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International Journal of Marine Science 2012, Vol.2, No.1, 1
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6
the SWM which was noticed in the distribution of the
monthly mean chlorophyll-a. This is due to an early
build-up of chlorophyll-a concentration off Somalia in
response to the SWM and a overlap with the fall
inter-monsoon period. Higher levels of chlorophyll-a
occur, once again, in this region during the NEM
season. The annual and semi-annual peaks are of
similar magnitude off Yemen but differ in their
spectral width. Off Salalah, we noticed a peak at the
annual frequency and another small peak but of
considerable magnitude at 90
-
day period. At Masirah
Island, the semi-annual peak is stronger than the
annual peak though its spectral width is shorter. It
shows that the on the semi-annual scale, the variability
and occurrence of high chlorophyll-a concentration is
more pronounced. In general Masirah is influenced by
both SWM and NEM equally. Further north, in the
Gulf of Oman, the chlorophyll-a concentration shows
multiple spectral peaks at annual, semi-annual, 120
-
day
and 30
-
day periods. The circulation in the gulf is
dominated by the NEM and meso-scale eddies which
presumably contribute to such wide spectrum of
variability in the Gulf of Oman.
In the northern and central Arabian Sea (NAS and
CAS), chlorophyll-a peaks show larger spectral width
and lower magnitude at the annual scale. However, at
NAS, the semi-annual peak is dominant followed by
another smaller peak around the 90
-
day period. The
spectral characteristics of the chlorophyll-a concentration
in the Gulf of Mannar stands out as clearly different
from the rest of the Arabian Sea. In the Gulf of
Mannar, the magnitude of chlorophyll-a concentration
is singularly dominant at the annual period and also
has broad spectral width.
1.5 Results from the empirical model
Figure 6 shows the SeaWiFS-observed chlorophyll-a
(dashed line) and the modeled chloroplyll-a (solid
line) at the selected locations. On the left Y-axis
chlorophyll-a concentration is shown as log (chloro-
phyll-a) while on right Y-axis it is shown in normal
units, both in mg/m
3
.
Off Somalia the model predicted the chlorophyll-a
variability with a primary maximum between June and
September and a secondary maximum during January-
February. The minimum chlorophyll-a concentration
occurred around April. The observed variability
agrees with the model. During 1998, the observed
chlorophyll-a concentration was less than model
predictions for the SWM season while it was higher
than the model during 1999. Departures from the
modeled values seen in the 1998 and 1999 SWM
periods were due to the anomalous conditions
prevailing during those years in the Indian Ocean.
During 2003, there was a sudden decrease in observed
chlorophyll-a during May but concentrations were
restored to model-predicted values during the SWM
season. In 2001 and 2002 the early rise in chlorophyll-a
concentration off Somalia in April was followed by
rapid fall in the following month.
The SWM and NEM peaks in chlorophyll-a off
Yemen agreed largely with modeled inter-annual
variability. In 1999 and again from 2001, the NEM
chlorophyll-a concentration off Yemen was marginally
higher than model predictions. During 2003 and 2004,
the continued higher levels of chlorophyll-a concen-
tration during the NEM delayed the occurrence of a
minimum level by about a month.
During 1999, the intra-seasonal variability off Salalah
was similar to that observed off Yemen. However,
during 2000, higher chlorophyll-a concentrations
during the NEM fell dramatically to a minimum level
by April and then anomalously increased in May when
the annual minimum is expected. During the following
year, 2001, the observed chlorophyll-a concentration
was marginally lower than the model during the SWM
but higher than the model prediction during the NEM
of the same year. We noticed that if an increase in
chlorophyll-a concentrations occur early, then there is
a rapid decrease in the following month. During 2003,
the observed chlorophyll-a concentration was higher
than the model in both the SWM and NEM seasons.
Nonetheless, the magnitude and phase of inter-annual
variability are within predicted levels. The chlorophyll-a
concentration off Masirah Island showed a bimodal
distribution with higher concentrations during the
SWM season than during the NEM season. This
location showed higher intra-seasonal variability. The
phase and magnitude of inter-annual variability is in
agreement with the model predictions.