Spatial and temporal variability of the remotely sensed chlorophyll a signal associated with Rossby waves in the South Atlantic Ocean

The present study focuses on the spatial and temporal variability of interactions between physics and biogeochemistry during the Rossby wave passage in the South Atlantic Ocean. The Rossby wave signature in sea level anomalies (SLA) and surface chlorophyll a concentration anomalies (CHLA) is analyzed using remotely sensed data from 1997 to 2006. Wavelengths between 400 and 1100 km, with westward propagating speeds up to 7.5 cm.s −1 , are observed. Using a theoretical model, three processes (meridional advection of surface chlorophyll a concentrations, uplifting of subsurface chlorophyll a maximum, and upwelling of nutrients) are likely to explain the chlorophyll a Rossby wave signature. A statistical assumption allows quantifying the relative importance of each process. Three zones are identified. The Subtropical Gyre is the only area where the contribution of the uplifting process reaches 20%. North and south of this gyre, the meridional advection process is responsible for an important part (around 60%) of the observed chlorophyll a signals. The temporal variability of this dominant process is studied using the phase relationships between CHLA and SLA and the surface meridional chlorophyll a gradient. A seasonal meridional shift (4°) is shown on both data sets on the area of negative meridional gradient. At 30°S–31°S, a clear seasonal cycle is observed in both data sets for the whole studied period, except in 2003 and 2004 where both data sets do not follow the usual seasonal cycle. These particular years can be related to anomalies in large scale atmospheric circulation over the South Atlantic Ocean.