Abstract
This study investigates how accurately the interannual variability over the Indian Ocean basin and the relationship between the Indian summer monsoon and the El Niño Southern Oscillation (ENSO) can be simulated by different modelling strategies. With a hierarchy of models, from an atmospherical general circulation model (AGCM) forced by observed SST, to a coupled model with the ocean component limited to the tropical Pacific and Indian Oceans, the role of heat fluxes and of interactive coupling is analyzed. Whenever sea surface temperature anomalies in the Indian basin are created by the coupled model, the inverse relationship between the ENSO index and the Indian summer monsoon rainfall is recovered, and it is preserved if the atmospherical model is forced by the SSTs created by the coupled model. If the ocean model domain is limited to the Indian Ocean, changes in the Walker circulation over the Pacific during El-Niño years induce a decrease of rainfall over the Indian subcontinent. However, the observed correlation between ENSO and the Indian Ocean zonal mode (IOZM) is not properly modelled and the two indices are not significantly correlated, independently on season. Whenever the ocean domain extends to the Pacific, and ENSO can impact both the atmospheric circulation and the ocean subsurface in the equatorial Eastern Indian Ocean, modelled precipitation patterns associated both to ENSO and to the IOZM closely resemble the observations.
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Notes
In our analysis we discarded the spin-up period and the first 30 years of integration.
The comparisons between ENS3 and ENS4, and between ENS5 and ENS6 allow us to draw the same conclusions. In the following we will concentrate on ENS4, without repeating figures and discussion for ENS6.
The AGCM performance is comparable at different levels in the lower troposphere, and the analysis of 850 hPa winds provides analogous results.
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Acknowledgments
The experiments described were performed as a contribution to the ENSEMBLES project funded by the European Commission’s 6th Framework Programme, contract number GOCE-CT-2003-505539. The authors wish to thank Ben Kirtman for useful discussions during the preparation of this manuscript, and Hari Annamalai and an anonymous referee for their valuable comments and suggestions that helped improving the manuscript.
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Appendix
Appendix
The goal of this appendix is to provide the reader with a quantitative assessment of the modelled surface fluxes.
A detailed analysis of the surface heat fluxes in the various ensembles and a complete understanding of the model response is beyond the goal of this work and will require further integrations and possibly more idealized set-ups. We think, however, that a presentation of the latent heat flux and of the surface solar radiation will allow for a better comparison between our integrations and results from recently published work.
In agreement with the findings of Wu and Kirtman (2004), ENS1, forced by observed SST, is characterized by evaporation anomalies that follow surface wind anomalies (Fig. 15a) during ENSO events. In the warm phase, positive SST anomalies in the eastern IO are associated to anomalous winds favorable to the advection of moisture into the Indian subcontinent, absent in the reanalysis. Correspondingly a strong peak in evaporation is centered off the Somali coast. The surface solar radiation, on the other hand, is directly correlated with the precipitation pattern, erroneously strong over India.
In the coupled cases (ENS3 and ENS5), latent heat flux anomalies in the eastern part of the basin are negatively correlated with SST anomalies [panels (c) and (g)]. Such a relations is preserved when the SSTs generated by the coupled model are used to force the ICTP AGCM [for ENS4 panels (e) and (f), ENS6 not shown]. The signal, however, has a stronger amplitude, evident especially in the latent heat flux regression map. The comparison between panels (c) and (e) confirms the role played by the interactive ocean model, which damps part of atmospheric variability. Such a damping is found also in the analysis of integrations performed with other models. The ICTP AGCM, however, differs from other models (e.g. Wu and Kirtman 2004; Wang et al. 2005) as it is able to preserve the main patterns of variability whenever is forced by SSTs previously created by the model heat fluxes.
From the analysis of the surface solar radiation variability, it appears that cooling anomalies associated with positive ENSO events over the Indochina peninsula in ENS3 are related to a decrease of surface solar radiation, associated with the excess of precipitation produced by the model in this region. The confinement of such anomaly over the West Pacific in ENS5, in better agreement with the observations, supports the conclusion of Wu and Kirtman (2004), who advocated the need of a fully coupled model over the West Pacific to properly simulate the latitudinal extension of convective precipitation in the Indo-Pacific basin.
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Bracco, A., Kucharski, F., Molteni, F. et al. A recipe for simulating the interannual variability of the Asian summer monsoon and its relation with ENSO. Clim Dyn 28, 441–460 (2007). https://doi.org/10.1007/s00382-006-0190-0
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DOI: https://doi.org/10.1007/s00382-006-0190-0