Satellite-based observation of ocean productivity in southeast Arabian Sea using chlorophyll, sea surface temperature and wind datasets

Abstract

The phytoplankton variability and ocean productivity have been studied with the retrieval of chlorophyll concentration (CC) from various satellite ocean colour missions. The phytoplankton distribution is related to ocean processes and physical parameters such as temperature, wind speed, currents, etc., at regional and global scales. In the current study, the phytoplankton productivity has been observed in the southeast Arabian Sea during southwest (SW, June–September) and northeast (NE, October–December) monsoon seasons using merged chlorophyll product datasets during the years 2017 and 2018. There has been an observation of high chlorophyll concentration (~5.0 mg m−3) in the southeast Arabian Sea (southern tip of India) during SW monsoon and less in NE monsoons (~1.0 mg m−3). The spread in CC has been observed maximum during July–September. High anomaly of CC was observed along north-eastern Arabian Sea during northeast monsoon months of 2017 due to the very severe cyclonic storm Ockhi. Similarly, during August 2018, high chlorophyll concentration was observed (18.8 mg m−3) in south-eastern tip as a result of heavy rainfall and discharge of nutrient rich riverine water. The sea surface temperature (Modis-Aqua SST) has been observed to be cooler (26–27°C) in the SE Arabian Sea coastal waters during July–August as compared to warmer (28–30°C) surrounding waters. During the SW monsoon (June–August), the wind speed has been observed to be of high magnitude (~10 m/s), which was not evident during September. The alongshore (north-westerly) wind has been observed during July–September in the near coastal water that causes coastal to offshore moving Ekman mass transport (EMT). Similarly, the alongshore (north-easterly) wind has been observed during December in the southern tip of India and off Sri Lankan west coast. The strong zonal EMT (–1500 to –500 kg/m/s) has been observed in the southeast Arabian Sea during July–September 2017 and 2018 which is weak to moderate (> –500 kg/m/s) in NE monsoon months. Statistical analysis has been made between SST, wind speed and CC which shows significant correlation coefficient (r > 0.6, p < 0.001). Hence, the present study confers that the alongshore wind and high wind speed regions are favourable to upwelling events and have been responsible for cooling of SST resulting in high productivity zones in the southeast Arabian Sea, which is important from the point of view of marine fishery and ecosystem assessment.