An assessment of Brazil Current surface velocity and associated transport near 22°S: XBT and altimetry data

Abstract

The use of Expendable Bathythermographs (XBTs) is widespread throughout the world’s oceans. 15 XBT high-density transects are located only in the Atlantic Ocean. However, the only transect that exclusively monitors the Brazil Current (BC) is the NOAA/AOML AX97 transect, running from Rio de Janeiro to Trindade Island. The BC is the Western Boundary Current linked to the circulation of the South Atlantic subtropical gyre. Surface velocity field based on the AX97 XBT data reference transect and two different altimetry datasets (AVISO and ATOBA) are compared with the objective of assessing the impact of different horizontal resolution altimetry products on averaged BC dynamical features. Furthermore, a sensibility study is performed considering different approaches to estimate BC volume transports. Data from 43 AX97 cruises, covering the period from Jan/2004 to Dec/2013 were used. AVISO (ATOBA) data are daily (weekly) and present a spatial resolution of 1/4° (1/12°). AX97 geostrophic velocity values were calculated based on XBT data using the isopycnal of σθ=26.8 kg/m3 as the reference level of no motion, whereas the altimetry surface geostrophic velocities were obtained based on maps of absolute dynamic topography. About 80.5% (74.4%) of the time, at least one altimetry dataset observed the BC core at AX97 region. ATOBA dataset presented a better performance if compared to AVISO when the BC is highly unstable. Also, in coastal areas, where the flow frequently changes direction, a high-resolution altimetry product (ATOBA) tends to represent the horizontal structure of the BC better than AVISO, where BC horizontal pattern is smoother. Finally, the merge between AX97 data and altimetry datasets presented some inconsistencies when surface currents are in opposite directions. It is recommended to combine AX97 data with another type of data acquired in deeper regions, where near-zero velocities will propagate a smaller error along the water column.