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International Journal of Marine Science 2012, Vol.2, No.1, 1
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11
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8
of chlorophyll-a. At this location, chlorophyll-a
concentration showed a single peak during the SWM
season, April through October. During 2002 through
2004, the intra-seasonal variability was large compared
to the other years in this study.
2 Discussion
Based on the inter-annual variability, the study region
can be grouped as upwelling affected (Group
-
1:
Somalia, Yemen and Salalah), regions of no significant
difference between monsoons (Group
-
2: CAS, NAS
and Masirah Island) and anomalous (Group
-
3: Gulf
of Oman and Gulf of Mannar). The variability of
chlorophyll-a concentrations is centered around the
two monsoons. Thus, it is adequate to examine the
chlorophyll-a variability during these periods to
understand the inter-annual variability of chlorophyll-a
in this region.
The Group
-
1 and Group
-
3 locations showed large
variability in chlorophyll-a levels between SWM and
NEM while the Group
-
2 locations showed nearly
equal concentrations of chlorophyll-a during these two
seasons. Though the Group
-
2 locations fall in the
same zonal belt, chlorophyll-a concentration at NAS
was always lower during the SWM season compared
to the NEM season. From 2000 onwards mean seasonal
chlorophyll-a concentration during the NEM appeared
to be higher than that during SWM at CAS and
Masirah Island. On year-to-year basis, there was
increasing trend in chlorophyll-a during the NEM at
all western Arabian Sea (Group
-
1) locations. A similar
observation was reported by Goes et al (2005).
The bi-modal variability of chlorophyll-a at Masirah
Island and distinct peaks pertaining to the SWM and
the NEM seasons observed in the SeaWiFS data in
the northern Arabian Sea at NAS, were predicted
appropriately by the model for the entire study period.
In the central Arabian Sea, the model performance
was better in predicting inter-annual variability of
chlorophyll-a. The central Arabian Sea showed an
anomalous drop in chlorophyll-a concentration during
the early SWM season of 1998 and 2000 and the
occurrence of chlorophyll-a maxima a month ahead of
the predicted maxima during 1999-2001. Though both
the observed and modeled periods of occurrence of
SWM and NEM peaks were generally in agreement,
the magnitude of the observed chlorophyll-a
concentration was marginally higher during certain
years.
The chlorophyll-a blooms during the winter period in
the Gulf of Oman and at NAS have resulted in
elevated levels of chlorophyll-a during February-
March at these locations. The winter blooms are
observed consistently toward the end of NEM season.
These elevated chlorophyll-a levels in the northern
Arabian Sea during winter are attributed to convective
mixing and subsequent nutrient enrichment in this
region during winter (Kumar and Prasad, 1996;
Barber et al., 2001). During the early SWM period,
June, variability is very large at NAS and in the Gulf
of Oman. During the SWM, a strong wind current
known as Findlater Jet deepens the mixed layer in the
central Arabian Sea (Baars and Oosterhuis, 1998). As
a consequence, the chlorophyll-a peak in this region
lasts through the entire SWM season. Banse and
English (2000) and Dickey et al (1998) also showed
that chlorophyll-a concentration increases during
the late SWM season and extends in to the fall
inter-monsoon.
The Gulf of Mannar stands out in this region as it
showed elevated chlorophyll-a concentrations only
during the SWM. These higher levels are created by
local upwelling in the Gulf of Mannar as well as by a
coastal front originating from the Bay of Bengal. A
coastal front also appears to enrich the local upwelling
along the southwest coast of India during the SWM
season (de Sousa et al., 1996; Muraleedharan and
Kumar, 1996).
During certain years, chlorophyll-a was low (except
at Yemen and Gulf of Mannar) during both seasons
due to anomalous climatic events (e.g., 1997-98 El
Nino, 1999 La Nina). During the anomalous SST
event of 1997-1998 in the Indian Ocean (Murtugudde
et al., 2000; Saji et al., 1999; Webster et al., 1999)
chlorophyll-a in the Arabian Sea also exhibited an
anomalous distribution. The lower levels of mean
chlorophyll-a observed during the 2001 NEM off
Salalah and during 2003 SWM at all the study
locations except off Salalah and NAS. It shows