Satellite Altimetry in the Gulf of Mexico: Techniques, Tools and Theory
Presented to the ASEN 6210 Remote Sensing Seminar on 4/14/04 by:
Robert Leben
Colorado Center for Astrodynamics Research
Reported by Gary Fager.
Images are from the speaker’s PowerPoint presentation.
Overview of the talk:
Mr. Leben presented a discussion of the use of satellite altimetry to estimate sea surface height. The discussion was set in the context of modeling the sea surface height of the Gulf of Mexico and how variation in sea surface height related to the Gulf Stream “Loop Current”. He also demonstrated several products available through the Colorado Center for Astrodynamics Research (CCAR). Among these products were the mesoscale altimetry and a joint product with John’s Hopkins Univ., the sea surface height/sea surface temperature (SSH/SST) image.
The circulation in the Gulf of Mexico and accordingly, the sea surface height is greatly influenced by the Loop Current. The Loop Current is a branch of the Gulf Stream that will periodically extend up into the Gulf of Mexico and devolve into eddies. These eddies move from east to west across the gulf and affect the currents. Satellite altimetry can track the progression of the Loop Current and associated eddies. Anticyclonic eddies form highs and cyclonic eddies form lows. Eddy pairs are also observed. In the SSH/SST images, the warm currents (anticyclonic) are more apparent since the cyclonic currents are at depth. The sea surface heights are determined using a radar scatterometer that is positioned very accurately using DGPS, laser ranging, and doppler shift beacons (DORIS). Several corrections need to be made to the raw data. Among these are uncertainties due to atmospheric effects, sea state effects, instrument effects, and external geophysical adjustments. One of the most intractable of the corrections is the geoid height. In order to bypass the effect of the geoid, a mean is determined which is the geoid plus the mean sea surface height. This mean plus the anomalous height is what is represented as the eddies. The mean reference surface has recently been updated to the Goddard Spaceflight Center MSS 00.1 (GSFC00.1_MSS or the “Goddard Mean”).
Several products are available from this system. The sea surface height and the blended SSH/SST maps referred to earlier as well as an altimetry blended SSH map that shows the surface height as contours (figures 1 through 4). These products are available for different temporal resolutions. Illustrations are given for specific days, three-day periods and recently, real-time products of the Gulf of Mexico are available. Also shown was an overlay product that illustrated the current velocities as vectors on top of the SSH (figure 5).
Figure 1 Goddard Mean image. /
Figure 2 CCAR SSH map
Figure 3 CCAR Anomaly map /
Figure 4 Altimetry blended map
Several real-time applications of these products were discussed. These were used in planning sport sailing routes, offshore drilling, oceanographic survey design and marine mammal habitat monitoring. Examples were given of each use. Sailing and oil platforms can be adversely impacted by currents so they want to know where and when these eddies are throughout the gulf. Monitoring the eddies is important for marine studies as marine life is strongly associated with upwelling cyclonic flow. Little life is associated with the anticyclonic flow.
Metrics of the loop current is also analyzed using the satellite altimetry. CCAR uses the 17 cm contour as an estimation of the high velocity core of the eddies in the eastern Gulf of Mexico (figure 6). A computer routine find the beginning and end of the 17 cm contour extending into the Gulf and is then able to calculate the maximum extent into the gulf of the current and the length of the loop. This produces an estimation of the maximum northward and westward extent, length, area, circulation and the volume of the Loop Current.
Figure 5 Geostrophic velocity viewer output /
Figure 6 Automated Loop Current tracking product
The periodicity of eddy current shedding is being analyzed due to the availability of a historical record going back 30 years. At this time it appears that primary peaks occur at 6 and 11 months with a smaller peak at 9 moths (figure 7). There is little or no power annually. Studies have found that once an eddy moves westward past 90 degrees west, they never move back to the east.
Figure 7 Periodicity of the Loop Current in the GOM /
Figure 8 Agreement between the CUPOM model and observations of the Loop Current.
Several models of the circulation in the Gulf of Mexico were discussed. The CUPOM (the CU version of the Princeton Ocean Model) apparently does a reasonable job of modeling the Loop Current dynamics but it is not perfect (figure 8). One possible deficiency in models was identified; the Campeche Shelf off of the Yucatan Peninsula may significantly affect Loop Current dynamics. The shelf may affect the geometry of the underlying principle of the momentum balance in the model. There may also be significant upstream dynamics that affect the Loop Current that are not reflected in the model.
In summation, models that accurately predict the behavior of the Loop Current are still lacking. With more observations, better models may be developed for these very complex phenomena. The altimetry products and other information are available at http://ccar.colorado.edu/~leben. Digital data is available through ftp (contact ).