Bio-optical response of surface waters to wind-induced upwelling off the Kanyakumari coast

Abstract

This paper investigates the impact of wind-induced coastal upwelling on bio-optical properties over waters off the Kanyakumari coast for two consecutive winters (October 2011 to March 2012 and October 2012 to March 2013). Level-2 wind datasets from Oceansat-2 Scatterometer (12.5 km) have been used to estimate rate of upwelling using a published approach. Moderate-resolution Imaging Spectroradiometer on-board Aqua and Terra space-platform has been used to obtain bio-optical datasets (level-3, 4 km) that include chlorophyll-a (Chl-a) and particulate organic carbon (POC) concentration, absorption due to phytoplankton at 443 nm (aph_443), absorption due to detritus and gelbstoff at 443 nm (adg_443) and remote sensing reflectance (Rrs) at multiple wavelengths. Wind speed resulting in upwelling is found to be in range of 6.95 m s−1–18.67 m s−1. Phytoplankton bloom occurs when upwelling intensity is ranging between approximately 0.90 m/day – 3.50 m/day at a lag of about 2–3 days. Chl-a, aph_443, adg_443 and POC concentrations show simultaneous increment after upwelling and Chl-a is found to be the major parameter controlling other bio-optical properties of the study region. The waters belong to Case-1 type mainly co-dominated by phytoplankton and CDOM with very low concentrations of suspended particulate matter. Additionally, a noticeable drop in SST ranging between 0.55 °C to 1.50 °C has been observed at the same day of upwelling when upwelling intensity is around 1.5 m/day or above.