Interannual variability of the tropical Indian Ocean mixed layer depth
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Interannual fluctuations of the mixed layer depth (MLD) in the tropical Indian Ocean are investigated from a long-term (1960-2007) eddy permitting numerical simulation and a new observational dataset built from hydrographic in situ data including Argo data (1969-2008). Both datasets show similar interannual variability patterns in relation with known climate modes and reasonable phase agreement in key regions. Due to the scarcity of the observational dataset, we then largely rely on the model to describe the interannual MLD variations in more detail. MLD interannual variability is two to four times smaller than the seasonal cycle. A large fraction of MLD interannual variations is linked to large-scale climate modes, with the exception of coastal and subtropical regions where interannual signature of small-scale structures dominates. The Indian Ocean Dipole is responsible for most variations in the 10 degrees N-10 degrees S band, with positive phases being associated with a shallow MLD in the equatorial and south-eastern Indian Ocean and a deepening in the south-central Indian Ocean. The El Niño signature is rather weak, with moderate MLD shoaling in autumn in the eastern Arabian Sea. Stronger than usual monsoon jets are only associated with a very modest MLD deepening in the southern Arabian Sea in summer. Finally, positive Indian Ocean Subtropical Dipoles are associated with a MLD deepening between 15 and 30 degrees S. Buoyancy fluxes generally appear to dominate MLD interannual variations except for IOD-induced signals in the south-central Indian Ocean in autumn, where wind stirring and Ekman pumping dominate.
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