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  • Cai, W., A. Sullivan, and T. Cowan, Rainfall teleconnections with Indo-Pacific variability in the IPCC AR4 models. J. Climate. Submitted.

The present study assesses the ability of climate models to simulate rainfall teleconnections with the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). An assessment is provided on 24 climate models used in the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC).

The strength of the ENSO-rainfall teleconnection, defined as the correlation between rainfall and Niño3.4, is overwhelmingly controlled by the amplitude of ENSO signals relative to stochastic noise, highlighting the importance of realistically simulating this parameter. Because ENSO influences arise from the movement of convergence zones from their mean positions, the well-known equatorial Pacific climatological sea surface temperature (SST) and ENSO cold tongue anomaly biases lead to systematic errors. The climatological SSTs, that are far too cold along the Pacific equator, lead to a complete “ENSO doldrums response” along the central and/or eastern equatorial Pacific in the majority of models. ENSO anomalies are also too equatorially confined and extend too far west, with linkages to a weakness in the teleconnection with Hawaii boreal winter rainfall, and an inducement of a teleconnection with rainfall over west Papau New Guinea in austral summer. Another consequence of the ENSO cold tongue bias is that the majority of models produce too strong a coherence between SST anomalies in the west, central and eastern equatorial Pacific. Consequently, the models’ abilities are reduced in terms of producing differences in the impacts by ENSO from those by the ENSO Modoki.

Similarly, the IOD-rainfall teleconnection strengthens with an intensification of the IOD relative to the stochastic noise. A significant relationship exists between inter-model variations of IOD-ENSO coherence and inter-model variations of the ENSO amplitude in a small subset of models, in which the ENSO anomaly structure and ENSO signal transmission to the Indian Ocean are better simulated. However, using all but one model (defined as an outlier) there is no systematic linkage between the ENSO amplitude and IOD-ENSO coherence. Indeed, the majority of models produce an ENSO-IOD coherence lower than the observed, supporting the notion that the Indian Ocean has the ability to generate independent variability, and that ENSO is not the only trigger of the IOD. Although models with a stronger IOD amplitude and rainfall teleconnection tend to have a greater ENSO amplitude, there is no causal relationship; instead this feature reflects a commensurate strength of the Bjerknes feedback in both the Indian and Pacific Oceans.


Last Updated: 2008-08-21

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