Effect of Seychelles Dome intensity on nutrient supply to the mixed layer: Insights from a coupled physical-biological model

The seasonal and interannual variability in the vertical nutrient supply above the Seychelles Dome (SD) in the southwestern tropical Indian Ocean is examined using a high-resolution coupled physical-biological ocean model. Above the SD region, seasonal phytoplankton blooms occur from May to August (from Austral fall to early winter). This unique seasonal variability is caused by a significant increase in the nutrient concentration, which coincides with a significant deepening of the mixed layer depth. A nutrient budget analysis revealed that the seasonal nutrient variability in the mixed layer above the SD is controlled mainly by vertical processes, including entrainment and vertical diffusion during the blooming season. The sum of the vertical nutrient supply is nearly equivalent to what is biologically consumed and is consistent with satellite observations of the seasonal variation in surface chlorophyll. Additionally, the influence of the SD intensity on nutrient concentrations above the SD region was investigated at an interannual time scale. Composite analyses of the nutrient budget imply that nutrient concentrations decrease (increase) because of negative (positive) vertical diffusion anomalies from the Austral summer to early fall in weak (strong) SD years. This negative (positive) vertical diffusion anomaly is caused by a gentle (sharp) vertical nutrient gradient owing to the weaken (strengthen) subsurface SD. Conversely, seawater nutrients and surface chlorophyll concentrations during blooms do not differ significantly between years with weak and strong SDs because negative (positive) nutrient concentration anomalies formed by anomalous vertical diffusion in the Austral summer are not maintained as a result of a damping effect, which is a positive (negative) meridional nutrient advection anomaly during the early blooming season and can last several months. During these months, anomalous meridional nutrient gradient plays an important role by inducing a meridional nutrient advection anomaly via a southwestward Ekman transport forced by climatological trade winds. These anomalous meridional nutrient gradients are caused primarily by vertical diffusion anomalies during the Austral summer during the SD-event years. The results of this study imply that interannual variations in mixed layer nutrient concentrations and surface chlorophyll during the Austral summer are strongly related to the intensity of the subsurface SD via the vertical gradient of nutrients inside and below the mixed layer.