ALBERT

Description: The Seychelles-Chagos Thermocline Ridge (SCTR) is a biologically important region of open ocean upwelling within the south west Indian Ocean (5-10°S and 45-90°E). The SCTR refers to an elongated feature which joins two local minima in thermocline depth; the Seychelles Dome (SD) and Chagos Domes (CD). We present the seasonality and physical drivers of the surface chl-a bloom for both the western (SD) and eastern (CD) extremes of the SCTR, using in situ observations and remotely sensed data. We find the buoyancy fluxes from the Indonesian Though Flow (ITF) strongly impacts surface chl-a concentrations over the CD. Here a significant correlation (r=0.33) was found between the altimeter-derived strength of the IFT and the surface chl-a concentrations. This correlation increases (r=0.58) when only the 90th percentile ITF events are considered, indicating the influence of the ITF may be overcoming other physical drivers for these years. We hypothesise the buoyancy flux of a strong ITF input suppresses the thermocline and ‘caps’ the CD with warm, less saline waters. This hypothesis is supported following a strong, significant relationship (r=0.66) between remotely sensed surface salinity and surface chl-a over the CD. This relationship is not found over the SD, where the ITF has a weaker direct impact over the bloom. The results suggest that the ITF may be an important factor for surface chl-a within the SCTR over longer-term time scales. Idealised model experiments will be used to investigate the relationship between the IFT and the spatio-temporal variability chl-a within the SCTR.

Description: The South-East Madagascar Bloom, one of the most compelling biogeochemical features of the Indian Ocean, occurs sporadically during austral summer in the oligotrophic waters south-east of Madagascar, where it can cover up to 1% of the global ocean surface area. Its spatial extension and its timing are highly variable. A high-resolution biophysical model is used to investigate a previous hypothesis that the onset of a particular circulation of the South-East Madagascar Current advects fresher and nutrient-rich waters eastward, feeding the bloom. The model is able to reproduce an intermittent phytoplankton bloom with large spatial variability but in the subsurface layers, as well as the presence of an irregular retroflection of the South-East Madagascar Current. The simulated bloom occurs within a shallow stratified mixed layer, with fresher waters at the surface, parallel to the water mass in an observed bloom. The model results suggest, from a nutrient flux analysis, that horizontal advection of low-salinity nutrient-rich Madagascan coastal waters can indeed trigger a phytoplankton bloom. The coupled model is also able to resolve a bloom that is atmospherically forced by cyclonic activity.