Coastal oceanographic regimes of the Northern Argentine Continental Shelf (34–43°S)

The oceanographic regimes of the Northern Argentine Continental Shelf (NACS, 34 e43 S) are derived from advected waters of subantarctic origin, local sources of continental run-off, and a locally generated salinity maximum. Based on 3690 CTD profiles, monthly mean wind fields at coastal stations, and river discharge data, we define the oceanographic regimes over the shelf by analyzing salinity characteristics and spatial distribution: (1) a maximum in salinity (33.7e34.2) originating from the Gulf of San Matı´as; (2) a relative salinity minimum (30.0e33.3) of the El Rinco´ n estuarine system; (3) a salinity minimum (0e33.0) originating in the Rı´o de la Plata; and (4) waters of the continental shelf (33.5 and 33.7). Temperature over the shelf is controlled by seaeair heat exchange coupled with bathymetry. An analysis of the Simpson parameter of stability (f) provided an objective definition of a vertically homogenous coastal zone separated from seasonally stratified shelf waters south of 37 S. Bottom temperature gradients and synoptic sections in the winter and spring indicate the presence of a shallow sea front at the 40e50-m isobaths south of 37 S, persistent throughout the year. We define two seasonal periods, autumnewinter and springesummer, based on seasonality in monthly mean winds fields, continental run-off, fresh water balance and the spatial distribution of salinity signals. Maximum seasonal variation in the extent and location of the oceanographic regimes occurs within the coastal zone. In the autumnewinter period, we observe a northward extension of the Rı´o de la Plata and Gulf of San Matı´as waters, as well as a reduction of the El Rinco´ n and Continental Shelf waters near the coast. The springesummer period is characterized by Rı´o de la Plata waters flowing to the south and east, a reduction of Gulf of San Matı´as waters and an invasion of El Rinco´ n and Continental Shelf waters into the coastal areas. In a general sense, waters across the NACS undergo a seasonal oscillation in distribution and extension that implies a springesummer reversal of the characteristic shelf-wide northenortheastward direction of flow within the coastal zone.