Time series (2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018) of a global monthly objectively analyzed fields of temperature and salinity were used to investigate the South Atlantic subtropical mode water (SASTMW). It is characterized as homogeneous layers in the temperature range of 13 to 16 • C and with low potential vorticity. Three different types of SASTMW were distinguished allowing us to evaluate their temporal volumetric variation. Each type has its distinct spatial and temporal distribution patterns and consequently dissimilar volumetric variation. This variability is seen in the volume anomaly and turnover time of each type. To examine these differences we compared two rates: the water formation in a given temperature range due to heat flux and the mode water formation. The first (13.1±2.4 Sv) is higher than the second (10.2±2.0 Sv), showing that although heat flux is essential for the formation of SASTMW, there are other factors that influence each type differently. Plain Language SummaryThis article aimed to investigate the South Atlantic subtropical mode water (SASTMW) to better understand how the volume of this mode water varies over the years (from 2002 to 2018) and in space (the South Atlantic basin). It is defined as a layer with very low vertical temperature, in a specific range (13 to 16 • C), and density variation. Three different types of SASTMW can been seen. This allowed us to independently assess the volume and its modifications over time for each one and we saw that each type has specific temperature/density characteristics and changes of its volume over time. It is understood that the formation of mode water is directly related to the processes that occur between the atmosphere and the ocean. Thus, we investigate whether the different volume variations of each type are related to these interactions. As a first approach, we investigated two rates: the rate of the amount of water that is cooled or heated to the 13-16 • C range per month and the SASTMW formation rate. By the difference between these rates, we understand that despite the importance of heat flux for the existence of SASTMW, other factors influence each type in a different way.
We investigated the formation and evolution of the South Atlantic subtropical mode water using data from profiling conductivity, temperature, and depth sensors (CTD) deployed in April–May 2015 and from two customized Argo floats that drifted from April 2015 to June 2017. From the CTD data, we observed a mode water layer below the seasonal thermocline that deepened from the southern side of the area to the north. The two Argo floats remained in the proximity of the cruise area for 2 years. Their slow displacement and recirculating patterns allowed us to observe the changes in the temperature and salinity structure before and after the formation period. We observed that the potential vorticity of newly formed mode water was O[10−1 to 10−2] of the mean value found in the whole mode water layer. There is a significant correspondence between the phases of the time integral of surface heat fluxes and the sea surface temperature. Mode water is observed to form at the integrated heat flux minimum phase. The relationship between the air‐sea fluxes and sea surface temperature promotes the necessary preconditioning for the mode water formation. Once this was established, the outcropping of the mode water, that was at about 100 m depth, coincided with the passage of an atmospheric cold frontal system. This event suggests that the mode water formation can be triggered by the passage of cold fronts.
This work describes the circulation over the continental slope and the São Paulo Plateau in the Santos Basin during the SANSED winter 2019 survey. The cruise consisted of four legs in the period between June, 11 and August, 03 2019. The observed circulation is dominated by the Atlantic southwestern boundary current regime and remotely-generated anticyclones and cyclones. The former is composed by the Brazil Current, the Intermediate Western Boundary Current and their mesoscale meanders; the latter are 300km vortical rings with origin in the eastern side of the South Atlantic Basin. A Lagrangian scheme applied over satellite altimeter maps indicate that the origin of these rings is primarily the Cape Basin off South Africa. The interaction between the boundary currents, their cyclonic meanders, and the anticyclonic rings is complex, and varies widely. During the SANSED winter 2019 survey period, three anticyclones interacted with the Brazil Current, instabilizing it, forming dipoles with the current cyclonic meanders, leading to their downstream propagation. Ancienter cyclonic eddies within Santos Basin may interfere with the propagation of the large anticyclones further south. In addition, the continuous arrival of remotely-originated anticyclones, the larger portion over the São Paulo Plateau presented a tendency of counter-clockwise circulation during the whole cruise period.
The South Brazil Bight is a section of the Brazilian margin mainly dominated by the poleward Brazil Current flow, their meanderings and eddies. We evaluated the mean mesoscale for the region, and an anticyclonic feature was highlighted over the São Paulo Plateau. Around that feature, cyclonic eddies were also accentuated. The combination of these structures dominate the region, forming an eddy corridor. Using eddy detection dataset, we reveal that the signal on the plateau was directly related to the presence of anticyclones. The cyclones in the region present both local and remote origins, however, most of the anticyclones are from remote sources. More than 95% of these anticyclones were Agulhas Rings, which could or could not have been subjected to splitting or merging processes. On the plateau we observe an average of 5.3 anticyclones per year. However, these rate is related not only to the number of anticyclones but also to the time they remain there. We observe that Agulhas Rings reside in the region for 50.8 days, consequently, they occupy the plateau for almost 75% of the year. During half of the residence time, there is a multi-pattern interaction with cyclones. This relationship between eddies of opposite polarity creates a shielding process. The anticyclones become shielded and trapped by the cyclones, have their progress delayed, and their course deflected toward the Brazil Current. This was the first observation of this process involving the Agulhas Rings and the first study of the subsequent eddy-current interaction in the region.
Subtropical mode water is formed in winter-time deep mixed layer due to variations in air-sea processes. In the South Atlantic, three formation cores are identified between 30oS and 40oS: in the west, in the east, and north of the Subtropical Front. Each one of these three types presents typical mean thickness and horizontal distribution patterns, mainly because of local dynamic and thermodynamic characteristics of each part of the basin. In this study we assess the effects of momentum, freshwater and radiative fluxes on the variation in volume and composition of the South Atlantic Subtropical Mode Water (SASTMW). Sensitivity experiments were designed using the National Center for Atmospheric Research Community Earth System Model. Multiple one-year simulations are forced with varying intensity of wind, precipitation and shortwave radiation. By comparing to a control run, we were able to determine that the water volume variations in the east (SASTMW type 1) and south (SASTMW type 3) are significantly affected by precipitation and shortwave radiation, and thus are more sensitive to thermodynamic processes. On the other hand, SASTMW type 2 has a greater relationship with dynamic processes and is influenced by the Indian-Atlantic interbasin exchanges.
Analyzed air-sea Fluxes (OAFlux). The D f showed a mean value (5.26±1.47 Sv) lower than the DF rate (8.97±1.40 Sv), indicating the need to evaluate a broader set of processes that affect the SASTMW formation, in addition to the heat balance through the surface.
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