Monday 22 August 2005

P1
1400-1520 hours

072
Intrinsic coupled variability in the tropical Indian Ocean
Hendon, Harry¹, Alves, Oscar¹, Zong, AiHong²
1 BMRC, Melbourne, Vic, Australia
2 NMOC
An intrinsic coupled mode of variability in the tropical Indian Ocean, the IOZM (Indian Ocean zonal mode), is highlighted in two coupled model runs where in one El Niño/Southern Oscillation (ENSO) is active and in the other it is purposely suppressed. The IOZM involves a feedback between anomalous equatorial zonal gradients of SST and rainfall with zonal wind anomalies, and is tightly tied to the seasonal cycle. In the presence of ENSO, the IOZM is promoted because equatorial surface zonal wind anomalies are induced in the central Indian Ocean when the Walker circulation is displaced eastward in the Pacific. When ENSO is suppressed, the IOZM still develops but its amplitude is 20-30% weaker. The simulated IOZM possess an intrinsic biennial timescale, which results from the combination of limited duration of ocean-atmosphere coupling (boreal summer and fall) and the subsequent adjustment time of the equatorial thermocline. In the absence of ENSO, the biennial variation in the thermocline is not amplified at the surface. This implies that thermocline variations on their own are not enough to instigate the IOZM. Besides ENSO, the IOZM can be triggered by a shift of the extratropical tropospheric circulation into the low phase of SAM (the southern annular mode), which leads to sustained low latitude easterlies in the southern Indian Ocean. Because SAM occurs randomly on seasonal timescales and with no particular tendency to be biennial, the biennial nature of the IOZM is not typically amplified in the absence of ENSO.

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