Sea Surface Height training course by liamei12345


                         training course

CATSAT Training Course

Part 1 - Oceanography
         Why the ocean surface is not plane?

         Dynamic topography and geostrophy
         Geostrophic current

Part 2 - Sea surface height measurement
         Observing the ocean with altimetry
         The altimeter measurement

CATSAT Training Course
                            Part 1

  Atlantic ocean                        Pacific ocean

                                        Indian ocean

  Arctic ocean                          Polar ocean

CATSAT Training Course
    Unequal repartition of the continents and oceans : this impact the
    atmospheric circulation and thus the oceanic circulation

                                 North Hemisphere : 61% of ocean

South Hemisphere: 81 % of
CATSAT Training Course
                    General atmospheric circulation

CATSAT Training Course
                    General oceanic surface circulation

  Wind induced circulation

CATSAT Training Course
                  Why is the surface not plane?

   Because of the ocean-atmosphere interactions :

           Winds and associated mixing.

           Heat exchanges with the atmosphere modify the temperature and

            salinity, thus water mass density.

           Seasonal cycles: ocean temperature and volume grow in
            summer, leading to the mean sea level expansion over a all
            hemisphere. The opposite occurs in winter.

CATSAT Training Course
               The ocean : a distorted surface
Sea level variations are due to:
    Tides
    Waves
    Water density :
      - the sea water density is related to the temperature and
      - the volume is related to the water density
      - If temperature decreases, density increases and volume
      decreases.         There is contraction of the water column.
     - If salinity decreases, density decreases and volume
      increases. There is expansion of the water column.
Density variations lead to appearance of bumps and holes on
the sea surface.
CATSAT Training Course
                                   H1 < H’1                   H’1
                                   S2 > S1
                    T1,S2                             T1,S1

     H                                                              H’>H

                                   H2 < H’2                   H’2
                    T1,S2          T1 < T2            T2,S2

         H3         T3,S3          H3 = H’3           T3,S3   H’3
                               Bottom of the ocean

                            Piling up different density
                                 water quantities

CATSAT Training Course
                           Pressure force

 The pressure at a given depth is essentially caused by
 the weight of the above water column.
 At two different locations in the ocean, at a given depth :
     if the above-water column have different density profiles, then the
       pressure will be different.
     an horizontal difference of pressure (“pressure gradient”) will
      produce a force which tends to push water from high to low

                                     --                        +++
                                          Pressure force

                                           Vertical section
CATSAT Training Course
                         Geostrophic current
 BUT, because of the Coriolis force due to the Earth
  rotation, a current perpendicular to the sea surface
  slope is created. This current is called the
  geostrophic current.

                                                      Pressure force

                             !   In the Northern hemisphere
CATSAT Training Course
The Coriolis force:

 CATSAT Training Course
          Dynamic topography and geostrophy

 The direction of geostrophic current is always
  perpendicular to the slope direction:
             In the northern hemisphere. The Coriolis force deflects
               movements to the right of the trajectory.

             In the southern hemisphere. The Coriolis force deflects
             movements to the left.

             WARNING : the Coriolis force vanishes at the equator. There
             is no geostrophic equilibrium at the equator : the slopes of the
             dynamic topography can NOT be related to geostrophic

CATSAT Training Course
        Calculation of the geostrophic currents

 Current speed:
 The current speed is proportional to the sea surface slope.

        g      H                                      A
U           X
      2sin   X

H         : height difference between 2 points
X         : distance between the 2 points
g          : 9.81 gravity constant
          : 7.27 10-5 Earth angular rotation speed
          : latitude of the 2 points

CATSAT Training Course
                No Coriolis force at the Equator

CATSAT Training Course
     The ocean surface shows whirling structures, meanders and
      fronts witch size vary from a few tens to a few hundreds of
     The geostrophic velocity is minimum at the center of the eddy
      and gets higher away from it.

      Rotation of eddies :
                                      Anticyclonic eddies or       Cyclonic eddies
                                            “bumps”            (depressions or “holes”)

                  Contain generally        Warm water                Cold water

                     Northern              Clockwise                anticlockwise
                    hemisphere              rotation                   rotation

                       Southern           anticlockwise              Clockwise
                     hemisphere              rotation                 rotation
CATSAT Training Course
    Fronts are found between positive eddies
     (anticyclones or “ bumps ”) and negative
     (depressions or “ holes ”), in areas of maximum
     current velocity.

    In the frontal areas take place convergence
     (sinking of surface water) and divergence
     (upwelling of cold water to the surface)

CATSAT Training Course

                  MOVY: 4 years of general circulation in
                  the Atlantic ocean, an animation.

CATSAT Training Course
                 Part 2
    sea surface height measurement

CATSAT Training Course
                         Observing the ocean

                                   In-situ : poor space/time sampling of
                                    the ocean, remote regions (southern
                                    oceans), costly.

                                   Satellite Data/: space data can
                                    provide long-time, repeated and
                                    global measurements of ocean key
                                    parameters in real time.

                                   Numerical modeling : the only way to
                                    forecast the oceanic situation.

CATSAT Training Course
                             Satellite altimetry

 Probably the most important satellite technique for oceanography.
 Very mature technique (20 years):
      GEOS-3 (1975), SEASAT (1978), GEOSAT (1985-1989), ERS-1
      (1991-1996), ERS-2 (1995- 2003) TOPEX/POSEIDON (1992 - ?), GFO,

 Major advances in sensor and processing algorithm performances
  over the last 20 years.

                         As a result, accuracy evolved from
                          several meters to a few cm only

CATSAT Training Course
                      The altimeter measurement

                                                     SSH = S - R = G + 

                                                     SSH: absolute sea surface
                                                     topography / ellipsoid
                                                     S: position of the satellite /
  LASER                                              R: range
 STATION                       RANGE
 DORIS                                               G: marine geoid height
STATION                                              (motionless ocean)
                 ALTITUDE                            : dynamic topography.
                             DYNAMIC TOPOGRAPHY

                                 GEOID     SURFACE

  CATSAT Training Course
                          The geoid

The Earth is a quasi sphere in space

Flatened around the Poles because
of it’s rotation, it’s an inhomogeneous

This creates an irregular gravity
field: the geiod.

This surface is roughly known

 CATSAT Training Course
     SSH = S -R = G + 

   • the quantity we want to know: the dynamic topography .
   • because of the geoid imprécision we calculate sea level anomaly
     to a mean.

             mean sea surface height calculation
                   <SSH>= <G>+ <>
             man can then deduces the sea level anomalies
                   SSH - <SSH>= (G+) - (<G>+<>)
                              = -< >  SLA

CATSAT Training Course
                  The radar altimeter : wave form

                Ideally calm sea             Rough sea

  Individual                                         Average of 90
return pulse:                                        return pulses

 CATSAT Training Course
                    Instruments characteristics

                            ENVISAT       GFO        JASON

              ALTITUDE       800 km      880 km     1300 km

              COUVERAGE     81°N-80°S   72°N-72°S   66°N-66°S

              INTER TRACk    85 km       160 km      300 km

              CYCLE         35 jours    17 jours    10 jours

CATSAT Training Course
                              JASON (blue), ENVISAT(red) and GFO (green) ground tracks

Multi satellite combination
to increase the precision
of the maps.

CATSAT Training Course
                          Sea level anomaly (cm)

“bump” of ocean
 topography, i.e
anticyclonic eddy
    (g. warm)

“hole” or cyclonic
  eddy (g. cold),
plankton can come
 up to the surface

 CATSAT Training Course
                    Part 3
          From scientific investigation
            Operational application

CATSAT Training Course
                          Scientific results

 High precision of altimetric measurements : radar capable of
  detecting sea level changes of 1-2 cm at an ocean basin
 Global coverage of the whole ocean
 allow a great improvement in observing/understanding
     the global ocean circulation and climate:
     the mean sea level changes
     the ocean variability (seasonal cycle, a.s.o…)
     inter annual changes (El Nino)
     tides, ...
 Besides applications in meteorology, geophysics, and

 CATSAT Training Course
                         Dynamic topography (cm)

CATSAT Training Course
               Ocean variability : El Nino 97-98

CATSAT Training Course
               Who’s customers for altimetry ?
   « sea people », for whom the ocean is the essential environment
    (marine, fishing activities, navigation, oil exploration, ...),

   « scientific people », who study the “ocean system”, its balances,
    mechanisms and variability.

   “climate people”, who consider the ocean as a fundamental element
    which regulates the climate system (coupling between Earth-Ocean-

CATSAT Training Course

                                    Fishing
                                        fishing helping tool: identification of
                                        the areas with higher fish

                                    Ship routing
                                        Optimization of maritime routes

                                       Scientific investigation
                                        Near real time help for campaigns
                                        at sea (in situ measurements)

                                       Oil exploration
                                         Help for planning operations at

CATSAT Training Course
                         Albacore tuna campaign
                               (June 1993)
                                                         June, 9
                                                         June, 19
                                                         May, 30

                            lag between map and date (in day)
CATSAT Training Course

To top