Seasonal variations in the South Atlantic Meridional Overturning Circulation from observations and numerical models

Monthly climatologies of temperature and salinity from observations and numerical models are used to estimate the Atlantic Meridional Overturning Circulation (AMOC) at 34°S. Observational estimates suggest that the geostrophic transport plays an equal role to the Ekman transport in the AMOC seasonal variations at this latitude, whereas in the models, the Ekman transport controls the AMOC seasonality. The seasonality of the geostrophic transport from observations is largely controlled by the seasonal density variations at the western boundary, but in the models, the eastern boundary dominates. The observed density seasonality at the western boundary is linked to the intensity of the Malvinas Current, which is poorly reproduced in the models. The results indicate that the weak seasonal cycle in the model geostrophic transport can primarily be attributed to excessively strong baroclinicity below the surface mixed layer, whereas the observations show a strong vertical coherence in the velocity down to 1200 m.