Multi-scale theories for the MJO

In the equatorial troposphere, the Madden–Julian Oscillation (MJO) is a planetaryscale wave envelope of complex multi-scale convection (see Figure 1 for a schematic illustration). It begins as a standing wave in the Indian Ocean and propagates eastward across the western Pacific Ocean at a speed of ≈ 5 m/s. Due to its planetary-scale circulation anomalies, the MJO significantly affects monsoon development, intraseasonal predictability in midlatitudes, and the development of the El Niño southern oscillation (ENSO) in the Pacific Ocean, which is one of the most important components of seasonal prediction (Zhang, 2005; Lau and Waliser, 2005). It is also known that the MJO is a superrotating planetary-scale wave [see Biello et al. (2007) and references therein], and recent simulations suggest there may be enhanced MJO activity in a warmer climate, possibly leading to a superrotating tropical mean circulation (Caballero and Huber, 2010). Basic understanding of the MJO’s features has been gained from observations, including statistical composites (Salby and Hendon, 1994; Hendon and Salby, 1994; Hendon and Liebmann, 1994; Wheeler and Kiladis, 1999; Roundy and Frank, 2004; Kiladis et al., 2005) and case studies of individual MJO events (Lin and Johnson, 1996; Yanai et al., 2000; Houze et al., 2000). From these observations, the following fundamental features of the MJO have been identified relatively clearly: