Brazil Current System: a proposal for using XBT Data in its dynamical investigation

Author(s)

Leandro Ponsoni

• Document type: Master's Dissertation
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Instituto Oceanográfico (IO/DIDC)
• Defense date: 2010-10-21

Examining board members

Ilson Carlos Almeida da Silveira; Luiz Bruner de Miranda; Alberto Ricardo Piola

• Advisor: Ilson Carlos Almeida da Silveira

Abstract

The Brazil Current (BC) and the Intermediate Western Boundary Current (IWBC) between 20°S and 28°S are perhaps two of the less studied subtropical boundary currents of the world ocean. Within this region, the BC develops vigorous meanders and rings. A combination of numerical simulations and observational studies are important tools to unravel these phenomena. Direct current measurements are rare and usually too short to depict the mean, long term circulation patterns. Similarly, quasi-synoptic hydrographic and ADCP data in the region are sparse. On the other hand, XBT data archived by the Brazilian Navy are abundant. Here we investigate how XBT temperature (T) profiles may improve the study region climatology and be used to generate initialization fields for numerical models. A method for constructing synthetic salinity (S) profiles is proposed considering the complex shape of the TS curve of the area, the shape of the in situ temperature profile and surface salinity. More specifically, surface salinity and nondimensionalized in situ temperature profiles are used for reconstructing salinity in the upper 200 m of the water column for locations with local depth greater than 1899 m. The mean nondimensionalized climatological salinity profile is used for the depth interval between 900 and 1300 m. The nondimensionalized profile is then redimensionalized using values estimated by the climatological TS relationship in its extrema. The salinity vertical profile of the remaining portions of the water column by linear fitting S to T each 25 m interval. It is also necessary to vertically extrapolate the temperature profiles from 1800 m (maximum depth reached by the XBT probes) to the ocean bottom. We propose using the nondimensionalized climatological T profile and the values of the temperature sampled at 1800 m as well as the bottom climatological temperature to redimensionalize it and extend the temperature vertical structure to the local depth. Geostrophic calculations using data from recent quasi-synoptic hydrographic surveys in the area are employed to test the methodology. A comparison between the currently proposed method and the geostrophic velocities calculated by other techniques is conducted. This comparison also includes geostrophic velocity estimates considering: constant salinity (S=35), an adaptation of classical techniques based solely on the TS curve shape, and by direct relating specific volume anomaly and temperature. The method proposed in this work is the one which best reproduces the current vertical structure and current volume transport in mean terms among all tested. In particular, the BC volume transport is virtually identical to the values obtained with the in situ fields. (AU)