Environmental Engineering Reference
In-Depth Information
Fields of altimetry-derived surface currents and associated frontal regions were
used operationally by the NOAA Office of Response and Restoration to monitor the
upper ocean dynamics. The real-time fields produced during the oil spill event are
located at
www.aoml.noaa.gov/phod/dhos
.
Altimetry observations have the advan-
tage that they are available year-round and are not affected by the near uniform SST
values often observed over the GOM during the summer months. The altimetry fields
also have the benefit of not being subject to cloud contamination. However, they can-
not provide the fine spatial and high temporal resolution to resolve surface features
obtained from satellite-derived fields of SST and ocean colour. Since altimetry fields
are constructed using the alongtrack satellite data, which may not necessarily run
along or across the region of LCR detachment, the exact date of detachment of a
LCR as seen from altimetry observations is only approximate.
1.3.3 Surface Features from Sea Surface Temperature
and Ocean Colour
Satellite observations of SST and ocean colour were used jointly to determine the
size and location of smaller mesoscale features that satellite altimetry fields cannot
observe or properly resolve. These smaller features were detected by adjusting the
contrast of the images and outline in the image. Ocean colour data were particularly
useful for mapping surface ocean circulation features during the summer months,
when SSTs in the Gulf of Mexico (GOM) have relatively weak gradients compared
with the winter months [
24
]. Ocean colour observations fromMODIS, SeaWiFS (on
SeaStar) and MERIS were routinely used to study regional dynamics. The gradi-
ents of ocean colour and derived surface chlorophyll concentration make these data
particularly useful in the GOM.
Several passive satellite sensors were available to estimate SST by measuring
thermal infrared (IR) and microwave radiation emitted by the ocean. During the
DWH event, SST imagery collected using IR sensors, allowed researchers to make
inferences about the distribution of surface features and frontal zones that could have
been associated with different water masses in the GOM. The satellite sensors used
in this study were the various AVHRR sensors, MODIS, and the European Advanced
Along-Track Scanning Radiometer (AATSR, onboard Envisat). The 6km resolution
SST fields from the geostationary GOES-12 were also used to obtain coverage when
clouds were present by virtue of its sub-hourly sampling. Of significance is that it is
still not understood howoil present at the surface of the ocean affects the emissivity of
the IR radiation, whether SST observations within oil-covered waters were accurate
or not, orwhether this signature changedwith patchiness in the oil orweathering of the
surface slicks. During the oil spill event, maps of SST (Fig.
1.2
), ocean colour fields,
ocean frontal zones and derived circulation were routinely prepared by ROFFSTM,
NOAA, and other laboratories to monitor the upper ocean conditions. Some of these
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