Inter-comparison studies between high-resolution HYCOM simulation and observational data: The South Atlantic and the Agulhas leakage system

Statistical analyses and model-data inter-comparisons are performed to evaluate the model's ability to reproduce the dynamics in the upper layers (< 2000 m) of the South Atlantic ocean. Outputs of an eddy-resolving Ocean General Circulation Model (OGCM) are analyzed and compared with observed data. The model, a 1/12-degree, 22-layer implementation of the Hybrid Coordinate Ocean Model (HYCOM) to the South Atlantic, was forced with monthly-mean products from the NCEP Reanalysis for the period 1960 to 2010. The numerical experiment was capable to reproduce the large scale and the mesoscale dynamic in the South Atlantic and in the Agulhas region. The vertical structure is in agreement with in situ data, the model has lower skill when compared with PIRATA lower temperatures, and is able to capture the seasonal and annual variability in the tropical Atlantic. Furthermore, sensitive change until 2007 is clear in the vertical structure, at 4°N-38°W; 0°-35°W and 10°S-10°W, suggesting an important change in the stratification. The primary results concern a significant change in the decadal anomalies of the temperatures and salinity, which exhibit a warmer and saltier water in the southeastern Atlantic. Furthermore, linear trends found in the transport time-series in the North Brazil Current, and the South Equatorial Current were seen to correspond with increasing trends of the warmer water from the Agulhas Current into South Atlantic. The integrated transport during the period 1960-2010, shows an increase in westward changes in the large-scale circulation south of Africa are show in the negative trends indicate a widening of the “Agulhas gap” and increase in westward volume transport since 1980. It is therefore suggested that variability in the Agulhas System on the last two decades is affecting the dynamic in the South Atlantic, namely the temperature and the volume transport, reach the tropical region of the Atlantic.