Coastal oceanographic regimes of the northern Argentine Continental Shelf (34 - 43°S).

LUCAS, A.J.; GUERRERO, R.A.; MIANZAN, H.; ACHA, E.M.; LASTA, C.

ESTUARINE COASTAL AND SHELF SCIENCE

Año: 2005 vol. 65 p. 405 - 420

0272-7714

The Northern Argentine Continental Shelf (NACS, 34º to 43ºS) is a broad, shallow system between the coastline and the shelf break at a depth of 200 m. The oceanographic regimes over the shelf 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 defined the oceanographic regimes over the shelf based on salinity characteristics and spatial distribution: 1) a maximum in salinity (33.7 to 34.2) originating from the Gulf of San Matías; 2) a relative salinity minimum (30.0 to 33.3) of the El Rincón estuarine system; 3) a salinity minimum (0-33.0) originating in the Río de la Plata; and 4) waters of the continental shelf (33.5 and 33.7). Temperature over the shelf is controlled by sea-air 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 shallow sea front at the 40-50 m isobaths south of 37°S, persistent throughout the year. We defined two seasonal periods, autumn-winter and spring-summer, based on seasonality in monthly mean winds fields, continental runoff, and the spatial distribution of salinity signals. Maximum seasonal variation in the extent and location of the oceanographic regimes occurs within the coastal zone. The Río de la Plata estuarine waters flow to the north along the Uruguayan coast in the autumn-winter and are forced to the south and east in the spring-summer by northeasterly and easterly winds. In the autumn-winter period, a seasonal peak in salinity and prevailing northwesterly winds cause a strong northeastern extension of Gulf of San Matías waters to the northeast, where the salinity maximum intersects the coast and is locally maintained by an excess of evaporation over precipitation and runoff. A minimum in net evaporation in Gulf of San Matías, a maximum in continental runoff, and the lack of a dominant forcing mechanism in the spring and summer weakens the high salinity signal and allows an eastward extension of El Rincón waters and a southward and westward intrusion of continental shelf waters into the coastal zone. In a general sense, waters across the NACS undergo a seasonal oscillation in distribution and extension that implies a spring-summer inversion of the characteristic shelf-wide north-northeastern direction of flow within the coastal zone.