GGE6022 knowledge

Lesson Plan: Dynamic Ocean Topography


Dynamic Ocean Topography (DOT), also known as Mean Dynamic Topography

Assumed background

  • understanding of ocean geophysics, including tides and currents, and
  • understanding of geodesy, especially difference between ellipsoid and geoid representations of earth

Lesson Objectives

Students will be able to…

  • Explain what DOT is, and the driving forces of DOT
  • Derive DOT from tidal benchmark datasheets


Example tide gauge data sheets for exercise

Lecture Outline

What is DOT?

  • Height of mean sea surface above the geoid.
  • Show model example of mean Sea Surface Height (SSH) note similarity with the geoid
  • Contrast SSH with DOT

Computing DOT

  • Discuss satellite altimetry minus geoid model

Driving Forces

  • Currents – geostrophic currents, eddy currents
  • Density – driven by salinity and temperature
    • solar radiation and evaporation
    • ice and freshwater input
  • Atmospheric
    • prevailing winds
    • pressure systems, aka inverse barometric effect

Importance and use

  • Modeling
    • example of HYCOM realtime used to estimate oceanographic data for a broad range including weather prediction to safe navigation


Datum worksheet – lookup and compute various datum heights and separations relative to mean sea level using published tidal datums and benchmark datasheets,

This is mostly looking up data and simple math but requires understanding the sign conventions between datums and separation values.

  • Published values to document
    • Gauge Datums
      • MLLW (datum data sheet) – should be 0.0
      • MSL (datum data sheet)
    • Benchmark
      • MLLW height (benchmark data sheet)
      • Ellipsoid height (published on OPUS, link from benchmark data sheet)
      • Orthometric height (published on OPUS, link from benchmark data sheet)
  • Calculated values
    • Geoid Separation – benchmark orthometric to the ellipsoid height
    • Mean sea surface height (SSH) – MSL to ellipsoid
      • Note to instructor: this is calculated as benchmark ellipsoid height minus benchmark MLLW height plus gauge MSL height
    • DOT – geoid to SSH
      • Note to instructor: this is the SSH minus geoid computed above

Discussion question:

  • Discuss what the DOT value indicates about the local mean water levels?
    • The answer should explain local water level relative to the global mean, above/below
    • The answer should mention the three driving forces, currents, water density, and atmospheric forcing, and contrast local versus global averages, i.e. more/less solar radiation, currents.

Lesson planning references used:

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