The El Nino-Southern Oscillation (ENSO) and Quasi-Decadal Oscillation (QDO) observed in global climate variability during the 20th Century (Allan, 2000) dominated tropical Indian Ocean climate variability from 1993-2002. Both signals shared a delayed action oscillator (DAO) similar to that shared by corresponding signals in the tropical Pacific Ocean (White et al., 2003), intensified in the South Indian Ocean. Each signal was characterized by a short-lived east-west tropical dipole in sea level height (SLH) and sea surface temperature (SST) variability, with the warm SST phase in the western ocean transitioning to the cool phase in response to slow, westward-propagating, upwelling coupled Rossby waves in the tropical South Indian Ocean. The cool phase in the eastern ocean transitioning to the warm phase in response to slow, eastward-propagating, downwelling equatorial coupled waves. At both boundaries, the simultaneous incidence of these coupled waves provides a delayed negative feedback to tropical SST variability, together producing a DAO that accounts for each signal, similar to those observed in the Pacific Ocean (White et al., 2003). This description allows us to construct a DAO model of the ENSO in the Indian Ocean that demonstrates how boundary reflections of the coupled waves, their intensification by wind stress forcing, and inherent delays in their propagation yield observed frequency and amplitude. Thus, warm SST variability in the western tropical Indian Ocean during 1995, 1998, and 2002 arose not from zonal atmospheric teleconnections originating with ENSO in the tropical Pacific Ocean, but from a DAO intrinsic to the tropical Indian Ocean that co-oscillated with that governing ENSO in the tropical Pacific Ocean.
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