Bimodal representation of the tropical intraseasonal oscillation

The tropical intraseasonal oscillation (ISO) shows distinct propagation characteristics between boreal summer and winter. To accurately represent the state of the ISO at a particular time of year, a bimodal ISO index was developed. That is the ISO index consists of two modes, the Madden-Julian Oscillation (MJO) and boreal summer ISO (BSISO). Each mode was identified with the extended empirical orthogonal function analysis of 31 years (1979-2009) OLR data during boreal winter and summer, respectively. By projecting each mode to the entire data period, a seamless ISO index in terms of the MJO and BSISO modes was obtained. The bimodal ISO index has advantage of objectively identifying the ISO type, i.e., MJO or BSISO. A high correlation of the ISO index with the Wheeler and Hendon's all season ISO (WH) index suggests that the ISO index can replace the WH index. With the ISO index, annual variation of the ISO was examined in detail to advance our understanding of how background field affects ISO behavior. In terms of the propagation characteristic, three seasons are clearly separated. During boreal winter from December through March (summer from May through October) only the MJO (BSISO) mode exists. April and November are climatological transitional months when predominant mode changes from one to the other. Zonal wind vertical shear over the northern Indian Ocean was suggested to be the key background component in the propagation characteristic of the ISO in the transitional months. Also the propagation features was suggested to link to surface boundary conditions over land rather than over ocean. The method and results provide useful information in assessing models' performance to reproduce the ISO and developing further research on predictability of the ISO.