Given these defining monsoon characteristics, there is understandably a large climatic influence on the lifestyles and practices of the people of the area. In particular, the monsoon seasonal cycle has a governing influence on agriculture, and in times past has had a large influence on navigation and trade. Such influences have undoubtedly played an important role for the highly populated islands of Indonesia (e.g., Java and Bali), making research on year-to-year variability of the monsoon obviously important. Indeed, there has been a long history of studies of interannual variability, with multiple examples from both the colonial period (e.g., Berlage 1927; de Boer 1947) and the modern research era (e.g., Nicholls 1981; Hastenrath 1987; Naylor et al. 2001 and references therein). Yet, as is the case for the other monsoon areas of the world (Chapters 2 to 4), intraseasonal variability (ISV) is also prominent. Research on ISV within the region, however, has had a comparatively shorter history, and for Indonesian meteorological parameters in particular, is relatively absent.
The beginning of published research on ISV in the region appears, to our knowledge, to be the seminal paper by Troup (1961). Troup, concentrating on the Australian component of the monsoon system, used the term "bursts" to describe spells of excessively wet or low-level westerly conditions occurring for periods shorter than the overall summer season. "Onset" was then naturally defined as being the beginning of the first westerly burst in each wet season. Such bursts, by definition, are the manifestation of subseasonal variability within the monsoon, for which a very large variance component falls within the range we classify here as ISV. Since Troup's paper, many authors have referred to active (burst) and break events in the context of the Australian monsoon (e.g., Murakami and Sumi 1982; McBride 1983; Holland 1986; Gunn et al. 1989; Drosdowsky 1996; McBride and Frank 1999), and the current operational methodologies to define onset and active versus break periods follow the framework Troup developed.
Yet there have been other influences upon the development of the region's research. Due partly to the utility of Darwin in northern Australia as a base for meteorological field experiments, the Australian monsoon has received focussed study in the last two decades (e.g., see review chapters by McBride 1987, 1998; Suppiah 1992; Manton and McBride 1992). In parallel with this development, internationally there has been a recognition of the importance of the variability on intraseasonal time scales in tropical weather and climate, and in particular on the role of the Madden Julian oscillation (MJO) as a large-scale control (e.g., Madden and Julian 1971, 1994; Weickmann et al. 1985; Lau and Chan 1985; Knutson and Weickmann 1987; Wang and Rui 1990; Hendon and Salby 1994; Chapter 1). Despite this, with the notable exception of the work by Hendon and collaborators (Hendon et al. 1989; Hendon and Liebmann 1990a, b; Wheeler and Hendon 2004), the research literature on bursts and breaks in the Australian monsoon has seemed to largely ignore or downplay the influence of the MJO. Indeed, a seemingly great diversity of views about the importance of the MJO can be found. This is particularly evident in the case of monsoon onset, for which Hendon and Liebmann (1990a) implied that nearly all onsets result from the passage of a convectively-active MJO phase, while others have implied that the MJO has little or no impact (e.g., Davidson et al. 1983; Drosdowsky 1996).
Even more recently there has been an increased recognition of other 'modes' of ISV within the monsoons, besides just the MJO. In particular, ever-increasing attention is being paid to the pure shallow-water-like equatorial waves, as have now been well-observed to couple to convection, and produce prominent perturbations in the near-equatorial monsoons (e.g., Takayabu 1994; Numaguti 1995; Wheeler and Kiladis 1999; Wheeler et al. 2000; Wheeler and Weickmann 2001; Straub and Kiladis 2002). Comparatively less, however, is known about such modes, especially within the Australian-Indonesian region.
Within this context then, this chapter provides a timely review and synthesis on the topic of ISV within this monsoon region. Included in the contents is a general description of the climatological seasonal cycle of the region, as it is this that forms the necessary background state about which ISV appears (Section 5.2). Although exceptions can be found, ISV is usually defined as covering all time scales of variability beyond the synoptic limit (~10 days) to less than a season (~90 days). We adopt this definition here. The earliest work concerning variability within this broad range in the Australian-Indonesian region is discussed in Section 5.3. Frequency-only power and coherence spectra of the standard monsoon variables are examined in Section 5.4. While the MJO is the only prominent spectral peak, the fact that much variance does exist in the intraseasonal band, and much of it is highly coherent, is important.
The local manifestation in the monsoon of the spectrally-broad ISV, irrespective of its source, are the bursts and breaks, the general meteorology of which is discussed in Section 5.5. Section 5.6 is then devoted to the character and influence of specifically the MJO in the region. The discussion of the previous sections is then contrasted and compared to the evolution of two individual monsoon years, 1983/84 and 1987/88 (Section 5.7). Importantly, the fact that all years show at least some degree of burst or break activity (and hence have ISV), but not necessarily show strong MJO variability, is highlighted.
Section 5.8 then addresses the contrasting views that have been expressed on the importance of the role of the MJO, especially with regards to monsoon onset. As such, it is our attempt to reconcile the above-mentioned differences of opinion. We then turn our attention to the other sources and modes of ISV in the region (Section 5.9), the modulation of extreme events in the region by ISV (Section 5.10), and the region's intraseasonal extratropical-tropical interaction (Section 5.11).