Spatial and temporal coherence between Amazon River discharge, satellite salinity and light absorption by colored organic carbon in Western Tropical Atlantic surface water. Joseph Salisbury1, Douglas Vandemark1, Christopher Hunt1, Janet Campbell1, Nicolas Reul2, Bertrand Chapron2 Dominik Wisser1

1. University of New Hampshire, Durham, NH, USA 2. Laboratoire d’Oceanographie Spatiale, Institut Francais de Recherche et d’Exploitationde la Mer, Plouzane´, France.

Abstract: The temporal evolution and spatial distribution of satellite-derived sea surface salinity and colored detrital matter (cdm) were evaluated in coastal waters adjacent to the Amazon’s discharge. Study objectives were: first, to document the spatial coherence between Amazon discharge, salinity, cdm, and the nature of the salinity-cdm relationship in a region adjacent to the Brazilian Shelf; second, to document the temporal and spatial variability of cdm along the trajectory of the low-salinity Amazon plume, and third, to explore the departure of cdm from conservative mixing behavior along the plume trajectory into the open ocean. Our work clearly demonstrates the value of satellite derived salinity for interpreting biology and biogeochemical data obtained from ocean color sensors.

Figure 2: Seasonal climatologies (2003-2007) of satellite variables: AMSR-E salinity (left) and MODIS acdm443 (right). The seasons (low flow, ascending, high flow, descending) were determined based on the climatological hydrograph of the Amazon River at Obidos, Brazil. Seasonal climatologies are estimated as the mean of seasonal NASA Aqua satellite data (2003-2007). Salinity is estimated using microwave brightness temperatures from the NASA AMSR-E sensor. Monthly acdm443 data are derived using monthly MODIS ocean color radiance data to which the QAA algorithm was applied (see text). The axis of the low salinity plume climatology (to ~36psu) is traced on the salinity data (black dash).

Figure 4: Monthly climatologies of salinity and acdm443 along the low-salinity plume trajectories (2003-2007). Salinity (lt. brown) and acdm443 (green). For each climatological month, all salinity and acdm443 data subtended by the low salinity trajectories were binned by distance from the Amazon mouth (50km/bin), averaged, and plotted as a function of distance.

Figure 1: Area of interes. The northwestern Tropical Atlantic. On land, the Amazon River and its major tributaries within our region of interest are shown, as are the Orinoco, Xingu and Tocatins (Toc.) Rivers. The location of the Obidos, Brazil discharge gauge is indicated as a solid circle. In the ocean, major currents are indicated: North Brazil Current (NBC), Northern Equatorial Current (NEC), North Equatorial Counter Current (NECC), Guyana tCurrent and Caribbean Current. The small flow vectors show the mean surface current direction and relative velocity during September. These were created from climatological ARGO float trajectory data.

Figure 3: Time series of monthly salinity, acdm443 and discharge. Data were retrieved and averaged for locations corresponding to the proximal (mouth to 600km), medial (600-1200km) and distal (1200 -1800km) sections of the low salinity plume trajectories and then plotted as a function of time. Salinity (lt. brown), modeled Amazon discharge (black solid), modeled discharge for the Rio Negro (black, 4a, second panel only), acdm443 (green), averaged acdm443 for the same region as Figure 3, (dashed green). Note the different axis scales for each plot.

Figure 5 : a) Distance (km) versus acdm443 residual (blue). b) Satellite CbPM data versus acdm443 residual (blue). Red lines represent the number of values in each bin: (a) binned by 50 km intervals. (b) binned by 50 mgC m-2 d-1 intervals. The dashed lines represent +/- 1 standard deviation of each bin average

Conclusions: 1. AMSR-E provides salinity data in time and space that is of sufficient accuracy to study biogeochemical processes in coastal regions. It complementary to ocean color data in these endeavors.2. Salinity is spatio-temporally coherent with satellite acdm data, but there are spatial and seasonal anomalies in conservative mixing of salinity and acdm.3. Anomalies below the mixing line tend to be related to distance along the plume trajectory, suggesting time-evolving photo-oxidation of acdm.4. Anomalies above the mixing are predominately found in high productivity regions, invoking the role of phytoplankton as a source of acdm.5. At the mouth of the Amazon, there is a lag between minimum salinity and maximum acdm. We speculate that the discharge of the Rio Negro is partly responsible for this observation.

Acknowledgements: We are grateful for the NASA support that funded our efforts: NASA Carbon Grants (NNX08AL8OG and NNX08AE59G), NASA Ocean Salinity Team Grant (NNX09AU69G), and NASA NEWS (NNX09AU89G). We deeply appreciate the technical support and advice from colleagues Tim Moore and Stanley Glidden. Observed discharge data has been provided by the Global Runoff Data Centre (GRDC) in Koblenz, GDR