Physical control of primary productivity on a seasonal scale in central and eastern Arabian Sea

Journal of Earth System Science (Impact Factor: 0.7). 04/2012; 109(4):433-441. DOI: 10.1007/BF02708331
Source: OAI

ABSTRACT Usingin situ data collected during 1992–1997, under the Indian programme of Joint Global Ocean Flux Study (JGOFS), we show that the biological
productivity of the Arabian Sea is tightly coupled to the physical forcing mediated through nutrient availability. The Arabian
Sea becomes productive in summer not only along the coastal regions of Somalia, Arabia and southern parts of the west coast
of India due to coastal upwelling but also in the open waters of the central region. The open waters in the north are fertilized
by a combination of divergence driven by cyclonic wind stress curl to the north of the Findlater Jet and lateral advection
of nutrient-rich upwelled waters from Arabia. Productivity in the southern part of the central Arabian Sea, on the other hand,
is driven by advection from the Somalia upwelling. Surface cooling and convection resulting from reduced solar radiation and
increased evaporation make the northern region productive in winter. During both spring and fall inter-monsoons, this sea
remains warm and stratified with low production as surface waters are oligotrophic. Inter-annual variability in physical forcing
during winter resulted in one-and-a-half times higher production in 1997 than in 1995.

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    ABSTRACT: The upper layers of the ocean play an important role in the regulation of the ocean–atmosphere as a coupled system. The Arabian Sea, situated in the western part of the northern Indian Ocean, participates in the intense air–sea exchange processes associated with the semi-annually reversing monsoons. During winter monsoon the weak northeast trade wind brings cool, dry continental air, while during the summer monsoon the strong southwest wind brings humid maritime air into the Arabian Sea. It is natural to expect that these semi-annual atmospheric forcings would modulate the thickness of the upper ocean. The seasonal variability of the mixed layer in the central Arabian Sea (equator to 25°N and 62–66°E) was studied using hydrographic data from NODC and INODC, while the atmospheric forcing responsible for the observed changes were explored using meteorological data from COADS and FSU winds. The mixed layer depth (MLD) and sea-surface temperature (SST) in the central Arabian Sea, to a large extent, were regulated by the incoming solar radiation and wind forcing. The Ekman dynamics associated with the Findlater Jet controlled the mixed layer depth during summer monsoon, while in winter the cooling and convective mixing regulated the MLD. We speculate that the deep mixed layer during February–March in the southern region is associated with that of the Rossby wave propagation. During spring and fall intermonsoons, the weak winds along with strong incoming solar radiation lead to the formation of a thin, warm and stratified mixed layer. We show that biological productivity of the Arabian Sea is rigidly coupled to the seasonal cycle of MLD, which regulates the nutrient levels in the euphotic zone.
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