Bimodal representation of the tropical intraseasonal oscillation
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The tropical intraseasonal oscillation (ISO) shows distinct variability centers and propagation patterns between boreal winter and summer. To accurately represent the state of the ISO at any particular time of a year, a bimodal ISO index was developed. It consists of Madden-Julian Oscillation (MJO) mode with predominant eastward propagation along the equator and Boreal Summer ISO (BSISO) mode with prominent northward propagation and large variability in off-equatorial monsoon trough regions. The spatial–temporal patterns of the MJO and BSISO modes are identified with the extended empirical orthogonal function analysis of 31 years (1979–2009) OLR data for the December–February and June–August period, respectively. The dominant mode of the ISO at any given time can be judged by the proportions of the OLR anomalies projected onto the two modes. The bimodal ISO index provides objective and quantitative measures on the annual and interannual variations of the predominant ISO modes. It is shown that from December to April the MJO mode dominates while from June to October the BSISO mode dominates. May and November are transitional months when the predominant mode changes from one to the other. It is also shown that the fractional variance reconstructed based on the bimodal index is significantly higher than the counterpart reconstructed based on the Wheeler and Hendon’s index. The bimodal ISO index provides a reliable real time monitoring skill, too. The method and results provide critical information in assessing models’ performance to reproduce the ISO and developing further research on predictability of the ISO and are also useful for a variety of scientific and practical purposes.
KeywordsTropical intraseasonal oscillation MJO Index
This research was supported by NSF Grant AGS-1005599 and NOAA Grant NA10OAR4310247. Additional support was provided by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), by NASA through grant NNX07AG53G, and by NOAA through grant NA17RJ1230 through their sponsorship of research activities at the IPRC. The interpolated OLR data are provided by the NOAA/OAR/ESRL PSD. The MERRA data are provided by the Global Modeling and Assimilation Office (GMAO) and the GES DISC. We also thank two anonymous reviewers for their constructive comments. Thanks also go to Dr. Nat Johnson for his editorial assistance and members of MJO working group, especially Dr. Matthew Wheeler, for invaluable feedback on the part of real time monitoring.
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