Satellite Altimetry in the Gulf of Mexico: Techniques, Tools and Theory

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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 PowerP
oint 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 Jo
hn’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 t
hat
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. Anticy
clonic 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 rada
r 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 a
nomalous 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 surfac
e 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 fo
r 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).


Figur
e
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 use
d

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 curr
ents 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 o
f 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 t
he 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 ava
ilability 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 westwar
d 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.


Sev
eral 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 up
stream 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
compl
ex phenomena
. The altimetry products and other information are available at
http://ccar.colorado.edu/~leben
. Digital data is available through ftp
(contact
leben@colorado.edu).