A NUMERICAL STUDY OF THE PLATA RIVER PLUME ALONG THE SOUTHEASTERN SOUTH AMERICAN CONTINENTAL SHELF

The Rio de la Plata, one of the largest rivers on E arth, discharges into the ocean waters from basin t hat covers a large area of South America. Its plume ext nds along northern Argentina, Uruguay, and souther n Brazil shelves strongly influencing the ecosystems. In spite of this, little is known about the mechan isms that control it. Here we report results of simulati ons with POM carried out to investigate the roles o f wind and river discharge in Plata plume dynamics. Differ ent outflows were explored, including an average climatological value and magnitudes representative of La Niña and El Niño. Forcing the model with rive r discharge the average plume speed was directly rela ted to the outflow intensity. The Plata northward extension varied from 850 to 1550 km and for averag discharge a band of low salinity waters formed from the estuary up to 30 N of South Brazilian Shelf. Upwelling and downwelli ng winds were applied after 130 days. The distribution of low salinity wa ters over the shelf was more sensitive to the wind direction than to the river outflow variability. Do wnwelling winds were very capable of advecting the low salinity signal downshelf. Upwelling winds were eff icient in eroding the plume, which was basically detached from the coast by Ekman drift. Abnormal pl ume intrusions toward low latitudes may be a result of the original plume position coupled with events of persistent strong downwelling favorable winds.


INTRODUCTION
appearance of penguins are some indicators.It's recurrence was studied by Campos et aL. (1999) and The intermittent intrusion of relatively fresh Lentini et al. (2001) through the analysis of a 13-year cold waters from higher latitude into the South Brazil time series of AVHRR images.Their findings revealed Bight (SBB), between 28 0 S and 22°S off the Brazilian the cold intrusions as seasonal phenomenon, which coast, is a phenomenon that has intrigued the scientific presents an interannual variability possibly related to community.This phenomenon has been known for a El Niflo/Southem Oscillation (ENSO).At the same long time but was first documented in the scientific time, Piola et al. (2000) based on the analysis of literature in the middle 90's (Campos et aL, 1995, historical hydrographic data described the presence of 1996a,b).Combining in situ measurements and a defined seasonal sign associated with the Rio de la satellite imagery, these studies demonstrated the Plata surface plume.As one of the largest rivers on presence of cold (T-18°C) low salinity (S-32) waters Earth, the Plata River (Plata hereafter) discharges into extending from south of the Cape of Santa Marta the ocean waters from a drainage basin that covers (28'S) to the middle of the bight, at approximately nearly 20% of South America.The influence of its 23'30'S (Fig. 1).The absence of major river systems plume, as suggested by the historical data, extends into in the region and the thermohaline characteristics of the continental shelf off northern Argentina, Uruguay the coastal waters excluded the possibility of influence and southern Brazil.Sometimes the plume reaches of local river runoff or upwelling (Campos et aL, latitudes as low as 27°S during the winter, while in the 1996b).
summer it is confined to 32'S (Fig. 2) (Piola et al., Based on satellite SST images and 1999, 2000, 2005).trajectories of surface drifters launched in February The Plata discharge presents interannual and late April of 1993, the unexpected water mass was variability related to El Nifto scale rainfall variations associated with advection from the vicinity of the over South America (Ropelewski & Halpert, 1987;Brazil-Malvinas Confluence (BMC) in the Argentinian Grimm et al., 1998;Mechoso & Iribarren, 1992; continental shelf (Stevenson et al., 1998;Campos et Depetris et al, 1996).This correlation was suggested al., 1996b).This water mass was suggested as the as a reason for unusual plume intrusions along the agent responsible for the failure of important Brazilian shelf, which under high outflow events would explain fisheries (Matsuura, 1996;Sunye & Servain, 1998) the appearance of cold and fresh waters to SBB and for the intrusion of organisms from different (Campos et al., 1999 andPiola et al., 2000).Surface biogeographic zones.The detection of cold water salinity observations from Guerrero et aL. (1997a) in foraminifera at unexpected low latitudes (Stevenson et late April and May of 1983 demonstrated how the 30 at, 1998) and the anecdotal reports of the occasional isohaline extended to about 150 km offshore and to 200 km along the coast of Uruguay beyond its average et at, 1997a,b;Simionato et at, 2004).The position during a high outflow event.The 1993 phenomenon of coastal buoyant plumes, on the other intrusion however, remains unclear.The change in the hand, is well explored in the scientific literature.extent was of the order of 800 to 1000 km, or nearly 4 Works related to the Amazon River plume (Lentz, times larger than observed by Guerrero et al. (1997aGuerrero et al. ( ). 1995;;Limebumer et al., 1995;Geyer et al., 1996), the Nevertheless, the anomalous northward penetration Delaware and Chesapeake systems (Malnchow and occurred about one year after a large river outflow Garvine, 1993;Rennie et al., 1999; Sanders and event associated with the 1992-1993 El Niflo.Garvine, 2001;Houghton et al., 2004), Columbia Despite its considerable dimensions and (Hickey et al., 1998) and the Rhine river plumes strong influence on nearshore ecosystems (Ciotti et al., (Simpson and Souza, 1995) are some examples.These 1995; Mianzan et al., 2001), little is known about the studies reveal the strong dependence that buoyant Plata plume variability or mechanisms that control it.
plumes demonstrate not only under river, but under A few numerical modeling studies investigated its wind forcing as well (Chao & Boicourt, 1986; Chao, seasonal variability (Simionato et al., 2001(Simionato et al., ), most of 1987(Simionato et al., , 1988a,b;,b;Kourafalou et al., 1996a,b; Garvine, the investigations focused on the estuarine domain 1999).(O'Connor, 1991;Framifian & Brown, 1996  Within this context, the objective of the set to dx-dy=10 km, which resulted in a 105x270 present study is to extend the understanding on the points grid (Fig. 2a).With this spatial resolution, the Plata plume variability and investigate the relative Nyquist wavelength is of 20km, a bit larger than the roles of river discharge and wind forcing on the local Rossby Radius of deformation.However, dynamics of the Plata river plume over the considering both the alongshelf and cross-shelf Southwestern Atlantic shelf Specifically, we strove to dimensions of the plume, the 10km horizontal explain plume intrusions to abnormal low latitudes.resolution is reasonable.Depths greater than 1000 m This research was based on simple numerical were set to 1000 m and the vertical structure was simulations of the Plata plume using the Princeton represented by 12 sigma surfaces (Fig. 2c).In the Ocean Model (POM).
numerical simulations, the domain was initialized with a homogeneous structure, with T=25 0 C and S=35 psu.

METHODOLOGY
The Plata River outflow was implemented as an open boundary condition at the estuary head on the western Buoyancy driven coastal plumes have been side of the domain.Since the domain was rotated 40' widely investigated by many researchers.These degrees, the estuary head fit well to the meridional features are nonlinear, with Coriolis force and friction domain axis (Fig. 2b).Thus, the horizontal barotropic playing important roles (Csanady, 1984; Mifnchow & velocities across the river mouth could be prescribed Garvine, 1993;Kourafalou et al., 1996 b,a;Garvine, as: 1999).Here we adopted the Princeton Ocean Model Q (POM), a three-dimensional, sigma-coordinate, U=-, 1'=0, (1) primitive equation model (Mellor, 1996;Blumberg & A Mellor, 1987).
where A is the area of the vertical section of the The Numerical Model estuary head, and Q is the river discharge [m 3 .s-'].The free surface elevation is computed by the model, The model dependent variables are the applying a no-gradient condition (la13i = T la, where B zonal, meridional and upward components of velocity is the boundary index).Hydrographic data show that (u, v, w, respectively), the free surface elevation 1I, the water column at the estuary head is quasi temperature T, salinity S, and two properties of the homogeneous in temperature and salinity (Guerrero et turbulence field (turbulent macro scale and kinetic al., 1997a,b).Temperature and salinity were specified energy).The density p is computed from the equation using a temporal relaxation based on historical of state.The system of equations include the nonlinear conditions of the Rio de la Plata winter temperature advective terms and the variables are computed (T=14°C) and salinity (S=5 psu).integrating the continuity equation, the momentum equation, and two equations for advection-diffusion of Oceanic Boundary Conditions heat and salt.For the horizontal grid, an "Arakawa C" differencing scheme is used.The horizontal time In addition to the river runoff specification differencing scheme is explicit, whereas the vertical as an open boundary condition at the estuary head, differencing is implicit, which eliminates time three oceanic open boundary conditions were constraints for the vertical coordinate and permits the implemented.At the cross-shore boundaries (north and use of fine vertical resolution in surface and bottom south) the elevation was treated with a no gradient layers.An imbedded second moment turbulent closure condition, while barotropic velocity components and sub-model (Mellor & Yamada, 1982)  In this set of experiments, we dealt with the performed, but with the addition of a 10-point buoyancy-driven force generated by the Plata hyperbolic sponge layer for the velocity components.
discharge, which mixes with the surrounding oceanic In that layer, the velocity components were waters.Our experiments were based on two major progressively damped from their original value (OB-I0) questions.
(1) What would be the extent of the inside of the domain, to 78% of their value near the alongshelf penetration of the plume for a steady boundary (0. 7 8 ýBa_).An Orlanski scheme was applied coastal buoyancy source?(2) How would the at the eastern domain edge (6a).For the temperature alongshelf penetration vary with increased or and salinity boundary conditions, an advection decreased river discharges?solution was used in order to allow the plume To address these questions, we first ran three movement through thecross-shoreboundaries: experiments with the following values of river discharge: Q=15xl 0 3 m 3 s-(low); Q=25x10 3 m 3 s-' (medium); and Q=50xI0 3 m 3 s-1 (high).According to a S O SMechoso and Iribarren (1992) and Depetris et al.
a t a y (1996) these values are typical of a La Nifla situation (dry season/lower discharge), to normal periods (average discharge), and to El Nifho Events (higher where v is the velocity component normal to the discharge).Hereafter, we refer to these experiments as boundary, and 8/8y is the derivative in the normal RI5, R25 and R50.The river outflow was set to zero direction.This equation was implemented through a at initialization and allowed to grow linearly to the scheme advanced in time and space, as described by target value within one inertial period.The Palma & Matano (2000): experiments were run for lengths of time enough for allowing the progression of the plume along the entire extent of the spatial domain.The results were analyzed by observing the spatial evolution of the 34.9 where isohaline, as depicted in Figure 3. Within the first 20 rl= 0. 5(ý t ilY) (Vn+[ VnB[)   days of integration, the plume remained confined to SB B 1 the estuarine domain, except for the higher outflow r 2 =O.5(•tily)(Vn-lVn)   case (R50), when the 34.9 isohaline extended beyond =B B Punta del Este during that period.In all cases, after reaching the shelf region, the plume turned In these equations, S is the salinity, Ati is the anticlockwise, following the direction of propagation internal mode time step, Ay is the meridional space of coastally trapped waves.From that point on, the resolution, vB is the normal velocity component of the main difference among the three experiments was the boundary and S. is the prescribed external value, northward extension of the plume.In the R15 Temperature is treated in an identical fashion.On the experiment, after 80 days of simulation with the low eastern boundary, a simple no-gradient condition was river discharge the plume extremity reached latitude applied for temperature and salinity, since it was far 34 0 S, an extent of about 623 km.In the R25 and R50 enough to ensure no influence on the plume.
experiments, the distance from the plume extremity to the estuary head was 764 km and 1099 km, NUMERICAL EXPERIMENTS respectively.On day 140, the R15 plume had reached the latitude of Rio Grande (32 0 S), while the R25 In the real ocean, estuarine plume dynamics plume extended to the northern limit of Patos Lagoon are dictated by several factors.The river discharge (30°S).In R50, the plume reached 270S, past the Cape interacts with the surrounding water, generating of Santa Marta Grande.The alongshelf plume thermohaline gradients that drive the density penetration, as determined by the distance of the 34.9 circulation; the wind works by establishing sub-isohaline from the estuary head, is represented in inertial coastal currents; the tides enhance vertical experiments are more or less similar, suggesting an increase in river discharge.This can be seen in the first estuarine channel flux.When the plumes reach the derivative for the velocity curve, depicted by the solid distance of 400 km from the estuary head, their line in Figure .4b.An upshelf plume penetration velocities become notably dissimilar.This leads to an (southwestward) was also noticeable in these increased distance between their frontal positions.To experiments, achieving significant distances (more verify the behavior of the plume as a function of river than 250 kin) along Argentinean coast (Fig. 3).The discharge in a more continuous fashion, we ran distance of this intrusion was also proportional to the another set of experiments, changing the river out-flow river discharge magnitude.None of these plumes fully intensity by smaller increments.The average speed of achieves an alongshore stationary position of the propagation of the plume bead, as a function of river plume head during the simulations.However, with outflow, can be followed in the starred graph in Figure respect to offshore expansion, the plumes seem to 4b; the speed of propagation increases as discharge achieve a limit.A considerable change in plume crossincreases.In our main experiments, the estimated shore dimensions was noticed with increasing river values were 5.88 km.day t • (0.07 m.s-A) for Rl5, 7.84 discharge, but all cases eventually displayed a km.day t • t0.09 m.s-t ) for R25, and 11.35 km.day-t tendency toward a cessation of the offshore (0.13 m.s) for R50.We note that the rate of increase development.The differences in offshore extension of the velocity decreases with increased discharge, can be verified by plotting plume cross-shore width as which suggests that for higher outflow regimes there is a function of time along two cross-shore transects, one a weaker response (in terms of plume velocity) to an at the estuary mouth, the other off the latitude of Lagoa Mirim (transects J=34 and J=80, in Figure 5c)... km to R50.Along the southern transect (J = 34), the cross-shore expansion stabilizes at approximately I -* 360 km for RI5, 420 km for R25, and 500 km for R50. Figure 5a shows the surface salinity distribution for the experiment with average river discharge .In this second group of simulations, This region displays a complicated meandering initialization files from the river forcing experiments pattern, showing vortices with diameters of were used to start the runs.For this new set, we approximately 35 km and velocities of about 0.125 selected all three (Ri5, R25 and R50) discharge m.s 1, as indicated by transect J = 34 in Figure 6.The experiments.These new simulations started on day transition between this meandering field and the 130.The river discharge was kept constant since the coastal body is smooth, within 200 km north of the initialization, while the wind forcing was ramped up estuary axis.In this zone, a surface coastal jet within one inertial period.After initialization, the develops.At the core of the J = 80 transect, winds were kept constant in time and throughout the northeastward alongshore velocities up to 0.2 m.s-t are domain.The plume response to different alongshore found.This velocity decreases to the north, decaying wind directions was tested.Since the grid was rotated to less than 0.1 m.s-i across transect J=122.
40o with respect to the north, we assumed NE and SW Underneath the surface jet, a weak counter current is winds to be coincident with the domain's meridional observed near the bottom on transect J = 80.It is axis.The main wind experiments were forced with 8 mainly composed of slightly diluted near-bottom m.si1 wind speed, for both northeasterly and waters, and -0.025 m.s-i speed southwestward.In southwesterly cases.This corresponds to simulations RI5 and R50 the circulation followed ityI0-4 xm 2 .s-2, where T, is the wind stress divided by almost the same general pattern, differing only in the water density.For convenience, we will refer to terms of magnitude, being 0.13 m.s-' and 0.25 m.s-' these experiments as R15NE, R25NE and RSONE, respectively, for surface velocities (across J = 80), and respectively, for low, medium and high discharge -0.023 m.si1 and -0.035 m.s-, respectively for bottom northeasterly wind simulations, and R15SW, R25SW velocities (J = 80).and R50SW, respectively, for low, medium and high discharge southwesterly wind simulations.In the high discharge experiment (R50SW) the head is southwesterly wind case, two other wind magnitudes located about 450 km farther upcoast, near 27 0 S. were used to force the model, referred to as weaker ( 6Ten days after southwesterly winds were m.s-I T =-5xl0-5m 2 .s-2)and stronger (15 applied, the plume in all three experiments extended m.s'-r-'T=-3x 10-4 m 2 .s-2 ) southwesterly winds approximately 240 km northeastward of their initial experiments.The T. component was set to zero in all positions, reaching Tramandai for R15SW, Santa simulations.
Marta Cape (28.3TS) for R25SW and 25.2'S for R50SW (near Paranagud).A significant modification Southwesterly Wind Experiments in the offshore expansion of the plume's bulge was noted, with a contraction of the low salinity region In this set of experiments, we applied the so-towards the estuary.After 20 days, the high discharge called downwelling winds over low, medium and high plume shifted about 275 km farther up-coast and discharge plumes for 50 days.We will first describe reaches the middle portion of the South Brazil Bight the evolution of the 34.9 surface isohaline along the at 23.8'S.In R25SW, the plume head was located coast as a function of time for the main wind at 26.2'S, 450 kIn downcoast, and on RI5SW it was experiment (Fig. 7).We will then focus on the found at the latitude of Santa Marta Cape.After 30 differences imposed by wind magnitude.As an initial days of southwesterly wind forcing, the plume in the condition, the plume in the low discharge experiment R25SW experiment entered SBB, east of Paranagud, (R15SW) extends 850 km from the estuary to Rio while'on R50SW experiment the plume occupied Grande (32'S); in the average discharge experiment almost all SBB domain.The plume head reached (R25SW) the plume head is near Tramandai (30°S), the middle of the bight in R25SW and R15SW, approximately 1100 km from the mouth; and in the after 40 and 50 days of simulation respectively.These .The contours depict the 34.9 isohaline position with 5-day increments.
differences in plume extension with and without winds are summarized in Figure 8.After day 130, the application of downwelling winds led to a substantial modification of the plume's propagation speed for all river discharge conditions, with an average velocity of 27.16 km.day-(0.31m.s-I, about 2 to 4.5 times faster than the river only cases).In an attempt to evaluate the influence of wind strength in alongshore velocity of the plume, two additional downwelling experiments were performed for all river outflows, with stronger and weaker southwesterly winds (15 m.s and 6 R25SW an 6 : f"t-40 days of Ilnd m.s-l).The plume propagation speed was 21.93 kmday-1 (0.25 ms-) and 42.22 km.day-t (0.49 m.s-n), respectively, for the weaker and stronger wind experiments.A summary of the results in all southwesterly cases is shown in Table 1.The plume propagation speed was independent of the river outflow.In Figure 4a we observe the along coast •s evolution in these two additional wind experiments for an average river outflow magnitude.The dashed line represents stronger (denoted by the numeral I) and weaker (denoted by II) wind cases.Since the propagation speed was independent of river outflow magnitude, similar paths for the other experiments can Ow Z3 daow be found by drawing parallel lines from the end of the simulation forced only with river outflow.A surface tow" salinity field, when the plume reaches the S. Sebastigo Fig. 8. Surface salinity fields for downwellinglatitude, is presented in Figure 8 for the main wind favorable wind experiments with average discharge experiments (8 m.s-1 southerly winds), where R25SW outflow after 40 days of wind forcing (top panel) and displays its 40th day of simulation (top panel) while extreme river discharge after 25 days of wind (bottom i it panel).Salinity is depicted by the contours of 10, 20, R50SW is in its 250 day (bottom panel).Both plumes 30, 32, 33, 34 and 34.9 PSU as well by the shade extended more than 2000 km from the Plata estuary.
ranging from fresh (black) to salt (white) waters.Although representing different situations, these experiments were very similar in a general way.In its central part, between the Uruguay coast and Florian6polis, the low salinity band was composed mainly by 20 to 30 PSU waters that displayed crossshore salinity gradients.Salinities below 20 PSU remained mostly confined to the estuary, whereas higher alongshore salinity gradients were observed --------near the head of the plume.Small low salinity pools I were frequently detached at the estuary and near the Uruguay coast in these simulations.Other " sotwestefreunly wids.aheatthe icnedsoniuary atind ofrte:' downwelling wind experiments produced only slightly II different surface-salinity configurations.In Figure 9 -|--we observe R25SW salinity transects after 40 days of southwesterly winds.The inclined configuration:of isohalines observed in the outflow only experiment was replaced by a cross-shore salinity gradient, with a .lack of vertical salinity stratification (see vertical ---....... isohalines in transects J=80,J=122 and J=202).The plume widened beyond the 60 mn isobath.At the estuary domain (J=34) there was a retraction of the plume body and an enhancement of the salinity gradients.This configuration was different from the I .outflow only simulations, where the front occupied the [.whole estuary.In transects J=34 and J=80, we observe surface buoyant pools detached from the plume's main Fig. 9. Salinity transects for R25SW after 40 days of body, with a length scale of 30 km.
This rapid extension of the low-salinity forcing.A definite barotropic signal is found on the signal along the coast is a result of advection promoted continental shelf, nearly bounded by the 250-300 m by a barotropic flux over the shelf The downwelling isobaths.This current started on the southern wind induced a surface Ekman transport towards the boundary, traveled through the north Argentinian, coast, accumulating water in the nearshore region.
Uruguayan, and Brazilian shelves.A greater This was enhanced near the estuarine zone, where intensification occurred on transects J = 80 and J = higher surface elevations were observed against the 122 for south Brazilian shelf Currents achieved Uruguayan coast, achieving magnitudes of 0.25 m, velocities of 0.8 m.s-1 and 0.7 m.s-', respectively, in 0.40 m and 0.8 m, respectively, for the weak, medium the core, but mostly reached the average value of 0.1and strong wind cases.Along the southern Brazilian 0.3 m.s-' on middle and outer shelves.Wider currents coast, the elevation varied between 0.15 and 0.5 m, in the northern and southern shelf transects achieved while along the SBB the largest elevation did not 0.45 m.s-' and 0.65 m.s-, respectively, in their core.exceed 0.45 m for the stronger wind case.The Transect J=202 had the smallest alongshore velocity elevation pattern followed closely the depths over this gradient across the shelf In other wind experiments, region for all cases, revealing the confinement of the currents achieved maximum values of 1.0 m.s-1 and flux on the shelf 0.55 m.s-1, respectively, for the weak and strong wind This piling up of water creates a positive cases.In the estuarine zone, the current followed the cross-shore pressure gradient which is governed by bathymetry, making a wide turn and entering the Coriolis adjustment producing a strong and well region immediately outside of the Plata estuary.This defined northeastward current.In Figure 10 we current shear was responsible for the detachment and observe the alongshore velocity component for four advection of small low-salinity pools (Fig. 9, J=34).transects in the domain after 20 days of SW wind Northeasterly Wind Experiments plume detachment continued, and at the end of the Northeasterly (upwelling favorable) winds simulation its body was completely separated from the were applied over developed plumes as in the previous coast.The plume offshore drift may be seen by the experiments, but for 6 m.s-t and 8 m.s-t wind values, evolution of the low-salinity signal in the estuary The model was run for 20 days.The surface-salinity mouth and Lagoa Mirim salinity transects on R25NE fields for days 5 and 10 are shown in Figure 11 for (Fig. 4c) for both wind magnitudes.The plume grew average and high outflow cases under 8 m.s7' NE offshore in an approximately linear fashion, doubling winds.The plume responded to wind forcing within a its width on the northern transect (J=80) after 10 days few days with a significant offshore spread.Within 5 of wind forcing.The average growth was similar for days, a considerable offshore dislocation was other river outflows on the estuarine transect: 8 produced.For both the river discharge cases the km.day-1 for 6 m.s-1 winds (dashed lines with plumes extended 45 km offshore.After 10 days of numerals III and IV) and near 15 km.day-1 for 8 m.s-i wind forcing, the coastal plume achieved almost twice winds (continuous lines).The velocities were the same its original cross-shore width.The plume separation for the high outflow case on J=80, although for the from the coast increased at the plume nose and on the average and lower outflow cases they diminished after southern Uruguayan shelf For the high discharge case, some time (i.e., once the plume nose started to get the separation also occurred on southern Brazilian closer to the transect position).In terms of along coast coast east of Rio Grande.On day 15 (not shown) the retraction, the modification is clear in all NE wind Fi.  [ by -0.20 and -0.30 m on the south Brazilian shelf The currents were about -0.35 m.s-1 for stronger winds (8 m.s-'), and -0.20 m.s-1 for weaker winds (6 m.s-1).
The barotropic current follows the bathymetry southwestward, and flows through the southern boundary.Northeasterly winds led to significant and rapid extension of low salinity water along the coast south of the estuary.

South
Fed by the second largest river system of South America with an average river runoff of 23x10 3 m 3 .s-'the Plata shelf plume's dimension .. surpass the internal Rossby radius and therefore is " subject to earth rotation effects.The funnel shape of the estuary provides a gradual change between an "estuarine channel and the inner shelf, with an increasing Kelvin number (K = b/ri, where b=inlet breadth; ri= internal Rossby radius of deformation on the shelf) (Garvine, 1999).We observed distinctive circulation types along the estuarine axis that agree with the regime differentiation found in the numerical . ,,0 ,studies of Chao & Boicourt (1986).These features ' ,,,, were also qualitatively consistent to observations of Mtinchow & Garvine (1993) for the Delaware plume, Fig. 12. Salinity transects for R25NE after 15 days of wind despite the differences in the spatial scale compared to forcing. the Plata.Inside the estuarine channel (first 200 kin), we found velocities consistently higher than those over current, this retraction could reach approximately -18 the shelf After leaving this "channel regime", the km.day-i for 8 m.sit winds and -12 km.day-i for 6 water flowed out of the estuary and traveled up-coast, m.s-1 winds, on average and high outflow cases during but a considerable amount of energy seems to be the first 10 days.But once these plume bodies accumulated near the estuary mouth, forming a large departed from the coast and from the shelf current, pool of mixed water.A "near flow field" characterized their southward rate of propagation was diminished, the anticyclonic surface flow and the formation of the For example, we can follow the alongshore path estuarine bulge.The "far field" was characterized by evolution measured by the model for the average river the plume alongshelf intrusion.discharge case under 8 m.s-1 Winds (Fig. 4a, depicted Near/far field separation can be as R25NE).The difference between river outflow-only characterized by. a strong cyclonic surface flow and and the northeastward experiment regimes was noticed downwelling (Chao & Boicourt, 1986; Oey & Mellor, by the curve break on this graph.About 10 days after 1993), and with a modification from a nearly inertial northeasterly winds were applied the main plume body regime in the turning region, to a nearly geostrophic was detached 100 km from the coast (Figs 1 lb and regime in the coastal current itself (Garvine, 1987; I ld), and only a narrow low-salinity strip remained Miinchow & Garvine, 1993).Although no sharp attached to the coast (Fig. 12).The main plume body distinction was observed in our simulations, the was 10 to 15 m thick, while the coastal strip extended differentiation between near and far fields was throughout the water column between the coast and apparent in river only experiments.In spite of this the 10 m isobath.In the estuary domain the regime differentiation, the bulge surface area was northeasterly winds dislocated the plume offshore in related to outflow magnitude, and a meandering the first 5 days.After that, a southwestward current pattern was a common feature in this location.developed and reversed the plume.As exemplified by The presence of upshelf intrusions or 10 and 20 isohalines (Fig. 11), it changed its natural massive bulges such as those observed in our up-coast propagation for a down-coast dislocation, simulations are usually absent in field observations This shelf current occurred as a result of the (Garvine, 2001).One exception is the upshelf geostrophic balance between the negative pressure intrusion observed of the Changjiang (Yangtze) river gradient (developed by the offshore surface Ekman plume, as described by (Beardsley et al., 1985, in transport) and the Coriolis force.After 20 days, for Garvine, 2001).Fong & Geyer (2002) attributed these both weaker and stronger winds, the near-coast water differences to the effect of ambient shelf currents, level decreased by -0.20 and -0.55 in at the estuary and which would be essential in the determination of the offshore plume dislocation were comparable to An important part of the shelf currents findings of Houghton et al. (2004) for Delaware variance along the Southeastern America continental plume.The detachment from the coast agrees well shelf is known to be attributed to the winds (Soares & with their observations and with the conceptual model M6ller, 2001;Zavialov et al., 2002;Pimenta et al., proposed by Fong & Geyer (2001).The persistence of 2004).Cold fronts are responsible for strong winds with a strong upwelling component are likely to southwesterly winds and for the generation of sea level be an effective mechanism for dissipating the Plata oscillations that travels equatorward along the SBS plume and transferring shelf waters to the ocean.
and SBB in the form of continental shelf waves (Stech • Kourafalou et al. (1996a) have shown how upwelling & Lorenzzetti, 1992;Castro & Lee, 1995).Unusual winds work on the exchange of materials from the strong events might occur.Melo et al. (2003) describe continental shelf and Piola (2002) suggested this an cyclone generated at Uruguay as responsible for transfer of light water pools to the Brazil Malvinas raising sea level about 1 m and for generation of fairly Confluence as an effective and irreversible process of strong northward currents (1.6 m.s-') at the Santa water mass exchange.Near the coast, simultaneously Catarina shelf Indirect drift measurements registered to the plume detachment occurred the upwelling of around 27.7'S have also revealed persistent northward subsurface waters.This process seemed to be currents for 10 to almost 20 days in early May and enhanced off the Uruguayan coast, where the effect of June 2001 (Pimenta et al., 2004).Assireu et al. (2003) winds upon the shelf currents was also capable of suggest that the drifter that accompanied the cold reversing the river plume direction.This result was water intrusion of 1993 winter (described in Campos consistent with Mianzan et al., (2001) observations in et al;1996a) was also driven by continental shelf the summer of 2000.In a period of anomalously low waves.Plata outflow (12x 10 3 m 3 s-) and strong northeasterly Rather than being the result of continuous winds, high salinity waters (> 34) and Southern wind forcing, plume intrusions might result from the Brazilian marine biota (plankton and fishes) were passage of a sequence of cold fronts.Rennie et al. observed in the outer Plata estuary.
(1999) have demonstrated the effect of winds in the In general, our results suggest that the creation of a pulsed buoyancy current downstream of distribution of low-salinity waters over the shelf is Chesapeake Bay & Hickey et al. (1998) have shown much more sensitive to wind direction than to the river sensitive responses of the Columbia plume to changes outflow variability.Alongshore winds seem to in wind speed and direction in the time scale of 2-10 determine the main role on the Plata plume alongshore days.growth or destruction.The effect of cross-shore wind Since the Plata outflow was normal during components, as investigated in Pimenta ( 2001) have the winter of 1993, the observations of anomalously shown smaller importance on the plume control.Once cold, low salinity waters are probably attributed to intrusion velocities did not depend on the river southwesterly winds.Studies demonstrated up to a discharge magnitude but mostly on the downwelling 50% increase in the formation of cyclones over wind intensity, the possible intrusion of cold and fresh Uruguay and San Matias Gulf (Argentina) during waters up to the SBB seems to be linked to three main ENSO events (Satyamurty et al., 1990; Gan & Rao, factors: the initial alongshelf plume position (that 1991).This conclusion is in agreement with recent might be determined by the river outflow), the analysis of historical hydrographic data and winds, intensity, and the persistence of the southwesterly which suggest that the winter 1993 southwesterlies wind events.Our simulations have show that the time were among the strongest since 1949 (Piola et al., scale necessary to advect the Plata waters to SBB by 2005).Extreme events may arise from certain steady wind forcing and average river discharge, combinations of alongshore winds and outflow.varied between 25 and 49 days, depending on the wind Extreme northeastward penetrations of the Plata plume strength (Table 1).When constant, strong will be associated with large outflow and strong, downwelling winds blow over a plume, with spatial sustained southeasterly winds, while extreme plume extensions similar to the mean winter field described retractions will occur under low outflow and by Piola et al. (2000), this time scale can be even northeasterly winds.reduced to 15 days.
Fig. I. a) Sea surface temperature (SST) derived from AVHRR/NOAA for July 1993.The dark colors indicate cold waters coming from the southwesi into the area.b) Surface salinity field derived from COROAS-WOCE hydrographic 1993 winter cruise (Modified from Campos e1 at, 1999).

Fig. 2
Fig. 2: a) Orthogonal rotated bathymetric domain, extending from south of Rio de la Plata estuary to Cabo Frio.b) Zoom at estuarine zone.c) Estuarine cross-section and sigma layers.

.•
The result is shown in Figure4c.On the northern i,-transect (J = 80), the 34.9 isohaline distance from the -. a .coast reaches 60 km for R15, 70 km for R25, and 110 . That figure reveals the freshening of a region __ .. ....... _______"__,_____ extending from the northern Argentinian coast to Ib .southern Brazil, in the form of a low salinity band.The main difference between this spatial distribution .the bottom (5b) and cross sections along three cross-shelf transects (5c).In C spite of the wider cross-shore expansion of the plume .... .......at the ocean surface, the salinity field near the" bottom was roughly confined by the 40-meter .R50 (not shown here).The .. vertical distribution was stratified with positive slope of the salinity isopleths, as seen in the three Fig.4.a) Alongshelf plume penetration from the estuary head transects J=34, J=80 and J=122 (Fig.6c).The as a function of time for all numerical experiments.b) estuarine salinity configuration is evident along J = 34Average velocity of plume propagation as function of river where the 34.9 isohaline extends 450 kin from the discharge only (denoted by the stars).The thin dashed line estuary head.To the north of the estuary mouth, the represents a regression to the data in the form of a power isohaline configuration is consistent with an curve derivative, (c) Cross-shore plume width for river only experiments and simulations under upwellinggeostrophic adjustment.favorablewinds (the cross-sections are indicated on Figure Near the estuary head, the flow is nearly 6).straight, following the main channel.The flow regime changes when low-salinity water reaches the Experiments Forced With Wind and River Discharge sea and starts to accumulate, forming a large bulge.

Fig. 5 :Fig. 6 .
Fig.5: Surface (a) and bottom (b) salinity fields for R25 river only experiment after 140 days of simulation.e) salinity transects for the same experiment The 40 m isobath is depicted by a dashed line on "b".Salinity is depicted by the contours 10, 20, 30, 32, 33, 34 and 34.9 PSU, as well by a shade from fresh (black) to salt (white) waters.

Fig. 11 .
Fig. 11.Surface salinity field after 5 and 10 days under upwelling-favorable wind forcing with average (a,b) and extreme outflow(c,d).The dashed line represents the initial 34.9 isohaline position.

Table 1 .
Necessary time under downwelling winds for the plume to reach S. Sebasti~o (t,) and alongshore average speed of propagation (Vp) under different wind and river magnitudes.