Development of an Ehux Bloom (Satellite Images)
Lasse H. Pettersson
Nansen Environmental and Remote Sensing Center
(NERSC)
Edv. Griegs vei 3a
N-5037 Solheimsviken
NORWAY
Email: lasse.pettersson@nrsc.no
The NOAA AVHRR sensor has daily coverage of the Norwegian waters, and
data may be provided in near real-time through the high latitude Tromso
Satellite Station (TSS). Since the AVHRR ground resolution is at least
1 kilometer, this sensor is not optimal for local coastal and fjord
studies of coccolithophorid blooms. However, the successfull use of
AVHRR data for open ocean applications has been widely documented by
others.
Five NOAA AVHRR scenes have been acquired for the 1993 Emiliania
huxleyi blooming period in May/June 1993. High quality data have been
obtained from the daytime passes on May 25th, and on June 2nd, 7th, 9th, and
10th.
25th: No particular high-reflectance signals are observed in the
AVHRR data in the coastal region west of Bergen. The in situ
observations from e.g Fauakangerpollen (too small to be resolved in the
AVHRR data) confirm that the bloom has started, but not at a spatial scale
and magnitude which can be resolved in the AVHRR data. Further south, near
Stavanger, the shoreline was cloud covered and no conclusions can be made in
this region.
2nd: Some increased surface reflection is observed in AVHRR channel
1. High reflection filaments are observed outside the Boknafjord,
Bomlafjord, Selbjornsfjord and Korsfjord. A region of higher reflectance,
presumably coccolithophore-laden, is also observed extending west off the
Joren coast between Stavanger and Egersund and northward to the Boknafjord.
Conclusions for this part of the image are limited by the fact that
gradients in the cloud and haze layer may effect the remotely sensed data.
7th: clearest indication of the spatial structure of the algae bloom
is evident in the data from this day. The boundaries of the bloom are
well-evident, and the varying concentration of coccoliths within the bloom
area are clearly shown in the enhanced channel 1 image. A comparison of the
channel 1 image with the channel 4 thermal image reveals a high degree of
correspondence in the thermal ocean front boundaries and the extension of
the coccoliths, which is limited to the extension of the Norwegian Coastal
Current. The colder water near the Joren coast is relatively algae-free,
while the warmer water is associated with the bloom region. Alongcoast
meandering in the off-shore extension is observed and, in particular, a wide
extension is observed west off Karmoy.
9th: general structure seen in the June 7th data is evident in the
June 9th data. Further, a part of the coccolithophore bloom is trapped
southward outside Karmoy. This part forms a cyclonic eddy circulation
pattern west of Boknafjorden. Close to the Joren coast the water has a
lower reflectance, hence the origin of the coccolithophorids in this region
may come from the off-shore North Sea region.
10th: the eddy west of Boknafjorden is still clear with a further
southward extension of the thong of coccolithophorids trapped in the eddy.
Summary: the origin of the coccoliths observed in the AVHRR images
(Channel 1) seem to have two sources of origin during the May/June bloom in
1993. One part originates from the fjords, as resolved in the data from
June 2nd, and then transported out in the Norwegian Coastal Current from the
fjords. The other part appears to originate from the North Sea and then
interacts with the coastal current west of Joren. In this region, the
boundary between the coccolithophorids are consistent with the temperature
boundary between the two regimes.
Ehux
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