Dandonneau¹, Yves, Andres Vega², Christophe Menkes¹, and Hubert Loisel³

¹LODyC, CNRS-IRD-UPMC, Paris, France, ²LEGOS, CNES-CNRS-IRD-Université Paul Sabatier, Toulouse, France, and ³LOA, CNRS-Univesité de Lille 1, Lille, France

Sea surface ecosystem and oceanic Rossby waves in the South Pacific

Sea color anomalies detected by SeaWiFS have been evidenced in the oligotrophic subtropical gyres with propagation characteristics similar to those of the slow Rossby waves that cross the oceanic basins from east to west. We have analyzed satellite data in the south Pacific (chlorophyll from SeaWiFS, altimetry from TOPEX-POSEIDON, sea surface temperature from TRMM) to explain these sea color anomalies. Chlorophyll increases generally respond to nutrients inputs into the photic layer. In the studied area (80 – 120° W, 15 - 25° S), the chlorophyll maxima that seem to respond to Rossby waves are not located at the troughs of dynamic height (crests of the pycnocline), nor in areas where the Rossby waves generate divergence of the currents. Thus, an explanation by nutrients inputs is not supported by these observations. Instead, we found that the chlorophyll maxima are located in areas where Rossby waves induce a convergence of the surface flow, and correspond to maxima of the sea surface temperature. We also found that the occurrence of chlorophyll anomalies associated with Rossby waves was blurred under strong wind conditions (during La Niña, or seasonally, in austral winter). This is coherent with floating material accumulated in convergence zones, and mixed inside the water column when the wind is strong. Indeed, using a radiative transfer model, we show that retrodiffusion of light by particles in excess increase the green to blue ratio of oceanic reflectances which is used by the SeaWiFS algorithm to estimate the chlorophyll concentration. We thus propose that the sea color anomalies that propagate along with Rossby waves result from the accumulation of floating particles near the sea surface in convergence zones, rather than from chlorophyll increases. Understanding the nature and role of such floating particles requires that more attention be paid to the upper first centimeters of the sea in future studies.