ADCP CURRENT DATA ANALYSIS IN THE RIO DE LA PLATA SALINITY FRONT: VARIABILITY FROM TIDAL TO SYNOPTIC TIME SCALES

SIMIONATO, C. G.; MECCIA, V. L.; DRAGANI, W. C.; GUERRERO, R. A.; NUÑEZ, M. N.

Perugia, Italia

Congreso; XXIV Asamblea General de la IUGG; 2007

International Union of Geodesy and Geophysics (IUGG)

The first ADCP current data collected at two locations of the Ro de la Plata salinity front during a period of around 6 month and salinity profiles gathered at and around those locations are used to study the vertical structure of currents response to wind variability in the tidal to synoptic time scales and its implications on stratification. Data shows that even though tides were though to dominate the system dynamics, they only account for 25% of the variance. Baroclinic currents provide the first evidence of the occurrence of internal waves, which can account for another 25% of the total variance. In the northernmost location, predominantly zonal oscillations with semidiurnal period, and oscillations with a dominant meridional component and diurnal period are found. Whereas the first ones can be related to the semidiurnal tide, the second ones seem to be atmospherically forced by the land/sea breeze. In the southernmost location, more rotational oscillations are observed, with periods around the inertial and diurnal ones. Inertial oscillations could result of wind relaxation, whereas diurnal oscillations also seem to be forced by breeze. Wave activity in the diurnal band was less frequent in the northernmost than in the southernmost location. This can be attributed to less frequent favorable stratification conditions in that area during the observed period. Wave activity in the southernmost location resulted weaker during the observed fall than during the summer. This could be a typical feature given that in autumn both, the number of storms destroying the thermohaline structure increases, and land/sea breeze is less frequent. This suggests a likely seasonal cycle in the diurnal wave activity in this area, given that those unfavorable conditions are even more marked during winter. In larger time scales, results indicate that the estuary rapidly responds to prevailing southwesterlies/northeasterlies with currents that decay towards the bottom with only little rotation in depth. For the less frequent southeasterlies/northwesterlies the estuary develops a strong vertical structure with a defined inversion in current direction between surface and bottom layers. These patterns derive from estuarys geometry and bathymetry. Results have important implications on the salinity vertical structure that are verified on the analyzed profiles. Firstly, the combination of the bathymetry and coastline with the prevailing wind variability is highly favorable to the maintenance of a salt wedge structure in this estuary. Secondly, weakening and eventually breakdown of stratification can only occur for intense and/or persistent southeasterly winds, which even can be very strong, are not frequent. This can explain why the Ro de la Plata displays the unusual feature of being an area of spawning and nursery for a number of coastal species that use the wedge as an essential element for their reproduction. Results show that stratification is highly affected by short term wind variability -which is its major characteristic in the area- changing the classical concept of summer-winter seasonality as the main feature of estuarine wind forced variability.