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International Journal of Marine Science 2012, Vol.2, No.1, 1
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11
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The seasonal distribution of chlorophyll-a off Somalia
showed higher concentration (above 4 mg/m
3
) during
SWM of all years except in 2003 when it dropped to
about 0.4 mg/m
3
. At this location the chlorophyll-a
concentration is nearly constant (0.4 ± 0.1 mg/m
3
) during
NEM for all the years.
The SWM season is the dominant influence on
enhanced chlorophyll-a levels at locations south of
about 20°N while NEM is dominant north of this
latitude (Figure 3). In 1998, when the rest of the
Arabian Sea and its marginal seas exhibited lower
concentrations of chlorophyll-a, the locations off Yemen
and Masirah Island showed relatively higher values. In
the Gulf of Mannar, the chlorophyll-a concentration is
considerably higher during the SWM during all years
compared to the NEM. In general the chlorophyll-a
concentration is either stable or marginally decrease
during SWM while during the NEM the chlorophyll-a
concentration showed increasing trend during period
of study.
1.3 Inter-annual variability of chlorophyll-a
The monthly chlorophyll-a data obtained from SeaWiFS
were plotted with a seven-year mean for the locations
shown in Figure 1. A logarithmic scale for the plots
was used in order to visualize smaller changes clearly.
However, to facilitate reading, we give logarithmic
and conventional units on the left and the right Y-axes
respectively.
The annual distribution of mean monthly chlorophyll-a
from the selected locations is presented in Figure 4.
As the Southwest Monsoon (SWM) and the Northeast
Monsoon (NEM) are the dominant forcing events in
this region, we shall discuss the inter-annual variability
in relation to these periods. The small increase in
chloroplyll-a concentration in February showed very
small inter-annual variability during this period
compared to the SWM period which shows a wider
peak spanning from May through September off
Somalia. The inter-annual variability is large during
both the early phase (May-June) and the late phase
(August-September) of the SWM compared to the
peak SWM period. The chlorophyll-a concentration
was generally low (<0.5 mg/m
3
) during inter-monsoon
periods with almost no significant inter-annual variability.
The chlorophyll-a distribution off Yemen showed two
maxima, a large-magnitude, prolonged peak during
the SWM and a very small-magnitude short duration
peak during the NEM, with the SWM being higher
in magnitude and longer in duration. The primary
maximum in chloroplyll-a concentration occurs during
the SWM with very high inter-annual variability
during the early phase of the SWM season. The peak
in chlorophyll-a occurred in September off Somalia
and Yemen with higher concentrations off Yemen.
Hitchcock et al (2002) suggest that chloroplyll-a from
Somalia is swiftly carried into the central Arabian Sea
and diluted by the fast current of the Great Whirl. The
minimum concentration of chlorophyll-a was observed
during April off Somali but in May off Yemen with
very small inter-annual variability. The regions off
Salalah and Masirah Island show similar variability in
chlorophyll-a except that the higher concentration off
Salalah occurs during the SWM while off Masirah
Island it occurs during the NEM. These locations fall
within Area
-
1 of Banse and English (2000) where
they reported highest pigment means up to 5 mg/m
3
during SWM; Brock and McClain (1992) reported the
chloroplyll-a concentration >11 mg/m
3
around these
locations.
The Gulf of Oman showed higher variability from
June to October and from January to March reflecting
the influence of the two monsoons. In the northern
Arabian Sea at NAS, the inter-annual variability is
considerable during March and June. The chlorophyll-a
showed a secondary maximum during the SWM in the
NAS region while such a feature is absent in the Gulf
of Oman. In the central Arabian Sea, relatively higher
levels of chlorophyll-a were associated with the SWM
period while winter blooms showed their presence in
increased chlorophyll-a in February. The chlorophyll-a
distribution in the Gulf of Mannar has an entirely
different pattern compared to the other regions of the
Arabian Sea. The chlorophyll-a concentration in the
Gulf of Mannar started increasing very early, from
April, and gradually increased until August, thereafter
a nearly uniformly high chlorophyll-a concentration
persisted until October. The high chlorophyll-a
concentration in the Gulf of Mannar lasts for about 6
months, the longest period of elevated chlorophyll-a