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International Journal of Marine Science 2012, Vol.2, No.1, 1
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Oman are not connected physically and are driven
by different physical dynamics (Smith et al., 1998;
Hitchcock et al., 2002). The local wind forcing
along the Yemen and Oman coasts generates isolated
chlorophyll-a rich filaments associated with local
upwelling. The veering of strong Somali Current away
from the African coast around 5°N and the Great
Whirl dynamics around ~10°N create two areas of
upwelling driven by vorticity balance in the Somalia
region. Thus, chlorophyll-a laden waters spread
throughout the Arabian Sea in July through September.
While increased cloud cover obscured many individual
satellite images during July and August, high
chlorophyll-a features could still be seen in occasional
images. By September a well developed anti-cyclonic
gyre in the western Arabian Sea along Somalia,
Yemen and Oman has reached Ras al Hadd near the
mouth of Gulf of Oman. The northern branch of this
gyre developed into a strong and narrow surface
current near Ras al Hadd (Manghnani et al., 1998;
Morrison et al., 1998). This current develops in response
to Southwest Monsoon forcing and forms a dynamic
barrier between Arabian Sea and Gulf of Oman waters.
The chlorophyll-a rich filaments originating from the
southern Oman coast extend far offshore into the
central Arabian Sea during the SWM.
By November, surface conditions have returned to a
low chlorophyll-a state which is maintained through
December. A narrow, coastally trapped front of higher
Figure 2 Typical annual distribution of the chlorophyll-a
concentration in the Arabian Sea derived from SeaWiFS
chlorophyll-a concentration originating in the Bay of
Bengal, along the east coast of India, is seen moving
into the Gulf of Mannar from July. It joins the local
upwelling in the Gulf of Mannar and southwest coast
of India during the SWM. This feature is noticed
during all the years in the present study. As this
chlorophyll-a rich front moved northward along the
west coast of India, the southern coastal waters
become depleted of chlorophyll-a. Thus the annual
cycle of the chlorophyll-a in the study region
distinctly shows two periods of elevated chloroplyll-a
concentrations, one during SWM (June through Sep-
tember) and the other during NEM (January through
March). In the Gulf of Mannar, however, only one
prolonged period of high chloroplyll-a was observed
from July through October.
1.2 Seasonal distribution of chlorophyll-a at selected
locations
The annual sequence of chloroplyll-a distribution
shown above is forced by the semi-annually reversing
monsoon winds, namely Southwest Monsoon (SWM)
and the Northeast Monsoon (NEM) seasons. Significant
changes in concentration of chlorophyll-a in the study
region occurred during these two monsoons periods.
For the purpose of the present study, January through
March is considered the NEM and June through
September is considered the SWM. The seasonal
means of chlorophyll-a for the NEM and SWM for 7
years (1998
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2004) are presented in Figure 3.
Figure 3 The seasonal mean concentrations of chlorophyll-a at
the selected locations (dashed line for the southwest monsoon
season and solid line for the northeast monsoon season)
Note: Left Y-axis shows chlorophyll-a in log units, right Y-axis
shows chlorophyll-a in normal units, both in mg/m
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