CCAR - Colorado Center for Astrodynamics Research

Satellite Oceanography : Applications :

Reading Oceanographic Maps

Chad Fox, Bob Leben, and Suzanna Barth

Sea Height Anomaly Maps

A Tool for Locating Eddies

The map above left was created
by the Near Real-Time Altimeter Data Viewer by specifying the region (lat and long) and selecting <<NO MEAN>> and <<SEND VALUES>>. Looking at this image, you see 'circles' that are red and blue. An eddy is a generic term used to refer to both cyclones and anti-cyclones, found in the ocean. The red 'circles' are eddies which rotate anti-cyclonicly (clockwise), and the blue 'circles' are called Cyclones, which rotate cyclonicly (counter-clockwise). The anti-cyclones have a positive surface signature (higher than average), while the cyclones have a negative surface signature. Anti-cyclones have a positive surface signature because anti-cyclonic rotation produces water downwelling. Downwelling means warm surface water is being pushed down into the water column.

Figure 1. Double click to view animation of Gulf Stream Topography

Because warm water expands more than cold water, more warm water in the water column means the sea surface will be higher in the anti-cyclone than elsewhere. The opposite of this is true for cyclones, which upwell cold sub-surface waters, and which stand lower than elsewhere. Double click Figure 1 and look at an animation of consecutive images, over a few days, and see how these features move around.

Anomalies vs.Sea Surface Height

Next, try creating some maps while selecting <<ADD MODEL MEAN>> to the sea surface anomaly. Notice how some features become stronger, others weaker, and some disappear altogether? One important thing to observe is the large positive values off the northeastern coast of the U.S.,down to the coasts of Cuba and South Florida. This is the Gulfstream. The Gulfstream is positive because it brings warm Caribbean waters from the south into the Gulf of Mexico. The Gulfstream is also responsible for the creation of eddies in the Gulf of Mexico. The Gulfstream will build up warm, high water along its northwest border, which --when enough water has accumulated-- will split from the Gulfstream and form an eddy. The dynamics and frequency of these events is a current research project. Again, take some time and look at different images and see how the Gulfstream evolves.

The reason we measure anomalies and not absolute sea surface height, is that the mean sea surface height has to be subtracted from the data set. This is done to remove errors in the data caused by insufficient knowledge of the Earth's gravity field, which affects the prediction of the satellites orbit. The gravity field effectively does not vary in time, so each pass of the satellite will be affected the same by the gravity field. By removing the mean, we remove these errors, but we also end up removing the mean ocean circulation. Currently, the only way to estimate the mean ocean circulation is through numerical models.

Geostrophic Current Velocity Maps

Figure 2. Geostrophic Current Velocity Map, with vectors overlayed on Sea Surface Height Anomalies (SSHA).

Geostrophic velocity vectors, calculated from satellite altimtery, are a widely used product. The image below right shows the vectors superimposed on a sea surface height map, and they may also be displayed on a height anomaly map. Geostrophic flow is a major component of ocean surface currents and are a function of wind forcing, tidal forces, buoyancy, and the Earth's rotation and gravity.

The current velocities are determined on the basic principal that water runs from a high location to a lower location due to gravity, until equilibrium is reached. This is true in the ocean, but the Earth's rotation, causes another force to act on the water --the Coriolis Force . The Coriolis Force causes the water to move at right angles to the direction it normally would, and it enables features like eddies and cyclones to remain coherent structures in the ocean. Geostrophic currents occur when this pressure component is balanced with the Coriolis component. Often in the deep ocean, a large percentage of the surface current is in geostrophic balance, and it can be calculated as a function of the latitude and change in sea surface height.

View an estimation of the velocity field, at the above link, and look at both the case with and without the mean. Notice how the velocity vectors move anti-cyclonicly or cyclonicaly around the features, depending on the existence of a high or low in the surface height anomaly.

Surface Wind Speed Maps

Figure 3. Surface Wind Speed Map.

Surface wind speed is measured indirectly by the altimeter. The measurement is based on the effect of small-scale surface roughness on the return signal. There is a reliable relationship between the energy scattered from near-nadir and the wind speed. However, this measurement does not indicate wind direction. Satellite scatterometer wind observations may be used to determine both direction and magnitude and are of greater practical use to the maritime community; however, wind speed calculated from radar altimetry may make useful comparisons.

Altimeter-derived wind speeds are also valuable for generating wind climatologies, due to the long time series available.

Significant Wave Height Maps

Figure 4. Significant Wave Hieght Map.

Significant wave height is calculated from altimeter data based on the slope of the leading edge of the altimeter waveform, similar to a binned (time) histogram of the power returned by the radar pulse after it bounces off the sea surface. A calm sea with low waves returns a short, sharply defined pulse, whereas a rough sea with high waves returns a "stretched" pulse. The significant wave height is the average height of the highest one-third of all waves in a particular time period. Wave direction is not provided by altimeter data. Maps of significant wave heights and wave height climatologies are useful for ship routing.

Chad Fox is a Postdoctoral Research Fellow at CCAR. Suzanna Barth is a Graduate Research Assistant at CCAR. Both are actively involved in satellite altimetry research at CCAR.

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