Climate Dynamics

, Volume 40, Issue 1, pp 493-509

Open Access This content is freely available online to anyone, anywhere at any time.

Real-time multivariate indices for the boreal summer intraseasonal oscillation over the Asian summer monsoon region

  • June-Yi LeeAffiliated withInternational Pacific Research Center, University of Hawaii Email author 
  • , Bin WangAffiliated withInternational Pacific Research Center, University of HawaiiDepartment of Meteorology, University of Hawaii
  • , Matthew C. WheelerAffiliated withCentre for Australia Weather and Climate Research (CAWCR), Bureau of Meteorology
  • , Xiouhua FuAffiliated withInternational Pacific Research Center, University of HawaiiDepartment of Meteorology, University of Hawaii
  • , Duane E. WaliserAffiliated withJIFRESSE, University of CaliforniaJet Propulsion Laboratory, Caltech
  • , In-Sik KangAffiliated withSeoul National University


The boreal summer intraseasonal oscillation (BSISO) of the Asian summer monsoon (ASM) is one of the most prominent sources of short-term climate variability in the global monsoon system. Compared with the related Madden-Julian Oscillation (MJO) it is more complex in nature, with prominent northward propagation and variability extending much further from the equator. In order to facilitate detection, monitoring and prediction of the BSISO we suggest two real-time indices: BSISO1 and BSISO2, based on multivariate empirical orthogonal function (MV-EOF) analysis of daily anomalies of outgoing longwave radiation (OLR) and zonal wind at 850 hPa (U850) in the region 10°S–40°N, 40°–160°E, for the extended boreal summer (May–October) season over the 30-year period 1981–2010. BSISO1 is defined by the first two principal components (PCs) of the MV-EOF analysis, which together represent the canonical northward propagating variability that often occurs in conjunction with the eastward MJO with quasi-oscillating periods of 30–60 days. BSISO2 is defined by the third and fourth PCs, which together mainly capture the northward/northwestward propagating variability with periods of 10–30 days during primarily the pre-monsoon and monsoon-onset season. The BSISO1 circulation cells are more Rossby wave like with a northwest to southeast slope, whereas the circulation associated with BSISO2 is more elongated and front-like with a southwest to northeast slope. BSISO2 is shown to modulate the timing of the onset of Indian and South China Sea monsoons. Together, the two BSISO indices are capable of describing a large fraction of the total intraseasonal variability in the ASM region, and better represent the northward and northwestward propagation than the real-time multivariate MJO (RMM) index of Wheeler and Hendon.


Boreal summer intraseasonal oscillation Madden-Julian Oscillation Real-time multivariate index Northward propagation Asian summer monsoon Monsoon onset