Eastward propagating decadal temperature variability in the South Atlantic and Indian Oceans

Abstract

The origin and structure of eastward propagating decadal temperature variability in the South Atlantic and Indian Oceans are investigated by using long-term output of coupled general circulation model. Composite analysis during the warm Southwest Indian Ocean (SWIO) years shows that decadal temperature anomalies in the SWIO region originate in the South Atlantic and are associated with density anomalies. Since the density anomalies propagate at the speed of a few cm s−1, slower than the speed of the background eastward Antarctic Circumpolar Current, and exhibit a surface-intensified equivalent barotropic structure, the eastward propagation of the density anomalies may be attributed to quasi-stationary oceanic Rossby waves. The density anomalies are also accompanied with anomalous Ekman pumping, indicating an important contribution from the overlying atmospheric variability. The role of atmospheric variability is further examined by evaluating the mixed-layer heat balance. It is found that the warm temperature anomalies in the South Atlantic are due to anomalous entrainment and meridional advection. These results suggest that the atmospheric variability plays an important role through the ocean mixed layer in generating the eastward propagation of decadal temperature variability from the South Atlantic besides the internal ocean variability as suggested in previous studies.