HR: 0800h
AN: GC51A-0155    [Abstracts]
TI: Simulation of Coupled Variability in the Tropical Indian Ocean
AU: * Zhong, A
EM: a.zhong@bom.gov.au
AF: National Meteorological and Oceanographic Centre, 700 Collins Street, Melbourne, Vic 3008, Australia
AU: Hendon, H H
EM: h.hendon@bom.gov.au
AF: Bureau of Meteorology Research Centre, 700 Collins Street, Melbourne, Vic 3008, Australia
AU: Alves, O
EM: o.alves@bom.gov.au
AF: Bureau of Meteorology Research Centre, 700 Collins Street, Melbourne, Vic 3008, Australia
AB: The coupled ocean-atmosphere variability in the tropical Indian Ocean is investigated by analysing three 100-year integrations of an Australian Bureau of Meteorology coupled seasonal forecast model. In its fully coupled (control) run, ENSO appears to be the leading mechanism that excites Indian Ocean coupled dipole/zonal mode. This involves a feedback between anomalous equatorial easterlies and zonal gradients in SST and rainfall, and is tightly tied to the seasonal cycle. The Indian Ocean zonal mode exhibits a dominant biennial periodicity, which is an amplification of the biennial ENSO mode in this model. In the second run, the local ocean - atmosphere coupling in the Indian Ocean is purposely suppressed by passing the climatological wind stresses derived from the control run to the ocean in the tropical Indian region. The dominant mechanism of SST variation in the Indian Ocean is investigated. A basin-scale surface warm anomaly is developed after the peak of El Niņo in the Pacific. It is found that this warming is driven by surface heat flux anomalies that are remotely driven by SST anomalies in the equatorial Pacific. In this run, the biennial periodicity of Indian Ocean zonal mode is significant reduced. In the third run, the ENSO is artificially suppressed by applying climatological surface stresses to the tropical Pacific Ocean. In that case, the Indian Ocean zonal mode still develops in the absence of ENSO but its amplitude is about 20-30% weaker, supporting the notion that the Indian Ocean coupled mode is an intrinsic mode of the variability in the Indian Ocean. Furthermore, the biennial variation, mainly apparent the subsurface, is not amplified at the surface in the absence of ENSO, suggesting that biennial variation in the thermocline itself can not trigger the Indian Ocean zonal mode. Besides ENSO, the model results also suggest that the Indian Ocean coupled mode can be triggered by an equatorward shift of the extratropical ridge/jet, which is associated with a shift of the SAM into its low phase. The results are consistent with observational study by Thomson and Lorenz (2004)
DE: 1620 Climate dynamics (0429, 3309)
SC: Global Environmental Change [GC]
MN: 2007 Fall Meeting