Variability of chlorophyll-a off the southwest coast of India
Coastal upwelling off the southwest coast of India during the southwest monsoon is a well-known phenomenon that enhances the chlorophyll-a (chl-a) biomass. The present study explores this property and examines the variability of surface chl-a using satellite data obtained from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) for the period from September 1997 to December 2010. Spatial variability showed substantial cross-shore as well as along-shore gradients during the southwest monsoon. Temporal variability in chl-a was studied in conjunction with satellite observations on sea-surface temperature, sea-surface height anomaly, winds, and currents. The results revealed the dominant influence of the West India Coastal Current on chl-a variability during the upwelling and downwelling periods. Moreover, noticeable intra- and inter-annual variability was observed in the parameter. Therefore, an empirical mode decomposition (EMD) method was used to identify the oscillations influencing variability. SeaWiFS chl-a data for the period 2008–2010 were omitted from this analysis due to gaps in the record. EMD analysis revealed oscillations ranging from seasonal to a five-year periodicity. Quasi-biennial oscillations are identified as the dominant factor causing inter-annual variability in chlorophyll in the study area, compared with the El-Niño Southern Oscillation and Indian Ocean Dipole. Chl-a was also studied in two smaller grids of size 0.5° × 0.5° separated by around 300 km and representing coastal and offshore areas, to understand the nature of variability in these areas. The annual range of variability was high (0.1–8.0 mg m− 3) near the coast consequent on high upwelling intensity, and very low (about 0.1 mg m− 3) in the offshore grid due to the absence of upwelling.
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Document Type: Research Article
Affiliations: Naval Physical and Oceanographic Laboratory, Defence Research and Development Organization, Ministry of Defence, Kochi, 682021, India
Publication date: July 18, 2014