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

Abstract

The oceanographic regimes of the Northern Argentine Continental Shelf (NACS, 34°–43°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 (33.7–34.2) originating from the Gulf of San Matías; (2) a relative salinity minimum (30.0–33.3) 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 ( ϕ) 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 40–50-m isobaths south of 37°S, persistent throughout the year. We define two seasonal periods, autumn–winter and spring–summer, 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 autumn–winter period, we observe a northward extension of the Río de la Plata and Gulf of San Matías waters, as well as a reduction of the El Rincón and Continental Shelf waters near the coast. The spring–summer period is characterized by Río de la Plata waters flowing to the south and east, a reduction of Gulf of San Matías waters and an invasion of El Rincón 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 spring–summer reversal of the characteristic shelf-wide north–northeastward direction of flow within the coastal zone.