• Separated from adjacent slower-moving waters are
  oceanic fronts, which are marked by changes in
  water color, temperature, and salinity

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Eastern boundary currents

• weaker than western boundary currents
• Broad, shallow, slow-moving - can readily flow
  over continental margins
• Often associated with up welling areas
• Arctic Ocean currents Fig 9.6 p 232
• Ocean currents follow the same hydraulics as
  rivers wide - slow narrow - fast

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• WESTERN INTENSIFICATION OF CURRENTS

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• Four processes acting together intensify western
  boundary currents Fig 7.8 a p 214
• SA 55
• 1. earth's rotation which displaces gyres toward
  the west, compressing them against the
  continents, thus surface slopes are steeper on
  the western side of basins (think about steep
  banks along rivers)
• 2. trade winds which blow generally westward
  along the equator, thus piling up the surface
  waters on the western sides of basins

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• Four processes acting together intensify western
  boundary currents
• 3. strong westerlies force surface waters in the
  midlatitudes to flow toward the equator as they
  move across the basin
• 4. current and apparent spin due to earth's
  rotation are in the same direction giving
  currents a higher velocity - Coriolis effect

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WIND INDUCED CIRCULATION

• Up welling and down welling Fig 7.12 p. 218 T
  87 SA 61 Caused by Ekman processes Fig 7.6, 7.7
  in the book
• Up welling occurs when surface waters move away
  from the coast exposing subsurface waters The
  equator is a divergence zone which is caused by
  the winds and by the changes in the sign of the
  Coriolis effect
• Down welling occurs where waters are moved on
  shore and causes currents to move parallel to the
  coast Fig 7.11 p 217 (book)

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GOOGLE

• Type rings and meanders animations in search
  line. Then go to the animation, and WALLA!! Many
  good things and pictures about ocean circulation.
  I am impressed.

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Rings and meanders

• Fig. 7.17 p 233 SA 58
• Pronounced meanders of western boundary currents
• Break off to form isolated rings that move with
  surrounding waters
• Best known associated with Gulf Stream
• cold core rings occur in Sargasso Sea warm core
  rings occur in slope waters
• Eddies are equivalent to atmospheric storms - are
  weaker than rings

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Langmuir circulation
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Langmuir circulation

• Near-surface phenomenon SA 59
• Caused by strong winds - water rotates at a very
  low rate
• Causes small-scale up welling and down welling
  forms straight rows parallel to the direction of
  the wind trapping plants etc. in zones of
  convergence between the cells (can be 100 m in
  length) - little plant material at zones of
  divergence (you can see these on Lake Michigan)

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Thermohaline circulation

• (temperature salinity) circulation - currents
  controlled by density
• Dense water masses form in high latitudes
• Water return to surface throughout the ocean and
  in up welling regions
• Studied by geostrophic calculations and tracer
  techniques
• Oriented generally north-south basins
• Current patterns in Atlantic are simpler than in
  Pacific

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Convergence/divergence circulation

• Fig 7.5 p 209 SA 60
• Water return to surface throughout the ocean
  and in up welling regions (divergence)
• Water circulates downward (convergence)

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Pacific Ocean surface currents

• Fig 7.19 p 228 (book)
• Fig 7 a p 206

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Atlantic Ocean surface currents

• Fig. 7.16 p 222
• Look again at Box 7.3 p 233

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Salt lenses

• Subsurface equivalent to rings and eddies
• Studied by acoustic tomography
• Seen in geostrophic circulation (drawing)
• Fig 7.8 p 214

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Density Structure

• Fig 7.26 p 238 SA 96

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Water column

• stable - dense below
• unstable - dense above (water tips over)
• We talked about this in the chapter on water.

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Depth changes

• pycnocline Fig 5.23 p 156
• density increases rapidly with depth
• thermocline
• rapid temperature change with depth (this can be
  several meters thick)
• thermohaline
• change in temperature and salinity with depth

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Depth changes

• Halocline Fig 5.22 p 154
• large changes in salinity with depth (read to
  class)
• (temperature salinity) circulation - currents
  controlled by density relationships
• Dense water masses form in high latitudes
• (convergence) Fig 5.22 p 154
• change in temperature and salinity with depth

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Putting it all together

• SA 97

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• Put in picture of thermalcline and helocline

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Surface Layer Temperatures

• see what wind does SA 98
• Go to google, type in thermocline changes
• seasons click on images
• summer - no strong winds
• fall and spring - wind mixing and shallow
  thermocline
• winter increasing winds - large mixed layer, no
  shallow thermocline

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Comparing Fig 8.7

• Atlantic Ocean SA 100
• salinity/temperature profiles
• Pacific Ocean SA 101
• salinity/temperature profiles

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Anatomy of the Atlantic Ocean

• SA 99

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