8.4.8 Madden-Julian Oscillation
The MJO (Madden and Julian, 1971) refers to the dominant mode of intra-seasonal variability in the tropical troposphere. It is characterised by large-scale regions of enhanced and suppressed convection, coupled to a deep baroclinic, primarily zonal circulation anomaly. Together, they propagate slowly eastward along the equator from the western Indian Ocean to the central Pacific and exhibit local periodicity in a broad 30- to 90-day range. Simulation of the MJO in contemporary coupled and uncoupled climate models remains unsatisfactory (e.g., Zhang, 2005; Lin et al., 2006), partly because more is now demanded from the model simulations, as understanding of the role of the MJO in the coupled atmosphere-ocean climate system expands. For instance, simulations of the MJO in models at the time of the TAR were judged using gross metrics (e.g., Slingo et al., 1996). The spatial phasing of the associated surface fluxes, for instance, are now recognised as critical for the development of the MJO and its interaction with the underlying ocean (e.g., Hendon, 2005; Zhang, 2005). Thus, while a model may simulate some gross characteristics of the MJO, the simulation may be deemed unsuccessful when the detailed structure of the surface fluxes is examined (e.g., Hendon, 2000).
Variability with MJO characteristics (e.g., convection and wind anomalies of the correct spatial scale that propagate coherently eastward with realistic phase speeds) is simulated in many contemporary models (e.g., Sperber et al., 2005; Zhang, 2005), but this variability is typically not simulated to occur often enough or with sufficient strength so that the MJO stands out realistically above the broadband background variability (Lin et al., 2006). This underestimation of the strength and coherence of convection and wind variability at MJO temporal and spatial scales means that contemporary climate models still simulate poorly many of the important climatic effects of the MJO (e.g., its impact on rainfall variability in the monsoons or the modulation of tropical cyclone development). Simulation of the spatial structure of the MJO as it evolves through its life cycle is also problematic, with tendencies for the convective anomaly to split into double ITCZs in the Pacific and for erroneously strong convective signals to sometimes develop in the eastern Pacific ITCZ (e.g., Inness and Slingo, 2003). It has also been suggested that inadequate representation in climate models of cloud-radiative interactions and/or convection-moisture interactions may explain some of the difficulties in simulating the MJO (e.g., Lee et al., 2001; Bony and Emanuel, 2005).
Even though the MJO is probably not fundamentally a coupled ocean-atmosphere mode (e.g., Waliser et al., 1999), air-sea coupling does appear to promote more coherent eastward, and, in northern summer, northward propagation at MJO temporal and spatial scales. The interaction with an active ocean is important especially in the suppressed convective phase when SSTs are warming and the atmospheric boundary layer is recovering (e.g., Hendon, 2005). Thus, the most realistic simulation of the MJO is anticipated to be with AOGCMs. However, coupling, in general, has not been a panacea. While coupling in some models improves some aspects of the MJO, especially eastward propagation and coherence of convective anomalies across the Indian and western Pacific Oceans (e.g., Kemball-Cook et al., 2002; Inness and Slingo, 2003), problems with the horizontal structure and seasonality remain. Typically, models that show the most beneficial impact of coupling on the propagation characteristics of the MJO are also the models that possess the most unrealistic seasonal variation of MJO activity (e.g., Zhang, 2005). Unrealistic simulation of the seasonal variation of MJO activity implies that the simulated MJO will improperly interact with climate phenomena that are tied to the seasonal cycle (e.g., the monsoons and ENSO).
Simulation of the MJO is also adversely affected by biases in the mean state (see Section 8.4.7). These biases include the tendency for coupled models to exaggerate the double ITCZ in the Indian and western Pacific Oceans, under-predict the eastward extent of surface monsoonal westerlies into the western Pacific, and over-predict the westward extension of the Pacific cold tongue. Together, these flaws limit development, maintenance and the eastward extent of convection associated with the MJO, thereby reducing its overall strength and coherence (e.g., Inness et al., 2003). To date, simulation of the MJO has proven to be most sensitive to the convective parametrization employed in climate models (e.g., Wang and Schlesinger, 1999; Maloney and Hartmann, 2001; Slingo et al., 2005). A consensus, although with exceptions (e.g., Liu et al., 2005), appears to be emerging that convective schemes based on local vertical stability and that include some triggering threshold produce more realistic MJO variability than those that convect too readily. However, some sophisticated models, with arguably the most physically based convective parametrizations, are unable to simulate reasonable MJO activity (e.g., Slingo et al., 2005).