Testing the application of coastal altimetry in two South-Eastern African bights: the relationship between mesoscale features and chlorophyll-a, cyclonic eddies in the Indian Ocean

Abstract

Enhanced coastal altimetry, adaptive Leading-Edge Subwaveform(ALES), was used to study wo well known eddies along the southeastern African coast, namely the Delagoa Bight Lee Eddy and the Durban Lee Eddy. I address a two part problem. Firstly, how well does the application of coastal altimetry techniques behave in the coastal region? Secondly, is there a relationship between the eddies’ presence and chlorophyll-a (chl-a). Sea level anomalies (SLA) were computed from the re-tracked ALES data of two satellite missions — Envisat (used for Delagoa Bight eddy) and Jason -2 (for the Durban eddy) with geophysical corrections removed. These datasets were compared with the original satellite, 1 Hz RADS, AND AVISO gridded data. Two regions were selected to see if there was a biophysical link. A centre region of the eddy and an outside region of the eddy were taken. Results indicate that coastal altimetrywas successful in delimiting both features. ALES was less noisey and able to recover more data that were missed by the 1 Hz RADS dataset. Hovmöller plots showed the Delagoa Bight eddy to be more of a transient feature than semi-permanent, as had previously been suggested. Results from the linear model indicate a negative correlation between SLA and chl-a. This influence could be through facilitation of chl-a in the bight or just retaining chla. This is hard to elucidate without in situ data. These results show a promising indication that coastal altimetry will be a useful and reliable product to further biophysical coupling research along the coast