Spatio-temporal variability of coastal upwelling using high resolution remote sensing observations in the Bay of Bengal

Abstract

The work investigated the tempo-spatial variability of coastal upwelling through high-resolution High-Frequency (HF) radar-derived ocean current and other remote sensing observations on the northwest coast of the Bay of Bengal in 2018. Sea Surface Temperature (SST) showed a coastal upwelling signature only during April and May despite upwelling-favorable winds throughout the southwest monsoon. Stratified coastal water and coastal-trapped downwelling Kelvin waves suppressed the upwelling on the western coast from June. Improved fidelity was available from sea surface temperature (SST) and cross-shore surface current (CSSC) components of HF radar-derived ocean surface current, which showed strong consistency in detecting actual coastal upwelling from April to the first week of June. Coastal upwelling events consisted of six phases; three active phases and each followed by three break phases. The active phases of the coastal upwelling from SST lasted for about 15, 6, and 9 days, with a horizontal extension of 105 km, 75 km, and 140 km. The horizontal extension of strong positive CSSC during the first and third phases was over 135 km, while being 75 km during the second active phase. The duration of the active phase and the strong CSSC directly influence the horizontal extension of the upwelled colder water. High resolution (6.25 km) swath data helps to show that the weakening of wind shear near the coast started two days before the start of the second break phase in SST, which could not be captured in 25 km gridded winds. The altimetry track data of sea level anomalies obtained from different sensors for different days also captured the different upwelling phases. A large concentration of Chl-a near the coast, as observed in the chlorophyll-a monthly mean result, suggested the occurrence of coastal upwelling. However, the temporal variability could not be elucidated due to the low temporal coverage of Chl-a data. The active and break phases of the coastal upwelling are explained by associated air-sea interactions. This study suggests that HF radar data is beneficial for monitoring the high spatio-temporal variability of coastal upwelling, and high resolution swath data from the scatterometer is crucial for understanding the variability of coastal upwelling.