## Topic/Overview

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

## Resources

- NASA Science – Ocean Surface Topography – topic overview
- DTU10 – Global Mean Dynamic topography – global DOT model, slides, and paper
- DTU10 – Global Mean Sea Surface – global SSH model and slides, used to compute DOT

Example tide gauge data sheets for exercise

- Datums for 9441102, Westport WA
- Bench Mark Sheet for 9441102, Westport WA
- OPUS Solution for Bench Mark 944 1102 K

## 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

## Assessment

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)

- Gauge Datums
- 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:

- Example of a Good Lesson Plan – Harrow College [pdf]
- Sample Lesson Plan: Intro to Satellite Altimetry | The Geodesy Corner [website]

### Other Posts in the Series

- GGE6022 – Advanced Topics in Ocean Mapping
- Dynamic Ocean Topography: An Introduction
- Inverse Barometric Effect: A Contribution to Dynamic Ocean Topography
- A Model of Dynamic Ocean Topography
- What is new in dynamic ocean topography?
- Dynamic Ocean Topography in a Flat Earth Perspective
- Lesson Plan: Dynamic Ocean Topography
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