Basic HTML Version
International Journal of Marine Science 2012, Vol.2, No.1, 1
-
11
http://ijms.sophiapublisher.com
9
significant inter-annual differences occur in Group
-
1
locations due to differences in local forces in this
region. It is obvious from the above discussion that
the high chlorophyll-a events occur during the SWM
and NEM seasons at different locations with varying
magnitude. The period from January 1998 to mid
-
1998 (with low productivity) and the SWM of 1999
(with higher productivity) were anomalous. The
significantly lower chlorophyll-a during NEM (January-
March, 1998) in the Arabian Sea (Wiggert et al., 2002)
coincided with uncharacteristically high chlorophyll-a
in the easter equatorial Indian Ocean during the same
period (Murtugudde et al., 1999). The El Nino and the
Indian Ocean Dipole during the 1997/1998 influenced
the low chlorophyll-a extending in to northern Arabian
Sea (Saji et al., 1999; Murtugudde et al., 2000;
Wiggert et al., 2002). During the SWM season of
1999, the western Arabian Sea showed very high (~12
mg/m
3
) concentrations of chlorophyll-a thought to be
a result of the prevailing La Nina condition. Thus, in
the present study, the neighboring observation points
of Salalah and Masirah showed vastly different
conditions in successive years in response to the
large-scale climatic events.
The model performance was evaluated by computing
the ratio of the modeled chlorophyll-a to the SeaWiFS-
observed chlorophyll-a concentration,
r
, for different
locations. Table 1 Shows the
r
values which vary from
0.740 53 off Salalah to 0.872 07 off Yemen. This may
be improved by including the deterministic trends of
high frequency variations which need an improved
observational network.
3 Summary
The analysis of seven years of monthly SeaWiFS-
derived chlorophyll-a in the Arabian Sea showed
considerable inter-annual and geographical variability.
The period of elevated chlorophyll-a levels in the
Gulf of Mannar is extended due the advection of
chlorophyll-a rich waters from the Bay of Bengal
along the east coast of India. A coastally trapped,
chloroplyll-a rich front from the east coast of India
joins the local upwelling in the Gulf of Mannar from
middle of the SWM season. As this front propagates
further north along the west coast of India, the
southern waters become oligotrophic (depleted in
chlorophyll-a). The empirical model based on the
monthly chlorophyll-a derived from SeaWiFS ocean
color images predicted the inter-annual variability of
chlorophyll-a reasonably well. Thus, the inter-annual
variability can reasonably be predicted using the
annual and semiannual variability of the chlorophyll-a
data from the ocean color images. However, the
SeaWiFS-observed chlorophyll-a values were higher
than the model predictions during the periods of high
productivity in the Arabian Sea. In this context, we
have not investigated the role of atmospheric dust
which varies due to high monsoon winds (Banzon et
al., 2004). The Somali Basin showed stronger annual
peaks and a relatively lower semiannual peak than
other locations. The northern Arabian Sea, which is
enriched both during the SWM and the NEM, showed
a predominant semiannual peak followed by another
peak at a shorter period. The locations CAS, Yemen,
Salalah, Masirah and the Gulf of Oman showed
spectral peaks at annual and semi-annual periods with
nearly equal magnitude. The spectral analysis showed
that the annual signal was dominant in the Gulf of
Mannar region, which is not affected by the NEM.
Thus, the semiannually reversing winds and associated
changes in the upper layer circulation control the
chlorophyll-a variability in the Arabian Sea and its
marginal seas. The chlorophyll-a concentration was
observed to be least in May in the entire Arabian Sea,
except in the Gulf of Mannar which recorded the least
concentration of chloroplyll-a during the March-April
period. The timing of the peak in chlorophyll-a during
the SWM showed a lead/lag of about a month
compared to the predicted increase in chlorophyll-a
concentration. However, the spread of chlorophyll-a
through the seasons agreed with model results. The
chlorophyll-a bloom during spring 1998 was absent
in response to the anomalous SST event during
1997
-
1998, and the other productive regions of the
northern Arabian Sea also showed extremely low
chlorophyll-a (<1 mg/m
3
) until mid
-
1998. In the
following year, 1999, the chlorophyll-a concentration
was high (~12 mg/m
3
) in the month of August over
large part of the western Arabian Sea. The occasional
differences in the observed and modeled chlorophyll-a
in phases and amplitudes of peak chlorophyll-a
concentrations at some locations during certain years,