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International Journal of Marine Science 2012, Vol.2, No.1, 1
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during the SWM. During events of intense mixing
in the upper layer, the DCM breaks down and
chlorophyll-a is redistributed into the surface layer
after the passage of a cyclone in the Arabian Sea
(Subrahmanyam et al., 2002). The surface chlorophyll-a
concentration is generally low during inter-monsoon
periods (March-May and October-December) with
values in the range of 0.03 to 0.05 mg/m
3
, while
during the NE and SW monsoons the chlorophyll-a
concentrations go up to 1.34 mg/m
3
in the eastern
Arabian Sea (Bhattathiri et al., 1996) and up to 11 mg/m
3
in the western Arabian Sea (Brock and McClain,
1992). The integrated water column values also vary
similarly. The study of inter-annual variability of
surface chloroplyll-a assumes significance as the
biomass exerts considerable influence on upper layer
thermodynamics (Frouin et al., 2000; Nakamoto et al.,
2000; Timmermann and Jin, 2000; Sathyendranath et
al., 1991) and biogeochemical dynamics.
In this article, we present the chlorophyll-a distribution
and its inter-annual variability in the Arabian Sea and
two adjacent gulfs; the Gulf of Oman and the Gulf of
Mannar, during a seven year period (1997
-
2004)
based on SeaWiFS data. We investigated the spectral
signatures of chlorophyll-a concentration and simulated
its variability by an empirical model. Since the
geographical variability was large, we selected a few
locations for the above analyses. The locations are
shown in Figure 1 and are designated as Somalia,
Yemen, Salalah, Masirah, Gulf of Oman, northern
Arabian Sea (NAS), central Arabian Sea (CAS) and
the Gulf of Mannar. The geographic coordinates of
these locations are shown in Table 1. The selection of
these locations was based on the studies of Levy et al
(2007), Banse and English (2000) and Colborn (1975)
that identified different regions in the Indian Ocean
based on productivity and the climatological sea
surface temperature.
1 Results
1.1 Typical annual distribution of chlorophyll-a in
the study region
Typical annual distribution of chlorophyll-a in the
Arabian Sea is presented in Figure 2. In January, the
Persian/Arabian Gulf and Gulf of Oman, along with
offshore regions of Oman and northern regions off the
Figure 1 Area of study with locations selected for analysis of
chlorophyll-a
Table 1 The ratio of the standard deviations of modeled to
SeaWiFS observed chlorophyll-a and the geographical coor-
dinates of the selected locations
Location
r
Latitude (°N) Longitude (°E)
Somalia
0.772 07 11
53.5
Yemen
0.872 07 13
53
Salalah
0.740 53 17
57
Masirah
0.765 88 20
60
Gulf of Oman
0.820 69 24.5
59
N. Arabian Sea 0.810 01 20.5
66.5
C. Arabian Sea 0.791 76 15
64
Gulf of Mannar 0.819 95 7
78.5
west coast of India, showed appreciable (~5 mg/m
3
)
chlorophyll-a concentrations while the rest of the
Arabian Sea had concentrations <0.02 mg/m
3
. A rapid
build-up of chlorophyll-a during February and
March was evident in the Gulf of Oman, northeastern
Oman coast and northern Arabian Sea. In April the
chlorophyll-a pattern changed dramatically and the
entire northern Arabian Sea had chlorophyll-a
concentrations <0.1 mg/m
3
except for a few pockets
along the central Arabian Sea coast of Oman and Gulf
of Khambhat on the west coast of India. By May
upwelling filaments appear off Somalia, signaling the
formation of the Great Whirl in response to SWM
forcing off Somalia, and a general increase in coastal
chlorophyll-a concentrations is conspicuous. The
upwelling off Somalia, driven by the SWM, further
increased its chlorophyll-a content in June, and
expanded toward Yemen and the open Arabian Sea.
However, the upwelling off Somalia and Yemen/