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The salinity signature of the cross-shelf exchanges in the Southwestern Atlantic Ocean: Numerical simulations.

Matano RP, Combes V, Piola AR, Guerrero R, Palma ED, Ted Strub P, James C, Fenco H, Chao Y, Saraceno M - J Geophys Res Oceans (2014)

Bottom Line: Dynamical analysis reveals that the cross-shelf flow has a dominant barotropic structure and, therefore, the SSS anomalies detected by Aquarius represent net mass exchanges between the shelf and the deep ocean.The net cross-shelf volume flux is 1.21 Sv.This outflow is largely compensated by an inflow from the Patagonian shelf.

View Article: PubMed Central - PubMed

Affiliation: College of Earth, Ocean and Atmospheric Sciences, Oregon State University Corvallis, Oregon, USA.

ABSTRACT

A high-resolution model is used to characterize the dominant patterns of sea surface salinity (SSS) variability generated by the freshwater discharges of the Rio de la Plata (RdlP) and the Patos/Mirim Lagoon in the southwestern Atlantic region. We identify three dominant modes of SSS variability. The first two, which have been discussed in previous studies, represent the seasonal and the interannual variations of the freshwater plumes over the continental shelf. The third mode of SSS variability, which has not been discussed hitherto, represents the salinity exchanges between the shelf and the deep ocean. A diagnostic study using floats and passive tracers identifies the pathways taken by the freshwater plumes. During the austral winter (JJA), the plumes leave the shelf region north of the BMC. During the austral summer (DJF), the plumes are entrained more directly into the BMC. A sensitivity study indicates that the high-frequency component of the wind stress forcing controls the vertical structure of the plumes while the low-frequency component of the wind stress forcing and the interannual variations of the RdlP discharge controls the horizontal structure of the plumes. Dynamical analysis reveals that the cross-shelf flow has a dominant barotropic structure and, therefore, the SSS anomalies detected by Aquarius represent net mass exchanges between the shelf and the deep ocean. The net cross-shelf volume flux is 1.21 Sv. This outflow is largely compensated by an inflow from the Patagonian shelf.

No MeSH data available.


Related in: MedlinePlus

EOF modes of SSS variability (a) first mode, the red line in the time series represents the alongshelf component of the wind stress; (b) second mode, the blue line in the time series represents the LPR discharge; (c) third mode, the green line in the time series represents the latitudinal variations of the BMC. The scale for the BMC location is shown in Figure 4a.
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fig03: EOF modes of SSS variability (a) first mode, the red line in the time series represents the alongshelf component of the wind stress; (b) second mode, the blue line in the time series represents the LPR discharge; (c) third mode, the green line in the time series represents the latitudinal variations of the BMC. The scale for the BMC location is shown in Figure 4a.

Mentions: To further characterize the SSS variability of the southwestern Atlantic region, we computed the Empirical Orthogonal Functions (EOFs) of the time series of the SSS anomalies (Figure 3). A Monte Carlo simulation indicates that the first three EOF modes are statistically significant at the 95% level [Preisendorfer, 1988]. These modes account for approximately half (52%) of the total SSS variance. The first EOF, which explains 30% of the variance, represents seasonal oscillations of the freshwater plumes along the shelf (Figure 3a). The spatial structure of this mode is characterized by a dipole structure, with lobes of opposing sign in the downstream and upstream regions. The upstream lobe, which extends from the mouth of the RdlP to 37°S, has a peak in the Samborombom Bay, which is a shallow (h < 20 m) indentation of the coast that is characterized by its large SSS variations [Möller et al., 2008, G14]. The downstream lobe extends from the mouth of the RdlP until Cape Santa Marta (∼28°S). The time series of the first EOF is dominated by seasonal oscillations that are correlated (r = 0.51, with a maximum correlation lag of 20 days, wind lead SSS, significant at the 99% level), with the seasonal oscillations of the alongshelf winds (Figure 3a). During the summer, northeasterly winds help to move salty waters from the north Brazilian shelf toward the downstream region and fresh RdlP waters toward the upstream region creating the dipole shown in the first EOF. During the winter, winds reverse direction and move saltier waters from the Patagonian shelf into the upstream region and relatively freshwaters from the RdlP into the downstream region reversing the sign of the first EOF. The lack of strong seasonal variations of the RdlP discharge contributes to the dominance of the wind forcing on the seasonal variations of its plume (the discharge from the Patos Lagoon is kept constant in our simulation). The discrepancies between the wind and the EOF time series are mostly related to the fact that the former includes strong intraannual and interannual variations while the later does not.


The salinity signature of the cross-shelf exchanges in the Southwestern Atlantic Ocean: Numerical simulations.

Matano RP, Combes V, Piola AR, Guerrero R, Palma ED, Ted Strub P, James C, Fenco H, Chao Y, Saraceno M - J Geophys Res Oceans (2014)

EOF modes of SSS variability (a) first mode, the red line in the time series represents the alongshelf component of the wind stress; (b) second mode, the blue line in the time series represents the LPR discharge; (c) third mode, the green line in the time series represents the latitudinal variations of the BMC. The scale for the BMC location is shown in Figure 4a.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4508930&req=5

fig03: EOF modes of SSS variability (a) first mode, the red line in the time series represents the alongshelf component of the wind stress; (b) second mode, the blue line in the time series represents the LPR discharge; (c) third mode, the green line in the time series represents the latitudinal variations of the BMC. The scale for the BMC location is shown in Figure 4a.
Mentions: To further characterize the SSS variability of the southwestern Atlantic region, we computed the Empirical Orthogonal Functions (EOFs) of the time series of the SSS anomalies (Figure 3). A Monte Carlo simulation indicates that the first three EOF modes are statistically significant at the 95% level [Preisendorfer, 1988]. These modes account for approximately half (52%) of the total SSS variance. The first EOF, which explains 30% of the variance, represents seasonal oscillations of the freshwater plumes along the shelf (Figure 3a). The spatial structure of this mode is characterized by a dipole structure, with lobes of opposing sign in the downstream and upstream regions. The upstream lobe, which extends from the mouth of the RdlP to 37°S, has a peak in the Samborombom Bay, which is a shallow (h < 20 m) indentation of the coast that is characterized by its large SSS variations [Möller et al., 2008, G14]. The downstream lobe extends from the mouth of the RdlP until Cape Santa Marta (∼28°S). The time series of the first EOF is dominated by seasonal oscillations that are correlated (r = 0.51, with a maximum correlation lag of 20 days, wind lead SSS, significant at the 99% level), with the seasonal oscillations of the alongshelf winds (Figure 3a). During the summer, northeasterly winds help to move salty waters from the north Brazilian shelf toward the downstream region and fresh RdlP waters toward the upstream region creating the dipole shown in the first EOF. During the winter, winds reverse direction and move saltier waters from the Patagonian shelf into the upstream region and relatively freshwaters from the RdlP into the downstream region reversing the sign of the first EOF. The lack of strong seasonal variations of the RdlP discharge contributes to the dominance of the wind forcing on the seasonal variations of its plume (the discharge from the Patos Lagoon is kept constant in our simulation). The discrepancies between the wind and the EOF time series are mostly related to the fact that the former includes strong intraannual and interannual variations while the later does not.

Bottom Line: Dynamical analysis reveals that the cross-shelf flow has a dominant barotropic structure and, therefore, the SSS anomalies detected by Aquarius represent net mass exchanges between the shelf and the deep ocean.The net cross-shelf volume flux is 1.21 Sv.This outflow is largely compensated by an inflow from the Patagonian shelf.

View Article: PubMed Central - PubMed

Affiliation: College of Earth, Ocean and Atmospheric Sciences, Oregon State University Corvallis, Oregon, USA.

ABSTRACT

A high-resolution model is used to characterize the dominant patterns of sea surface salinity (SSS) variability generated by the freshwater discharges of the Rio de la Plata (RdlP) and the Patos/Mirim Lagoon in the southwestern Atlantic region. We identify three dominant modes of SSS variability. The first two, which have been discussed in previous studies, represent the seasonal and the interannual variations of the freshwater plumes over the continental shelf. The third mode of SSS variability, which has not been discussed hitherto, represents the salinity exchanges between the shelf and the deep ocean. A diagnostic study using floats and passive tracers identifies the pathways taken by the freshwater plumes. During the austral winter (JJA), the plumes leave the shelf region north of the BMC. During the austral summer (DJF), the plumes are entrained more directly into the BMC. A sensitivity study indicates that the high-frequency component of the wind stress forcing controls the vertical structure of the plumes while the low-frequency component of the wind stress forcing and the interannual variations of the RdlP discharge controls the horizontal structure of the plumes. Dynamical analysis reveals that the cross-shelf flow has a dominant barotropic structure and, therefore, the SSS anomalies detected by Aquarius represent net mass exchanges between the shelf and the deep ocean. The net cross-shelf volume flux is 1.21 Sv. This outflow is largely compensated by an inflow from the Patagonian shelf.

No MeSH data available.


Related in: MedlinePlus