Double-celled subtropical gyre in the South Atlantic Ocean: Means, trends, and interannual changes

摘要

We present here a study of the mean upper layer structure of the South Atlantic Subtropical Gyre (SASG) and its modes of variability based on Archiving, Validation, and Interpretation of Satellite Oceanographic data sea level anomaly data, Gravity Recovery and Climate Experiment-based mean dynamic topography (MDT) models, Reynolds sea surface temperatures, and two tide gauge time series from Permanent Service for Mean Sea Level. Analysis of the MDTs shows that the SASG mean structure is represented by a double-cell circulation, the southern cell being a narrow quasi-zonal strip centered around 33°S. Its southern limb is limited by the recently discovered Saint Helena Current/Tristan da Cunha Current (StHC/TCC) and the North Subtropical Front around 35°S. This system is shown to be well separated from the South Atlantic Current/South Subtropical Front around 42-45°S. A method based on singular spectrum analysis was used to determine the principal modes of variability through the decomposition of the data into band-passed data sets. The altimeter sea level trend analysis suggests that the SASG pattern is slowly migrating southward and has been growing in amplitude between 1994 and 2006 with local growth rates of 5-10 mm/yr occurring in the southern cell. Flow intensifications in this period (2-3 cm/s in the South Equatorial Current and North Brazil Undercurrent, 3.5 cm/s in the Benguela Current), and weakening by 4.5 cm/s in the StHC/TCC are recorded. The altimeter-derived sea level trend is coherent with the growing phase of a bidecadal oscillation found from both tide gauges.