THE ATMOSPHERIC FORCING AS A SOURCE FOR OCEAN MESOSCALE ACTIVITY AT THE BRAZIL-MALVINAS CONFLUENCE ZONE.

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Resumen *

The South Atlantic Ocean is a relatively small fraction of the worl ocean but it is very active for its exchange with neighbouring oceans, and for the important formation of water masses that occur in this ocean. The western South Atlantic is a unique region of the ocean in the sense that water masses produced in very distant regions merge there and that it is the only ocean that transports heat from poles to equator.   The circulation of the western South Atlantic is an important factor in determining the climate of the neighbouring region of Argentina, not only because of the regulating influence that the ocean has in the local climate, but also because synoptic systems generated at souther latitudes are though to become intensified when they reach the region of warn surface temperature of the Brazil Current.   The confluence between the warm and salty Brazil Current and the cold and fresh Malvinas Current (hereafter refered as Convergence) occurs near the 39º south, with the presence of a strong frontal region, and very high dynamic activity. The temperature gradients in that area reach values as high as 1º/100 m (Legeckis and Gordon, 1982). These characteristics are shown in satellite images (Legeckis and Gordon, 1982; Brown, Olson and Evans, 1986; Olson et al, 1986; Olson et al, 1993; Figueroa and Olson, 1988; Provost el at, 1992) and in oceanographic observations (Roden, 1986; Garzoli et al, 1987, 1989, 1990, 1994). After joining at the Convergence both currents turn eastward and flow offshore in a serie of large scale meanders (Gordon and Greengrove, 1986; Reid, 1989). Warm eddies detach from the southern extension of the Brazil Current; cold and warm core addies are formed in the eastward flowing meandering boundary between the two currents.   The position of the Convergence fluctuates: the Malvinas Current shows northward intrusions and the Brazil Current shows variability in the latitude of its southermost extension. Observations and numerical models indicate that the latitude at which the Brazil-Malvinas Convergence is located varies seasonally, lying farther north during the austral winter (July-September) than during the austral summer (January-March) (Balech, 1994; Olson et al, 1988; Garzoli and Garraffo, 1989; Provost et al, 1992;  Matano et al, 1993).   Although there is no general consensus on what might determine the mean position of the Brazil-Malvinas Convergence all evidence suggest that main factors might be the basin integrated effect of the wind stress curl (Smith et al, 1994) and transport balance of the two colliding jets (Veronis, 1973; Matano, 1993; Agra and Nof, 1993). With regard to the large, seasonal oscillations of the latitude of the Convergence it has been hypothesized that they are related to seasonal changes in the Brazil and Malvinas currents transports (Matano et al, 1993).   The position of the Convergence seems to be sensitive to wind anomalies. Garzoli and Giulivi (1993) analysed the time series for the latitude of separation obtained from the IES Confluence data. They found that this latitude of separation presents a seasonal cycle, and two strong anomalies during the observed period November 1988 through February 1990. They relate the anomalous northward position for the separation latitude occurring on November 1988, to a negative (cyclonic) anomaly for the curl of the wind stress present on the South West Atlantic, centered at 50ºS: this cyclonic anomaly would accelerate the Malvinas Current and displace northwards the Convergence system. An anomalous southward displacement of the Convergence occurs during July 1989, which seemed related to a positive (anticyclonic) wind curl anomaly on the South Western Atlantic.   The oceanic transition zone between the area of subtropical influence (Brazil Current) and the area of subpolar influence (Malvinas Current) has its continental counterpart in the area of climatic transition located between  the upper Rio Colorado Basin and the lower Rio Negro Basin. This narrow continental zone separates the area of prevailing meridional flux to the north from that of dominant zonal flux to the south. The area of transition in the southwestern Atlantic Ocean  is the scenario of a number of weather fenomena with remarkable influence upon the climate of the coastal strip, affecting the life conditions of densely populated areas and productivity of the main agricultural region in South America. It has been pointes out that the South Western Atlantic Ocean between 30ºS and 45ºS is one of the with highest ciclogenetic activity within the southern hemisphere. The occurrence of cut-off flows, associated with typical circulation patterns (sudestadas), extended precipitations and occasional floods along the Rio de la Plata River bank, is one of the weather fenomena in the transition zone whose causes and dynamics are not yet well understood.   When this situation takes place, a strong cyclonic atmospheric circulation occurs over the southermost part of the Brazil current, which is usually asociated to an anticyclonic atmospheric circulation over the argentinian continental shelf. Figure 1 correspond a case that occurred on november 1989  that was analysed and model simulated by Seluchi and Saulo (personal comunication). Figure 1 exhibits the model’s wind on 1000 Hpa showing the cyclonic and anticyclonic circulation patterns associated to the sudestada; Figure 2 shows the “pseudo” wind stress vectors computed as                                       .   This “pseudo” wind stress is proportional to the aceleration component introduced by the wind into the ocean. Figure 3 shows the curl of this “pseudo” wind stress which gives an idea of the vorticity introduce into the ocean by the wind. This wind stress would  generate a similar circulation pattern in the ocean, with an associated southward advection of warm and salty water from brazilian coats, a northward advection of cold and fresh water from argentinian continental shelf, and an upstream advection of water onto the Río de la Plata, that would produce a level increase and occasionaly a flood. This combined effect would modify the thermal gradient alredy present in the Convergence area and could alter the frontal position.   An analysis of SST data for the same period (Figure 4a & 4b) suggest that this hypothesis is correct. Figure 4a shows the mean SST for November 1989, and Figure 4b shows the SST difference between the week when the sudestada takes place and the previous week. The data correspond to satellite, drifter bouys and in situ SST observations analysed and bias corrected at the NMC (IGOSS Data Set). Daily data can’t be used for this analysis because cloud cover during the cyclogenesis inhibits oceanic satellite observations in the area, which are the main component of the data set for the region. It can be seen in the figure that between the two weeks occurs a heeting of the area associated to cyclonic atmospheric circulation and a cooling of the area associated to to anticyclonic atmospheric circulation. The SST gradient increase that can be related to the cyclogenesis is about 1ºC in a week while it is expected from the climatologies for the whole November month a heating of around 2ºC (Figure 5).   That the sudestada has an effect on the frontal gradient and/or its position is though to be analysed by using a numerical regional model. A regional model will allow to isolate separte wind anomalies, and to analyse their effect on in the different fenomena. The model will be the Cox’s model (Cox, 1984) modified as a regional model. Boundary conditions from a coarser resolution model for the South Atlantic will be imposed. A first case of solution with a regional Cox’s model was presented in Garraffo et al (1989), for the Subtropical Convergence region. Figure 6 shows the circulation obtained in a coarse model, and with the higher resolution regional model; a big improvement in the regional solution was obtained with the regional model. The barotropic transport, temperature, salinity, and barotropic velocities were fixed at the boundaries of the regional model, interpolated from the coarse solution; a sponge layer was introduced near the boundaries to allow for a smooth adjustment between the external and external fields.   Información suministrada por el agente en SIGEVA