versión On-line ISSN 0717-6538
Gayana (Concepc.) v.68 n.2 supl.TIIProc Concepción 2004
| Gayana 68(2): 482-486, 2004 |
ANALYSIS OF SPACE-TIME VARIABILITY OF THE PLATA RIVER PLUME
Alberto R. Piola1,2,3 & Silvia I. Romero1,2
1. Departamento Oceanografía, Servicio de Hidrografía Naval, Av. Montes de Oca 2124 (C1270ABV) Ciudad Autónoma de Buenos Aires, Buenos Aires, Argentina.
Satellite ocean colour data from the Sea-Viewing Wide Field-of-View Sensor are analyzed to monitor the space-time variability of the Plata river plume over the continental shelf and slope. SeaWiFS Chlorophyll-a (CSAT) product is used to understand the nature of changes in the colour of waters and how this relates to variations in wind direction and river outflow. We use a 5-year data set (1998-2002) of Global Area Coverage monthly mean composites. The analysis of CSAT data reveals strong seasonal variability over the continental shelf with maximum northeastward penetrations in the austral winter (June to August). The plume may reach ~27S, about 1000 km northeast of the Plata estuary. In summer the plume is confined to south of 32°S and the Plata waters are frequently exported to the open ocean through the Brazil/Malvinas Confluence. This seasonal shift is similar to the variation of surface salinity observed in historical hydrographic data and is presumably associated to the variable influence of the along-shore wind stress. Interannually, 2002 showed the maximum northeastward penetration and the strongest anomalies (>6mg/m3). From late 1999 to early 2000, a strong negative CSAT anomaly (<6mg/m3) within the estuary could be linked to a period of very low Plata outflow (<12,000m3/seg). Results suggest that CSAT geophysical product is a good indicator of the presence of the Plata diluted waters. Other Case II ocean colour products are being explored (CDOM).
Key Words: ocean color, river plumes, Plata plume, SeaWiFS.
The Plata and to a minor extent the Patos/Mirim river systems (Figure 1), discharge substantial amounts of continental borne constituents, strongly affecting biological, chemical and physical properties as well as the circulation of the adjacent ocean. The northward extension of low salinity waters derived from the Plata impacts on the coastal sea surface temperature (Campos et al. 1999), the vertical stratification on the shelf (Piola et al. 2000; Zavialov et al. 2003) and presumably also on the coastal weather. The variability of near coastal phytoplankton biomass between 28°S y 34°S has been related to the availability of nutrients supplied by the freshwater outflow from the Plata and from the Patos Lagoon and to the seasonal displacements of the Brazil/Malvinas Confluence (Ciotti et al.,1995). Seasonal variations in the distribution of bluefish larvae Pomatomus saltatrix, the most important fishery off southern Brazil, have been related to meridional water mass fluctuations over the shelf off Southern Brazil (Muelbert and Sinque, 1996). Also Sardine (Sardinella brasilensis) catches have been related to the presence of coastal waters (Sunye y Servain, 1998). Numerical simulations (Kourafalou et al., 1996; Pimenta, 2001; Simionato et al., 2001, Palma et al., 2004) suggest that the fate of the Plata discharge depends primarily on the wind direction and on the magnitude of the freshwater outflow. When describing the Plata impact over the Atlantic Ocean, Piola et al. (2004a) point out that the Plata river discharges on average 23,000m3/seg, computed over a 50 year period, present significant variations without a well defined seasonal character. In 1983 and 1998 the Plata outflow increased beyond three times the mean value and between December 1999 and January 2000 it was less than 12,000m3/seg. Piola et al. (2004a) show that over the continental shelf, predominant winds are from the NE during summer and from the SW during winter. Historical surface salinity data (Figure 1) suggest large seasonal changes of several hundred kilometers in the northward and offshore extension of the river plume, associated to the Plata and Patos/Mirim river systems (Piola et al. 2000). However, historical surface salinity data do not present significant variability correlated to river discharge, instead this seasonal shift is presumably associated to the variable influence of the along-shore wind stress (Piola et al., 2004b).
Most of the current knowledge on the water mass distribution and circulation over the western South Atlantic is based on this historical hydrographic data (Piola et al., 2000). However, the sparse in situ observations, collected since the 1950s do not adequately resolve the space-time structure of the seasonal and interannual variations, in addition the space-time variability of the river outflow patterns is so large that the use of ship-borne observations alone may lead to misconceptions about the circulation (Müller-Karger et al. 1988). Because the river outflow introduces suspended matter, buoyancy and nutrients into the shelf, it induces a strong radiance signal in the visible band related to the concentrations of phytoplankton pigments, coloured dissolved organic material and suspended particulate matter. A preliminary analysis of monthly mean satellite chlorophyll-a (CSAT following Yoder, 2000) from SeaWiFS and monthly mean minimum sea surface salinity from historical observations, from Cabo Santa Marta Grande (~28S), reveal a linear correlation between both variables (Figure 2).
Our work hypothesis is that CSAT is a good indicator of the Plata river discharge patterns and their time variability. Thus, monthly mean satellite SeaWiFS chlorophyll-a (CSAT) distributions for the period 1998-2002 are used to characterize the space-time variability of the Plata river plume.
MATERIAL AND METHODS
A 5-year data set (1998 - 2002) of Global Area Coverage (GAC) Level-3 monthly binned CSAT imagery are used to study the seasonal to interannual variability of the Plata river plume. SeaWiFS CSAT data were available by NASA's Goddard Space Flight Center after the fourth reprocessing of the SeaWiFS global data set completed in July 2002. Level-3 monthly binned CSAT product is based on a fixed global grid of equal-area elements (bins) of approximately 9km x 9km resolution. On each bin, the accumulated CSAT data from all Level-2 GAC and for a binning period of one calendar month are averaged using the Maximum Likelihood Estimator (MLE, Campbell et al., 1995). In generating Level-3 binned CSAT imagery, only Level-2 data of the highest available quality are retained, while the remainder are discarded (IOCCG, 2004). The choice of this space-time resolution is adequate for the analysis on a regional scale and to reduce the effects of clouds, which seriously hamper the data availability in the study region in winter.
Monthly binned chlorophyll a concentrations from 21S to 41S and from 64W to 45S were used to construct CSAT mean distributions for summer and winter. Seasonal and interannual variability of the Plata river plume are described based on monthly CSAT extracted from an along-shore section AB (Figure 1) of ~1300km length that starts within the Plata estuary and ends close to 26S.
CSAT mean distributions for summer and winter (Fig. 3) show a significant seasonal variation in the Plata river plume waters. In austral winter, CSAT maximum, associated herein to regions of greater Plata influence, occurs mostly as a northeastward coastal band along the coast of Uruguay and southern Brazil, reaching near 26S, more than 1000 km away from the Plata estuary. In the austral summer the Plata waters influence is along two relatively thin filaments, one coastal plume to the northeast that reaches ~32S and one to the south. The later is exported to the open ocean towards the continental slope and into the Brazil/Malvinas Confluence. Though seasonal climatologies based on historical hydrographic data correspond to observations carried out over almost a century (Piola et al, 2000), seasonal changes in sea surface salinity are qualitatively similar to the seasonal shift observed in CSAT data. For example, the position of the 33.5 isohaline (Fig. 1) mentioned above as a sensible indicator of diluted waters due to continental runoff, varies between 31.5S in summer and 27S in winter.
It is not possible to describe the space-time evolution of the Plata river plume based on the sparse in situ sea surface salinity observations collected since the 1950s. Thus, we study the space-time variability of the northeastward coastal jet along the coast of Uruguay and southern Brazil based on five years of monthly CSAT extracted from an along-shore section AB (Figure 1). The analysis of CSAT data confirms the strong seasonal variability over the continental shelf with maximum northeastward penetrations in the austral winter (June to August) and minimum penetrations in summer. This seasonal shift is presumably associated to the variable influence of the along-shore wind stress which responds to predominant winds from the NE during summer and from the SW during winter.
Interannually, 1998, 2001 and 2002 showed similar CSAT distributions along the section, though the maximum northeastward penetration and the strongest anomalies (>6 mg/m3) occurred in 2002. From late 1999 to early 2000, a strong negative CSAT anomaly (<6 mg/m3) within the estuary could be linked to the above mentioned period of very low Plata outflow (<12,000 m3/seg). Also 2001 and 2002 were similar in that they present coastal plumes that reach approx. 500km from March to December. In summer 1998 and early fall 2001 positive anomalies (5 mg/m3) at 700 km from the Plata river probably indicate a greater influence from the Patos/Mirim system.
The analysis of CSAT for the period 1998-2002 reveals maximum northeastward penetrations during austral winter and minimum penetrations during summer qualitatively similar to those observed in the surface salinity distribution inferred from historical hydrographic data. This marked seasonality is presumably associated to the variability of the along-shore wind stress. CSAT data reveal strong interannual variability. The maximum northeastward penetration and the strongest anomalies (>6 mg/m3) are observed in 2002. From late 1999 to early 2000, a strong negative CSAT anomaly (<6 mg/m3) within the estuary could be linked to a period of very low Plata outflow (<12,000 m3/seg). These results suggest that the CSAT geophysical product is a good indicator of the presence of the Plata diluted waters. Other Case II ocean colour products are being explored (CDOM).
This work is supported by the Project SACC-CRN N° 61 from the Inter American Institute for the study of Global Change. The authors also acknowledge the SeaWiFS Project (Code 970.2) and the Distributed Active Archive Center (Code 902) from the NASA's Goddard Space Flight Center for the production and distribution of the data used herein.
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