Intraseasonal and interannual zonal circulations over the Equatorial Indian Ocean
Résumé
El Niño Southern Oscillation (ENSO) and given phases of the Madden-Julian Oscillation (MJO) show similar regional signatures over the Equatorial Indian Ocean, consisting in an enhancement or reversing of the convective and dynamic zonal gradients between East Africa and the Maritime Continent of Indonesia. This study analyses how these two modes of variability add or cancel their effects at their respective timescales, through an investigation of the equatorial cellular circulations over the central Indian Ocean. Results show that (i) the wind shear between the lower and upper troposphere is related to marked regional rainfall anomalies and is embedded in larger-scale atmospheric configurations, involving the Southern Oscillation; (ii) the intraseasonal (30-60 days) and interannual (4-5 years) timescales are the most energetic frequencies that modulate these circulations, confirming the implication of the MJO and ENSO; (iii) extreme values of the Indian Ocean wind shear result from the combination of El Niño and the MJO phase enhancing atmospheric convection over Africa, or La Niña and the MJO phase associated with convective activity over the Maritime Continent. Consequences for regional rainfall anomalies over East Africa and Indonesia are then discussed.