An All-Season Real-Time Multivariate MJO Index: Development of an Index for Monitoring and Prediction

Matthew C. Wheeler and Harry H. Hendon

2004: Monthly Weather Review, 132, 1917-1932.

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A seasonally-independent index for monitoring the Madden-Julian oscillation (MJO) is described. It is based on a pair of empirical orthogonal functions (EOFs) of the combined fields of near-equatorially-averaged 850 hPa zonal wind, 200 hPa zonal wind, and satellite-observed outgoing longwave radiation (OLR) data. Projection of the daily observed data onto the multiple-variable EOFs, with the annual cycle and components of interannual variability removed, yields principal component (PC) time series that vary mostly on the intraseasonal time scale of the MJO only. This projection thus serves as an effective filter for the MJO without the need for conventional time filtering, making the PC time series an effective index for real time use.

We call the pair of PC time series that form the index the Real-time Multivariate MJO series 1 (RMM1), and 2 (RMM2). The properties of the RMM series and the spatial patterns of atmospheric variability they capture are explored. Despite the fact that RMM1 and RMM2 describe evolution of the MJO along the equator that is independent of season, the coherent off-equatorial behaviour exhibits strong seasonality. In particular, the northward propagating behaviour in the Indian monsoon and the southward extreme of convection into the Australian monsoon are captured by monitoring the seasonally independent eastward propagation in the equatorial belt. The previously described interannual modulation of the global variance of the MJO is also well captured.

Applications of the RMM series are investigated. One is through their relationship with the onset dates of the monsoons in Australia and India; while the onsets can occur at any time during the convectively-enhanced half of the MJO cycle, they rarely occur during the suppressed half. Another application is the modulation of the probability of extreme weekly rainfall; in the ``Top End'' region around Darwin, the swings in probability represent more than a tripling in the likelihood of an upper quintile weekly rainfall event from the dry to wet MJO phase.