Seasonal Surface Chlorophyll a Variability in the Seychelles–Chagos Thermocline Ridge

Abstract

-Chagos Thermocline Ridge (SCTR, 5-10S, 50-75E) in the southwestern tropical Indian Ocean is a unique area that experiences year-round upwelling. This is a response to the upward Ekman pumping prevalent in the region. Satellite data, model data and objectively analysed Argo temperature/ salinity data have been used to study the seasonal surface chlorophyll a (chl a) variability in SCTR. Variability of surface chl a concentration in SCTR showed a weak semiannual signature. The western part of SCTR (WSCTR, 50-62E) is characterized by higher chl a concentration than the eastern part (ESCTR, 63-75E). Average chl a concentration in WSCTR/ESCTR showed a primary peak in July-August (~0.26/~0.16 mg/m 3 ) and a secondary peak in January (~0.14/~0.12 mg/m 3 ). Minimum chl a concentration (~0.12/~0.1 mg/m 3 ) was observed during March-April and December-January. The high amplitude of chl a variability observed during July-August is associated with weak stratification and deep mixed layer depth (MLD). Deep MLD reaching to nutrient-rich thermocline entrains nutrients to the surface and thereby increases the surface chl a concentration. However, the low surface chl a concentration is a result of shallow MLD in the region. The deep MLD (30-40 m) observed during June-October is dominated by wind mixing and supported by buoyancy mixing. Shallow MLD (<30 m) observed during rest of the year is due to weak wind mixing and high surface buoyancy. The high surface buoyancy is a manifestation of ocean surface warming and presence of low saline surface waters in the SCTR region.