An International Consortium for the Study of Oceanic Related Global and Climate Changes in South America

Ana AngeleriSeptember 13, 2017

Jun – 21 – 2011
An International Consortium for the Study of Oceanic Related Global and Climate Changes in South America

“Here the remarkable circumstance of the passage of the current of the Plata, across and over the southerly current, takes place: beyond which, to the south, the Brazil Current again appears, and is felt all the way to Staten Land, (off the eastern tip of Tierra del Fuego), although slow” (Rennell, 1832)

Plata impact on the adjacent shelf

The Plata river (Río de la Plata) and the Patos/Mirim lagoons are the two major sources of freshwater into the western subtropical South Atlantic. The along-shelf extent of the low salinity plume associated with these systems undergoes large seasonal changes (Piola et al., J.Geophys.Res., 105, 2000). In Austral fall and winter the plume may reach the Cabo Santa Marta Grande, about 1000 km northeast of the Plata estuary while in summer it is confined to south of 32°S (Fig.1).

Figure 1: Winter (left) and summer (rigth) climatological surface salinity distributions in the western subtropical South Atlantic based on historical hydrographic data. The heavy arrows indicate the northernmost extension of the 33 isohaline (adapted from Piola et al. 2000).

Summer northeasterly winds (Kalnay et al., Bull.Amer.Met.Soc, 1998) induce off-shore Ekman transport and oppose the northward plume penetration. In winter, however, northeast winds decrease or reverse in response to more frequent passage of frontal systems (Gan and Rao, Mon.Wea.Rev, 119, 1991), and induce further penetration of the coastal plume. Numerical simulations (Miller et al., Oceanogr. 14, 2001; Pimenta, MS Th., USP, 2001; Simionato et al., Geophys.Res.Let., 28, 2001) and theory suggest a northeastward extension of the low salinity waters in response to increased favorable winds and outflow. Thus, summer retreat may also be related to a decrease in river runoff. The largest fluctuations in the Plata discharge occur at interannual time scales and are correlated with shelf sea surface temperature anomalies and El Niño/Southern Oscillation (ENSO, Depetris et al., Naturwissenschaften, 83, 1996; Campos et al., Geophys.Res.Let, 26, 1999, Lentini et al., Cont.Shelf Res, 21, 2001). Though theory and models predict a significant effect of outflow magnitude on the plume extension, this is not confirmed by observations. This probably reflects the inadequate space-time coverage of the existing data base. The low salinity water increases the vertical stability of the water column and is presumably associated with low winter sea surface temperature anomalies observed along the continental shelf of Uruguay and southern Brazil (Campos et al., Ann.Acad.Bras.Cien., 68, 1996). The realm of the pelagic environment is closely related to the dynamics of the water structure.

In response to the thermohaline property and stratification changes, associated to the seasonal fluctuations of the Plata/Patos mixtures over the shelf, a strong biological impact is expected. For instance, seasonal variations in the distribution of fish larvae are related to meridional water mass fluctuations over the shelf off Southern Brazil (Castello et al., 1992; Muelbert and Sinque, Mar.Fish.Res., 47, 1996). In winter the abundance of bluefish larvae extends beyond Cabo Santa Marta Grande, while in summer it is restricted to the region south of 33ºS. This variations closely resemble the seasonal salinity fluctuations described above. A sharp subsurface transition zone exists between the southern source shelf waters, of subantarctic origin, and the northern source shelf waters, of subtropical origin (Piola et al., 2000). The mean location of the Subtropical Shelf Front (STSF, Fig.2a) falls along a north-south line from the mouth of Patos Lagoon to the outer shelf. It is also expected that this temperature, salinity and nutrient front also has a strong impact on the species distribution. Possible dynamical links between the STSF and the continental runoff and the western boundary currents of the South Atlantic are unknown.

Applying high resolution sigma-coordinate numerical models, that include the effects of the Plata outflow, E. Palma (UNS, Argentina), and R. Matano (OSU, USA) reproduce the STSF structure suprisingly well (right panel in Fig. 2).

Figure 2: Winter observed (left) and modeled (right) vertical salinity sections along the 100 m isobath on the continental shelf between 39°S and 29°S. The sharp, subsurface salinity (and temperature) front is referred to as the Subtropical Shelf Front. The front marks the transition between cold-fresh subantarctic shelf waters (S<34) and warm-salty subtropical shelf waters (S>35). The waters diluted by Río de la Plata mixture are evident above the STSF. The data were collected from R/V ALMIRANTE SALDANHA in winter 1977 (adapted from Piola et al., 2000) the model output was prepared by E. Palma.

The effect of the wind and river outflow fluctuations on the distribution of the low salinity waters is being investigated by means of several high-resolution modelling efforts. The Princeton Ocean Model (POM) is being used by the groups leaded by Elbio Palma (UNS, Argentina, and Edmo Campos (IOUSP, Brazil, Figure 3 (from a POM application at IOUSP: Pimenta, MSc Thesis, USP, 2001; Miller et al., Oceanography, 14, 2001 and Pimenta et al., in preparation), illustrates the dramatic impact of changes in wind forcing over the surface salinity distribution over the shelf under normal river outflow conditions (25000 m3/s). In these simulations wind forcing is applied after 130 outflow-only runs (no winds). Downwelling southeasterly winds push the low salinity waters onshore and a coastal low salinity jet develops, while upwelling northeasterlies advect low salinity waters offshore.

Figure 3: Surface salinity distributions for southwest winds, typical of the winter conditions (left), and northeast winds, typical of summer conditions (right). These distributions are snapshots from POM simulations (Pimenta, 2001).

The Plata project

The main goal of this project is to characterize the seasonal variations of the Plata plume and the STSF, their impact on the circulation and on the chemical and biological processes of the continental shelf. This requires determination of the relative importance of the main forcing factors on the distribution of the low salinity waters derived from the Plata and Patos Lagoon. To attain these goals we will conduct two surveys, one in Austral winter and one in summer. Each survey will combine airborne salinity measurements and in situ three-dimensional determination of physical, biological and chemical water mass properties. The surveys will collect the first quasi-synoptic and multidisciplinary set of observations of the low salinity plume, the STSF, and the background water masses (Figure 4). Satellite remote sensing of sea surface temperature and color and modeling will complement the above surveys. This project is being financed by the Inter-American Institute for Global Change Research and the Office of Naval Research (USA). A proposal has also been submitted to the Fundaçao de Amparo e Pesquisas do Estado de São Paulo (Brazil).

Figure 4: Background: Summer surface salinity distribution in the southwest Atlantic shelf. The black lines show the planned summer cruise hydrographic + biological sections across the continental shelf. The nominal station separation will be 15 km. The airborne salinity survey will cover the same area. Ongoing and planned experimental efforts in the region are also indicated: PRONEX-FURG, Brazil (red line), LETR-FURG, Brazil (magenta line) and PRONEX-IOUSP (hatched area).

Principal Investigators

Edmo Campos – IOUSP (Brazil) –
Joao Lorenzzetti – INPE (Brazil) –
Carlos Martinez – FCIEN/UdelaR (Uruguay) –
Jerry Miller – NRL (USA) –
Osmar Möller – FURG (Brazil) –
Alberto Piola – SHN/UBA (Argentina) –
Joel Wesson – NRL (USA) –