COMPARATIVE ANALYSIS OF SST AND SST GRADIENTS FROM PATHFINDER AND MODIS SENSORS

JUAN PABLO PISONI; ANDRÉS L. RIVAS; ALBERTO R. PIOLA

Arraial do Cabo, Brasil

Simposio; VII SIMPÓSIO SOBRE ONDAS, MARÉS, ENGENHARIA OCEÂNICA E OCEANOGRAFIA POR SATÉLITE (VII OMAR-SAT); 2007

INSTITUTO DE ESTUDOS DO MAR ALMIRANTE PAULO MOREIRA

Satellite data are useful for the analysis in regions where in-situ information is poor. Due to their spatial and temporal resolution, and the variety of available sensors this information becomes a powerful tool to analize daily to interannual variability and to observe small-scale features, even in near coastal regions. Sea surface temperature (SST) data presently available are derived from advanced very high resolution radiometer (AVHRR) referred to as Pathfinder, MODIS from Terra and Aqua, GOES (geostationary) SST data, among others. We present examples of SST analysis from the Argentinean North-Patagonian coastal area. The region includes the San Matías, San Jose and Nuevo gulfs. These are semi-enclosed basins, in which the SST distribution and its evolution are governed by surface heat fluxes and by the exchange with the open sea, which is limited due to the restricted flow through their mouths. The region is marked by very high semidiurnal tidal amplitudes (~ 5 meters or more in some cases). Tidal energy dissipation leads to the formation of tidal fronts. The area holds important fishery and tourism resources and it is therefore important to understand the physical conditions governing its variability and its relation to the biological system. Four years (2003-2006) of monthly, 4 km SST data were used. To characterize the study area NOAA/NASA Pathfinder v.5 and MODIS/Aqua images were selected as they provide high spatial resolution compared to other SST products. Our main objective is to determine which data set is more suitable to represent the SST distributions and the variability of fronts. Thus, it was convenient to use monthly images which minimize cloud coverage. In addition, to better observe the typical summer frontal features, SST images and SST gradients derived from each sensor were compared. The study also include seasonal variability of the SST and SST gradients. Although none of the sensors tested is ideal to analyze the SST field, we were able to identify advantages and disadvantages of each for a particular zone. An important problem of the Pathfinder data, is the relatively well known tendency to discard data from high SST gradient regions, which are incorrectly interpreted as cloud edges. This shortcoming is associated to the fact that the Pathfinder processing was designed as a global product. In turn, MODIS SST presents some differences with Pathfinder SST, such as relatively smooth SST distributions and a different pattern of SST gradients that are not apparent in Pathfinder derived SST gradients. To minimize small-scale features (noise) in the SST distributions and non oceanic features, thresholds of SST gradients images were calculated so as to only to observe the most intense gradients. This simple proceeding reveals the characteristic patterns of tidal fronts, mainly at the mouth and interior of San Matías Gulf and east of Valdes Peninsula. To illustrate the results of these SST product comparisons, we also present application to Cabo Frio, a region frequently dominated by intense coastal upwelling, which leads to the generation of relatively strong SST gradients separating the relatively cold South Atlantic Central Water near the coast from tropical surface waters further offshore.