Environmental Engineering Reference
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information on the extent of the oil at the surface. A potential third approach using
sea surface temperature (SST) observations was also explored during the monitoring
efforts and showed good potential for analysis of changes in surface oil extent.
No results using this type of analysis are presented in this chapter because they fall
beyond the scope of the work carried out during the oil spill monitoring efforts.
However, SST observations were also used to classify water masses with different
and often unknown concentrations of oil as part of evaluating the possible pathways
for dispersal.
1.3.2 Surface Currents
After the Seasat and Geosat satellites proved the concept that ocean currents could
be monitored and studied from space using radar, a number of space-based altime-
ters have flown beginning in late 1992. Data obtained from an altimeter onboard a
satellite, with the proper atmospheric corrections, represents the distance between
the satellite and the sea surface, and indirectly serves to estimate the sea surface
height anomaly (SSHA) along the altimeter groundtracks. The observations pro-
vided by altimeters are accurate estimates of the SSHA away (
50km) from coastal
zones [
3
], and are referenced to a mean sea height, which may be generated from his-
torical hydrographic observations and/or numerical models during a period of several
years. This process smears out mesoscale ocean features smaller than approximately
100km and those that move at speeds faster than a few kilometers per day.
During the DWH oil spill, fields of surface circulation were constructed using
data from various sources, both in situ and remote. Results presented here focus on
those fields obtained at the time of the oil spill using real-time satellite altimetry
observations. However, this work presents results that use delayed-time altimetry
observations, which generally become available with a several month delay. Two
main data sets were used to derive the upper ocean circulation from satellite altime-
try: (a) delayed-time altimetry-derived sea surface height (SSH) anomaly along-track
data from the Jason-2 and Envisat satellite missions, whose groundtracks are sepa-
rated by approximately 3 and 1 degrees in longitude, and are repeated approximately
every 10 and 35 days, respectively; and (b) a synthetic mean dynamic topography
or sea surface height [
29
]. Horizontal gradients of regularly gridded SSH fields
derived from the alongtrack satellite altimetry data were used to estimate daily sur-
face geostrophic currents following a well-established methodology [
4
]. The spatial
gradients of these geostrophic currents were then used to determine the location of
the fronts associatedwith the cyclonic and anticyclonic features, such as the LC, rings
and eddies. Results regarding the separation of the LCR from the LC, based on these
surface currents alone, may often differ from those obtained from satellite-derived
SST estimates, as the boundaries of the mesoscale features observed from dynamic
and temperature fields may not necessarily coincide. The fields of surface currents
are used in this work to understand and assess how ocean dynamics delineate the
outer boundary of the oil extent.
∼
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