The rivers in the northern region (north of Florianópolis, Fig. 2a) have discharges by the end of the year (from September to March) about twice as large as in the drier months around July. The variability is, however, large: the first year, 2013, experienced some weeks of high discharge in June and July, reaching 2000 m3 s− 1, associated with peaks from river Iguape. In 2014, the highest peaks occurred in February, June, and December and were not registered in the other years (except for a small peak in July 2016). The main river is Iguape, with close to 60% of the discharge in this subregion, located about 350 km north of Florianópolis (following the coast). In this region, the wind blows mostly towards the coast, converging to the bay (SBB), being predominantly directed southward along the coast (Figs. 8a and 9). The number of episodes of equatorward alongshore wind is variable. While in 2014 and 2015 they occurred only a couple of times per year, in 2016, this was the predominant regime during the winter months, from May to September. Over the slope, the currents are strong and are associated with the BC (Figs. 8a and 9). The flow is predominantly southward with limited sensitivity to wind reversals. Many episodes of northward slope flow events are however not associated with the wind but probably with eddy activity known in the region (Campos et al. 2000; Oliveira et al. 2009). Over the shelf, the currents are much weaker and more correlated with the wind (Fig. 8a).
A core of low salinity, centered at the mouth of river Iguape is present along the year, with salinity values under 34 covering practically all the coastal region of the SBB (Fig. 9). The northernmost latitude reached by this salinity value occurs in the winter (July) when it extends until the vicinity of Ilhabela. Salinities on the range 35–36 cover most part of the shelf in autumn and winter (April and July), retreating to the inner shelf in the summer. This seasonal variability is not related to wind regime or riverine discharge, but with the seasonal variability of evaporation, as can be inferred from the variability of the sea surface temperature. Temperatures higher than 28 °C are found at the coast during the summer along the entire SBB. The sea surface temperature decreases in the following months reaching by July values over the shelf ranging from 20 °C to 22 °C, with an along coast gradient. The temperature starts increasing towards the year-end having already by October values above 23 °C in a great part of the SBB (north of 27°S).
Rivers’ plume occupies the inner shelf of the SBB along the year (Fig. 10, 1st column). In January, the plume has the greatest southward extension reaching latitudes south of Cape Santa Marta, occupying the central region in front of Florianópolis mostly in January, but also in October, between the isobaths 50 and 100 m. This southward spreading of the plume is consistent with the southward wind regime in the summer months. Due to the orientation of the Santa Catarina Island, the meridionally flowing freshwater increases the separation from the island towards the south. This separation also seems to be associated with the presence of the plume from the southern region (Fig. 10, 1st line—see 1st and 4st columns). Model results show no evidence of the plume from the northern region being able to reach coastal locations around Florianópolis, namely the bay of river Tijucas and the North and South Bays (the bays between the Santa Catarina Island and mainland).
The vertical transects of salinity, temperature, current speed, and dye concentration further clarifies the seasonal variability of the region (Fig. 11, 1st column). Salinity values above 36 are found at the shelf in January, under the plume waters at the inner shelf and reaching the surface at the middle shelf. This salinity band is displaced offshore in the following months, being almost absent from the shelf by July. The higher salinity values are transported southward by the BC on the shelf break with maxima speeds near the surface above 0.3 m s− 1. The temperature stratification at the shelf is higher in January (16–28 °C, with plume temperature above 24 °C) and is lower in July (16–22 °C), when the upper 70 m is well mixed (20–22 °C). Note that in Fig. 11 the temperature contours are shown mostly every 5 °C for clarity and this analysis is based on contours with higher resolution, not shown. In April (autumn), the equatorward current contour of 0.05 m s− 1 is visible. The plume is thinner in the summer (January) with a maximum depth of 25 m, while by July, it reaches the bottom at 45-m depth. This vertical variability of the plume depth is associated with the plume displacement southward during the summer. The offshore extent of the plume does not however vary significantly throughout the year. The vertical stratification of the plume offshore, thus, decreases in the winter season when the interaction with the bottom is higher. The plume is illustrated by the contours 0.005 and 0.02 of dye concentration, using the sum of all dyes. These contours follow very well the contours of salinity, i.e., in this region, the freshwater is highly correlated with dye concentration. Local river discharge is thus the main driver of buoyant waters in the inner shelf.
The rivers discharging in the central region have the largest peaks of the whole domain, reaching values higher than 4000 m3 s− 1, Fig. 2b (in reality this may not be true since about 95% of the discharge in the southern region, with a much higher mean discharge, is climatological, and its daily values may also have strong discharge events). This region is dominated by river Itajaí-Açu with 83% of the discharge. The months of higher discharge are variable but occur typically between June and October. The wind is more intense than in the northern region and has also stronger northward periods (Fig. 8b). While in 2013, the northward winds occurred in several periods until October, in the other years they took place mostly during the autumn. The stronger northward winds originate stronger northward currents at the inner shelf, much stronger than in the region north of Florianópolis. While still weaker than that at slope, the shelf currents here are more intense (than in the northern region) and in many periods comparable to the slope currents. The seasonal surface maps of wind and currents (Fig. 9, right panels) show the wind converging to the coast at Florianópolis during the spring and summer months (October and January), associated with strong southward alongshore currents. During the autumn, the wind turns from northeast to southeast and the shelf currents reverse, moving equatorward. In the winter (July), the average wind blows from the northwest with spatially variable intensity, resulting in also spatially variable, but weak, currents, mainly diverging from the coast, moving offshore and southward over the shelf.
Salinity (Fig. 9, left panels) shows very little seasonal variability, with values of 34–35 in the inner shelf, except in the vicinity of the rivers, and 35–36 in the outer shelf, year-round. Surface temperature oscillates 3 °C, from 19–20 °C in the winter to 22–23 °C in the summer. The plume of Itajaí-Açu (Fig. 10, 2nd column) has its greater extension northward in the winter (July, 25°S) and southward in October, reaching the latitudes of Cape Santa Marta one season before the plume from the rivers north of Florianópolis. The separation from Santa Catarina island, as the plume disperses southward, is also observed (analogously to the northern rivers’ plume). During the spring, the plume enters the bay of Tijucas, and by the summer, it enters the North Bay, though in a very small amount. January has the lowest area occupied by the plume, probably due to the strong inner shelf currents by year-end (Fig. 8b, ii) and also due to the river discharge smaller than during the winter. The other three rivers of the central region (Tijucas, Biguaçu, and Cubatão do Sul, Fig. 10, 3rd column) are very regional, with a total mean discharge of 93 m3 s− 1. Biguaçu, in the North Bay, and Cubatão do Sul, in the South Bay, with very low discharge (33 m3 s− 1 in average, together), have influence only locally in the bays they drain into. Tijucas, the largest of the three rivers (average discharge of 61 m3 s− 1), occupies the coastal region from the Bay of Tijucas to the north of Santa Catarina Island. In the winter, the currents off coast disperse the plume a few km until the Arvoredo reserve (see Fig. 1c).
The vertical transects (Fig. 11, 2st column), illustrating the variables west of Santa Catarina Island, show the small seasonal variability of the shelf and slope salinity. Near the island, the salinity is higher than in the northern region, in the range 34.5–35.5 in all the seasons except autumn (April) when it has values of 35–35.5. The shelf temperature ranges 14–28 °C in January, with plume temperature above 22 °C. This stratification decreases along the months, and in July, the temperature range becomes 14–22 °C (14–20 °C at the inner and middle shelf). The equatorward current (contour of 0.05 m s− 1) in April is also visible in this region. The dye contour 0.02 is practically absent all year. In this transect, the freshwater comes from the rivers of the northern region, from Itajaí-Açu, and from the southern region (possibly La Plata and the rivers included in the model south of Florianópolis). And indeed, from the maps of the spatial distribution of dyes (Fig. 10), April has the lowest presence of freshwater in the central region, with contributions from the southern rivers only. Being under the influence of rivers discharging away from Florianópolis justifies the higher salinity in this region, and also justifies the small interaction of the plume with the topography. On the other hand, the dye contour 0.005 does not match the contours of salinity as closely as in the northern region, which may be associated with air-sea fluxes as the plume sources are far away from the transect. It may be also associated with the presence of freshwater from sources not included in the domain, i.e., the La Plata river plume.
In the southern region, the highest discharge takes place from June to October, similarly to the central region. Eighty-six percent of the discharge in this region occurs in Lagoa dos Patos (rivers Guaíba, Camaquã and São Gonçalo), which is connected to the ocean about 600 km south of Florianópolis. The region has important spatial variability of the wind. The wind time series at the shelf by the north of Lagoa dos Patos (on the transect South1 in Fig. 1b) has the strongest southward regime of the whole domain and has few northward periods, except in 2016 with strong northward wind events between mid-April to September (Fig. 8). In front of Mirim Lagoon (South2), the southward wind is much weaker and the northward wind is much stronger and very common in the autumn, winter, and spring. At both locations, the inner shelf currents respond to the wind relaxation and inversion. As happens through the domain, the inner shelf currents are weaker than over the slope, where a couple of strong current reversion events (moving northward) are observed and not associated with northward wind events. They may be associated with the BMC influence in the origin of the BCC (Palma et al. 2008; Dalbosco 2019).
Freshwater with salinity values lower than 30 is found around the mouth of Lagoa dos Patos (Fig. 9). Low salinity waters are however present over the shelf in the south of the domain and in part provenient from the La Plata plume through the southern boundary. This low salinity band disperses northward during the winter, due to the northward wind regime, until the latitude of Cape Santa Marta. It then retracts southward in the summer months (e.g., Piola 2005; Soares et al. 2007b) until the mid-latitude of Lagoa dos Patos. South and in front of Lagoa dos Patos it occupies the whole shelf, narrowing north until the coast. Sea surface temperature also oscillates seasonally due to the different wind regimes together with the seasonally variable surface thermal fluxes. In July, waters colder than 16 °C extend north until close to 30°S. By October, the 16 °C isoline has already retracted to the latitudes of Mirim Lagoon (∼33°S). Temperature continues increasing during the summer to values of 23–26 °C in January. The shelf waters contrast with the warmer BC moving southward over the slope.
The plume of the southern rivers (Fig. 10, right panels) disperses north until the latitude of Florianópolis the whole year, but in greater volume during the winter season (July). The region in front of Tijucas Bay, the North Bay, and the South Bay of Florianópolis is however not affected by this plume. Also, in July, the plume has the smallest southward extension, reaching about 33°S, while during the other seasons it reaches the south model boundary. The plume offshore extension follows the upwelling/downwelling wind regime, with the plume occupying a large part of the shelf south of 30°S during spring and summer (upwelling regime), and confined to the inner and middle shelf during autumn and winter (downwelling regime). This seasonal response of river plumes on SB has been described in Soares and Möller (2001) and Soares et al. (2007b), which focused on plumes from Rio de la Plata and Lagoa dos Patos and its dispersion on the shelf south from 30°S.
The vertical transects (South1 and South2, 3rd and 4th columns in Fig. 11) show that while the seasonal variability of ambient salinity is small, the salinity and dye concentration near the coast are very variable in the southern region. At South1, waters with salinity values lower than 35.5 are more confined to the coast in the summer and expand offshore through the shelf during the winter. The dye concentration, however, does not follow this pattern. In January, the dye occupies the shelf (at the surface), which indicates the riverine waters moving south in the summer already lost part of their freshwater characteristics, probably because of spending many months away from the discharge location. On the other hand, during the winter the amount of fresh water is much higher than the amount of dye, showing the influence of the low salinity waters from La Plata moving north during the winter wind regime. The depth reached by the plume from local rivers (dye) shows low variability along the year, contrarily to its offshore extent, in opposition to what happens in the northern region. The temperature stratification varies between 16 and 26 °C in January (with plume waters of SB rivers above 22 °C) and 16 and 20 °C in July, when colder waters are present in the upper 50 m of the shelf (16–18 °C). The equatorward BCC is stronger in April, and absent in October.
At South2 (Fig. 11, 4th column), closer to the southern boundary, the difference between low salinity and dye concentration contours is even larger than that at South1. At these latitudes, water with salinity lower than 34 covers most of the shelf year-round but with a greater extent in the winter, associated with the northward transport of freshwater from the southern boundary. In July, however, the dye is almost absent at the latitudes of the South2 transect. In the summer, the extent of dye concentration is closer to the extent of the low salinity waters because of the freshwater at the shelf, originated from local rivers, moving south with the summer wind regime. The temperature stratification at the shelf ranges 15–24 °C in January (dye plume waters above 22 °C) and 14–16 °C in July in most of the shelf (15–18 °C at the outer shelf). Flowing along the 100 m isobath, the BCC is more intense in this transect.
La Plata river plume
A Lagrangian approach was used to assess the possible migration northward of waters near the southern boundary, under the influence of the La Plata river plume. Floats were continuously released over the shelf and slope at the south of the model domain and its position was followed through the entire simulation period. The result is shown in Fig. 12. The northward migration pattern is variable along the years, but the higher latitudes are consistently reached during late winter/early spring. In 2014 and 2015, the floats reached latitudes around 30°S, but in 2013 and 2016, they reached the region of Florianópolis (2013) and latitudes north of river Paranaguá (2016, ∼25.5°S). In 2013, Florianópolis was reached about 180 days after release, while in 2016, it was reached twice as fast, in about 90 days. The northernmost latitude in 2016 was reached in June, 100 days after release. In this year, the northward migration was very fast and until higher latitudes due to the strong northward wind in the autumn and winter. This means the southern waters spent 3 to 6 months migrating northward. The southernmost locations (i.e., the percentiles 90% and 95% of the northernmost floats) is reached by the summer months and the migration north restarts by the end of summer/early autumn, when the age of the particles is lower than 50–60 days. When the floats start migrating back southward, their age continues increasing and then decreases rapidly as the water mass moving south mixes with the southern waters, increasing the number of younger floats in the mean age percentile 90%.