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International Journal of Marine Science 2012, Vol.2, No.1, 1
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11
http://ijms.sophiapublisher.com
1
Research Report Open
Inter-annual Variability of Chlorophyll-a in the Arabian Sea and its Gulfs
Y.V.B. Sarma
1
, Adnan Al Azri
1
, Sharon L. Smith
2
1. Department of Marine Science and Fisheries, College of Agricultural & Marine Sciences, Sultan Qaboos University, P.O. Box. 34, P.C. 123 Al Khod,
Muscat, Oman;
2. Rosenstiel School, University of Miami, FL 33149, USA
Corresponding author email:
sarma@squ.edu.om;
Authors
International Journal of Marine Science, 2012, Vol.2, No.1 doi: 10.5376/ijms.2012.02.0001
Received: 01 Apr., 2012
Accepted: 04 Apr., 2012
Published: 10 May, 2012
This is an open access article published under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any
medium, provided the original work is properly cited.
Preferred citation for this article:
Sarma, 2012, Inter-annual Variability of Chlorophyll-a in the Arabian Sea and its Gulfs, International Journal of Marine Science, Vol.2, No.1 1-11 (doi:
10.5376/ijms.2012.02.0001)
Abstract
The analysis of chlorophyll-a for a 7 year period from the ocean color images of SeaWiFS in the Arabian Sea and its
marginal seas showed considerable inter-annual and geographical variability. The variability in chlorophyll-a was essentially centered
on the Southwest and the Northeast monsoon seasons. The mean seasonal chlorophyll-a of the Northeast Monsoon in the western
Arabian Sea showed an increasing trend from 1998 to 2004. The period of elevated chlorophyll-a levels in the Gulf of Mannar is
apparently extended due to advection of chlorophyll-a rich water from the Bay of Bengal along the east coast of India. The mean
distribution of chlorophyll-a from the SeaWiFS images shows a coastally trapped chlorophyll-a rich front from the east coast of India
that enriches the Gulf of Mannar and continues northward along the west coast of India. As this front moves northward along the
coast, the southern coastal waters turn oligotrophic. The chlorophyll-a concentration in the region is also influenced by the
large-scale climatic events such as El Nino/La Nina during 1998
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1999.
The chlorophyll-a data from selected locations were subjected to spectral analysis to extract periodic signals. The annual signal was
found to be dominant in the areas affected by only one of the two monsoons such as the Gulf of Mannar. The semiannual signal was
predominant in areas affected by both the monsoons such as western and northern Arabian Sea. We built an empirical model
incorporating the annual and semiannual modes of chlorophyll-a from the ocean color data. The ratio of the standard deviations of
SeaWiFS-observed to the modeled chlorophyll-a, taken as a measure of model suitability, varied between 0.71 and 0.86. Thus the
chlorophyll-a variability in this region can reasonably be predicted using the empirical model developed in this study.
Keywords
SeaWiFS; Arabian Sea; Indian Ocean; Chlorophyll-a; Inter-annual variability; Empirical model
Background
The Arabian Sea and its northwestern gulf are
constrained by the landmass in the north and the
seasonally reversing winds. The surface currents of
this region have unique dynamical features associated
with physical, chemical and biological characteristics
(Varkey et al., 1996; Banse and English, 2000). The
semi annually reversing monsoon forcing leads to a
semi-annual signal in mixed layer depth variation
over most of the Arabian Sea (Rao et al., 1989). The
thermocline depth variation, in turn, controls the
surface layer productivity in the Arabian Sea during
the Northeast Monsoon (NEM) season (Wiggert et
al., 2002). Several recent studies have shown that
high chlorophyll-a concentrations develop during the
Southwest Monsoon (SWM) and NEM seasons due to
different factors (Bhattathiri et al., 1996; Wiggert et al.,
2005; Levy el al., 2007). The timing of peak productivity,
however, differs from region to region within the
northwestern Indian Ocean (Levy et al., 2007). Banse
and English (2000) have presented an exhaustive
review of geographical differences in seasonality of
phytoplankton productivity in the Arabian Sea using
CZCS data. The geographic differences in phytoplankton
seasonality in as diverse a region as the Arabian Sea
needs biophysical modeling. The studies of Bhattathiri
et al (1996) during JGOFS reported that a deep
chlorophyll maximum (DCM) is almost always present
around the base of the mixed layer in the eastern
Arabian Sea except along the southwest coast of India