STUDY OF THE RIO-DE-LA-PLATA TURBIDITY FRONT .1. SPATIAL AND TEMPORALDISTRIBUTION

The Rio de la Plata is a riverine system located on the east coast of South America at 35 degrees S. It covers 35,000 km(2), it is 320 km lo ng and 230 km wide at the mouth. The Rio de la Plata drains the second largest basin in South America with an average discharge of 22,000 m( 3) s(-1). The processes associated with the interaction of the fresh r iver water and the saline shelf water and tidal stirring generate a tu rbidity front in the outer region, which is clearly defined in the sat ellite images as a strong gradient in reflectance and a sharp change i n water color. We use a 4 yr span of NOAA-AVHRR daily images, from Sep tember 1986 to August 1990, to determine the spatial and temporal dist ribution of the turbidity front. Channels 1 and 2 (visible and near-in frared) are used to digitize the fronts, and channel 4 and sea surface temperature are used as complementary information for cloud detection . From a total of 2578 images, 1274 daytime images allowed us to deter mine 333 positions of the front. This information is used to estimate the distribution of the frontal density, a probabilistic measure of fr ontal occurrence. Monthly, seasonal and. annual analyses were performe d. The relation of the turbidity front patterns to bathymetry, tidal d ynamics, river discharge and winds were investigated. A comparison bet ween the frontal density distribution and stratification parameters is presented. The results show a high degree of variability of the front al distribution at the northern coast of the estuary. In this region,t he frontal position varies between 57 degrees W and 54 degrees 12'W. T he westernmost location occurs in the summer months, which is coincide nt with minimum river discharge, predominance of easterly winds and mi nimum occurrence of southwesterlies. The easternmost location occurs d uring the spring, with strong winds from the southwest. Ar the souther n coast the modal position of the front coincides with the 5 m isobath , although great variations to this position have been observed during years of large river discharge. The fall and winter are seasons of ma ximum river discharge. During these periods there is a bimodal frontal distribution with maximum values of frontal density at the northern a rea of Bay of Samborombon and south of Montevideo, and higher variabil ity in the center of the river. (C) 1996 Elsevier Science Ltd