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OCEAN CIRCULATION

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					OCEAN CIRCULATION

    As the world turns.
             Ocean currents
• Three kinds
  – 1. surface currents - are driven primarily by
    winds Fig 7.5 P 198
         Ocean currents cont.
• Three kinds
  – 2. sub-surface currents - Fig 7.28 p 225 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
         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. 6.11)
          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 214 (book) (remember
  monsoons from last time)
• A body in motion tends to remain in motion
  unless acted upon by another force.
• Think continents and ocean waters.
             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
         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
          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
    Ekman spiral Fig 7.7 P 201
• 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
          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
            Ekman transport
• is perpendicular to wind direction S&A 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
• Take out time to carefully read the section
  on ocean gyres Table 7.1 p 199 Fig. 7.5 p
  198
         Geostrophic currents
• currents where water flow due to gravity is
  balanced by deflection of Coriolis effect
  (difficult concept) Fig 7.8 P 203, S&A 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.
    Geostrophic currents cont.
• 2. Water piles up in the center of the gyres
  - can be > 1 meter above the water level at
  the margins of the gyres T 85
     Geostrophic currents cont.
• 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 203. Coriolis
  effect deflects the water flowing down the
  slope to the R in Northern Hemisphere
     Geostrophic currents cont.
• 4. Sargasso Sea is the classic example of
  gyre in geostrophic balance (home work
  print off a page showing the 1. location of
  the Sargasso Sea and 2. 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
      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
   BOUNDARY CURRENTS

more changeable than the major
 currents Fig 7.5 p 198 currents
Table 7.2 p 204 (remember what
 currents I want you to know)
    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 36%o
• Deep, narrow, swift - can not come up on
  continental shelf (western intensification)
• Intensified by Earth's rotation (clock wise)
Western boundary currents cont.
• Often meander and spin off rings that move
  separately (we will see these later, but keep
  them in mind)
• Separated from adjacent slower-moving
  waters are oceanic fronts, which are marked
  by changes in water 1. color, 2. temperature,
  and 3. salinity
     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
• Ocean currents follow the same hydraulics
  as rivers: wide - slow; narrow - fast
WESTERN INTENSIFICATION
      OF CURRENTS
• Four processes acting together intensify western
  boundary currents Fig 7.8 a p 203
• S&A 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
– 3. strong westerlies force surface waters in the
  mid-latitudes 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
 WIND INDUCED CIRCULATION
• Up welling and down welling, Fig 7.13 p. 207 T
  87 S&A 61 Caused by Ekman processes Fig 7.6,
  7.7 in the book p 201
• 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 Fig 7.10 p 206 (book)
• Down welling occurs where waters are
  moved on shore and causes currents to
  move parallel to the coast Fig 7.11 p 206
  (book)
        GOOGLE/ You tube
• 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.
          Rings and meanders
• Fig. 7.17 p 213 S&A 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
Langmuir circulation
         Langmuir circulation
• Near-surface phenomenon S&A 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 Langmuir
  circulation on Lake Michigan)
        Thermohaline circulation
• (temperature salinity) circulation - currents
  controlled by density See this on YouTube
   – Dense water masses form in high latitudes
   – Water return to surface throughout the ocean
     and in up welling regions Fig 7.27 p 224
   – Studied by geostrophic calculations and tracer
     techniques
   – Oriented generally north-south basins
   – Current patterns in Atlantic are simpler than in
     Pacific
     Convergence/divergence
           circulation

Fig 7.5 p 198 A&S 60
– Water return to surface throughout the
  ocean and in up welling regions
  (divergence)
– Water circulates downward (convergence)
  Pacific Ocean surface currents
• Fig 7.20 p 216 (book)
• Box 7.1 p 206 Read this in the book.
 Atlantic Ocean surface currents
• Fig. 7.16 p 211
• Look again at Box 7.2 p 210
               Salt lenses

• Subsurface equivalent to rings and eddies.
  Mediterranean Water eddies, or “meddies”,
  are large, warm, isolated lenses of highly
  saline Mediterranean Water that are found
  in the North Atlantic ocean.
• Studied by acoustic tomography
• Seen in geostrophic circulation
   Fig 7.8 p 203
Salt lenses
          Density Structure
• Fig 7.27 p 224 S&A 96
             Water column
• stable - dense below
• unstable - dense above (water tips over)
• We talked about this in the chapter on
  water.
             Depth changes
• pycnocline Fig 5.24 p 153
  – density increases rapidly with depth
• thermocline Fig 5.25 p 154
  – rapid temperature change with depth (this can
    be several meters thick)
• Thermohaline Fig 5.25 p 154
  – change in temperature and salinity with depth
              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
      Putting it all together
• S&A 97
• Put in picture of thermalcline and helocline
   Surface Layer Temperatures
• see what wind does S&A 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
                Comparing
• Atlantic Ocean S&A 100
  – salinity/temperature profiles
• Pacific Ocean S&A 101
  – salinity/temperature profiles
 Anatomy of the Atlantic Ocean
• S&A 99

				
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