Two Independent Triggers for the Indian Ocean Dipole/Zonal Mode in a Coupled GCM
Résumé
We 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) variability
in the Pacific, in a comparison of twin 200-year runs of a coupled climate model. The
first is a reference simulation, the second has ENSO-scale variability suppressed with a constraint
on 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 simulations
is virtually identical. Its growth depends on a positive feedback between anomalous equatorial
easterly winds, upwelling equatorial and coastal Kelvin waves reducing the thermocline depth
and sea surface temperature off the coast of Sumatra, and the atmospheric dynamical response
to the subsequently reduced convection.
Two IOZM triggers in the boreal spring are found. The first is an anomalous Hadley circulation
over the eastern tropical Indian Ocean and Maritime Continent, with an early northward
penetration of the southern hemisphere southeasterly trades. This situation grows out of cooler
sea surface temperatures in the southeastern tropical Indian Ocean left behind by a reinforcement
of the late austral summer winds. The second trigger is a consequence of a zonal shift in
the center of convection associated with a developing El Niño, a Walker cell anomaly. The
first trigger is the only one present in the constrained simulation, and is similar to the evolution
of anomalies in 1994, when the IOZM occurred in the absence of a Pacific El Niño state. The
presence of these two triggers—the first independent of ENSO, the second phase-locking the
IOZM to El Niño—allows an understanding of both the existence of IOZM events when
Pacific conditions are neutral, and the significant correlation between the IOZM and El Niño.
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