Two Independent Triggers for the Indian Ocean Dipole/Zonal Mode in a Coupled GCM

International audienceWe address the question of whether and how tropical Indian Ocean Dipole or Zonal Mode(IOZM) interannual variability is independent of El Niño-Southern Oscillation (ENSO) variabilityin the Pacific, in a comparison of twin 200-year runs of a coupled climate model. Thefirst is a reference simulation, the second has ENSO-scale variability suppressed with a constrainton the tropical Pacific wind stress. The IOZM can exist in the model without ENSO,and the composite evolution of the main anomalies in the Indian Ocean in the two simulationsis virtually identical. Its growth depends on a positive feedback between anomalous equatorialeasterly winds, upwelling equatorial and coastal Kelvin waves reducing the thermocline depthand sea surface temperature off the coast of Sumatra, and the atmospheric dynamical responseto the subsequently reduced convection.Two IOZM triggers in the boreal spring are found. The first is an anomalous Hadley circulationover the eastern tropical Indian Ocean and Maritime Continent, with an early northwardpenetration of the southern hemisphere southeasterly trades. This situation grows out of coolersea surface temperatures in the southeastern tropical Indian Ocean left behind by a reinforcementof the late austral summer winds. The second trigger is a consequence of a zonal shift inthe center of convection associated with a developing El Niño, a Walker cell anomaly. Thefirst trigger is the only one present in the constrained simulation, and is similar to the evolutionof anomalies in 1994, when the IOZM occurred in the absence of a Pacific El Niño state. Thepresence of these two triggers—the first independent of ENSO, the second phase-locking theIOZM to El Niño—allows an understanding of both the existence of IOZM events whenPacific conditions are neutral, and the significant correlation between the IOZM and El Niño