Sea surface salinity variability in response to the Congo river discharge

Abstract

Sea surface salinity (SSS) variability associated with the Congo River discharge is examined using Aquarius satellite-retrieved SSS data and vertical profiles of salinity measured by the Argo floats. The Congo River and its adjacent coastal ocean region are selected for study because of their global importance in ocean-freshwater dynamics undermined by the lack of observational data and coordinated efforts to make in situ measurements in this region. With a weekly repeat orbit, Aquarius provides a unique opportunity to routinely map the SSS in this relatively remote and understudied region. The Congo River plume can be clearly identified in the Aquarius SSS data with a northwestward extension of 500–1000km off the coast of the Democratic Republic of Congo (DRC). The peak amplitude of the SSS variability associated with the Congo River discharge exceeds 3.0psu, significantly greater than the designed Aquarius SSS retrieval accuracy of 0.2psu. Using the first two years of Aquarius data from September 2011 to August 2013, a well-defined seasonal cycle is described: maximum freshwater anomalies are found in the boreal winter and spring seasons. The anomalies during the 2012–2013 winter and spring seasons are significantly fresher than the 2011–2012 winter and spring seasons, suggesting a strong year-to-year variability.A strong correlation is found between month-to-month variations in upper ocean salinity (as revealed by Aquarius satellite and Argo observations) and month-to-month variations in the freshwater discharge from the Congo River over a region off the coast of West Africa that is large enough to be easily observed by the Aquarius satellite. Vertical profiles of salinity derived from Argo floats reveal that these freshwater anomalies can be traced to 30m below the sea surface. Combining the spatial area characterized by strong negative correlations between Aquarius SSS data and Congo discharge with the Argo vertical profiles of salinity, the volume of the freshwater anomalies is inferred and used to estimate the Congo River discharge. Reasonably good agreement is found between the Congo River discharge as observed by a stream gauge at Brazzaville and that estimated from the combined Aquarius and Argo data, especially during the freshening portion of the seasonal cycle (during this phase the correlation is 0.84, implying ~70% of the variance can be explained). The precipitation minus evaporation portion of the freshwater flux is found to play a secondary role in this region.