Annual distribution and variability of remotely sensed sea surface temperature fronts in the Southwestern Atlantic Ocean

BAVA, J.; GAGLIARDINI, D. A.; DOGLIOTTI, A. I.; LASTA, C. A.

Buenos Aires, Argentina

Simposio; 29th International Symposium on Remote Sensing of environment; 2002

Comisión Nacional de Actividades Espaciales (CONAE)

The occurrence and persistence of satellite-derived sea surface temperature (SST) fronts, over the Southwestern Atlantic Ocean, is analyzed using a 11 year times series (1985-1995) of NOAA-AVHRR Global Area Coverage images. A total of 800 five-day composites of AVHRR-GAC data were processed using an edge-detection algorithm to obtain monthly multi layer images of SST gradients, each layer corresponding to one of the six images for that month. A threshold value of maximum local standard deviation was used in this process to mask false gradients corresponding to cloudy pixels previously misidentified as SST pixels at the moment of generating the SST maps. After detection of fronts in individual images, the visual analysis revealed a permanent occurrence of fronts in the oceanic region influenced by the confluence of the Malvinas and Brazil currents. However, the complexity of the interaction between both currents produces high variability in the position of fronts between images of the same month, and the entire region is observed then as a ´frontal area´. Because of this, image processing was carried out differentially for the continental shelf region and for the oceanic region.Over the continental shelf, temporal permanence of fronts was evaluated using monthly images of persistent gradients. These images basically consisted in a composite of all fronts detected in at least the 50 percent of the images for the month under analysis. A single image was finally produced for each month by considering only those fronts which always showed occurrence in that month during the 11 year analyzed. For the oceanic region, coherence of detected fronts in each five-day composite image was analyzed and maps with all relevant fronts detected for each month were obtained. Most of the fronts observed over the continental shelf are relatively well known fronts, which appear during spring and are evident until autumn (San Jorge Gulf, San Matías Gulf, Península Valdés). However, in the coastal waters of the Buenos Aires province surface gradients are always visible around Bahía Anegada and appear during April to July east of the Bahía Blanca estuary. In Uruguay, a coastal front off Punta del Este occurs from December to February. Detection of the shelfbreak front is found to vary seasonally. It is weakly detected in late spring between 38ºS and 44ºS, well-detected in summer and decreasing its length northward until late autumn, when it essentially disappears. The extension and magnitude of SST gradients in the oceanic region is higher in winter, when the main fronts are detected from 32.5ºS to 44ºS. During summer, the distribution of maximum magnitude fronts is narrow, beingdetected mostly between 38ºS-42ºS.