A previous study [Meehl and Arblaster, 2011] noted that the mid-1970s climate shift [e.g.,Trenberth and Hurrell, 1994; Wang and An, 2001], when the Interdecadal Pacific Oscillation (IPO) transitioned from negative to positive (i.e. somewhat cooler to somewhat warmer tropical Pacific SSTs), was associated with interannual climate variability in the Indo-Pacific region becoming less biennial. In other words, the Tropospheric Biennial Oscillation (TBO) [e.g.,Meehl, 1987, 1997; Meehl et al., 2003; Li et al., 2001] became weaker. The IPO is the dominant feature of decadal variability of Pacific basin SSTs [Power et al., 1999], and is typically defined as the second empirical orthogonal function (EOF) of low-pass-filtered SSTs of near-global SSTs [e.g.,Parker et al., 2007], with the first EOF associated with the trend. In long climate model control runs, the IPO defined in this way is typically the first EOF, thus being the dominant mode of internally-generated decadal timescale variability in the Pacific region [Meehl et al., 2009; G. A. Meehl et al., Externally forced and internally generated decadal climate variability associated with the Interdecadal Pacific Oscillation, submitted to Journal of Climate, 2012]. The IPO is closely related to the Pacific Decadal Oscillation (PDO) [e.g., Mantua et al., 1997] and has a similar pattern, though the PDO is defined based on North Pacific SSTs, and consequently doesn't have as much power in the tropical and South Pacific as the IPO. The IPO has been connected to modulating the El Niño Southern Oscillation (ENSO) teleconnections from Australia to the tropical Pacific [Power et al., 1999; Arblaster et al., 2002], as well as to Indian monsoon precipitation [Meehl and Hu, 2006] and precipitation over the southwest U.S. [Meehl and Hu, 2006; Dai, 2012]. Though there have been studies making the case that the IPO is the low-frequency aspect of ENSO [e.g.,Jin, 1997], there have been papers that have outlined a distinct mechanism that can produce such decadal timescale variability in the Pacific [e.g., Meehl and Hu, 2006]. However, as with all internally generated variability of the climate system, the IPO operates in the context of changes in external forcing. In particular, the mid-1970s shift was shown to likely have involved both an internally generated component associated with the IPO, and an externally forced component related to increasing greenhouse gases in the atmosphere [Meehl et al., 2009].
 The mechanism of the TBO involves large-scale dynamically coupled interactions [Meehl, 1987, 1997; Meehl and Arblaster, 2002a, 2002b; Loschnigg et al., 2003]. It is based on the premise of air-sea coupling being particularly strong one season per year in the Indo-Pacific region during the passage of the “convective maximum”, a mass of convection and precipitation that moves with the seasonal cycle from the Indian monsoon in the northern summer southeastward to the Australian monsoon in southern summer and involving processes in the western tropical Pacific [Li et al., 2006; Annamalai and McCreary, 2005; Kug and Kang, 2006; Kug et al., 2006].
 The TBO has been noted to be the dynamical framework for regional processes related to meridional temperature gradients affecting the monsoon in the Indian sector [Meehl, 1994; Chang and Li, 2000], patterns of SST anomalies that involve El Niño and La Niña in the Pacific, and the Indian Ocean Dipole (IOD) [e.g., Webster et al., 1999; Saji et al., 1999]. Izumo et al.  made use of the biennial transition processes in the TBO to document El Niño forecast skill by monitoring the state of the IOD in the northern fall.
 Given these seemingly self-sustaining processes,Meehl and Arblaster asked the question as to why the Indo-Pacific climate system isn't perfectly biennial. Among the possible causes, decadal-timescale internally-generated variability associated with the IPO was noted to have the potential to disrupt or modify biennial variability associated with the TBO in the Indo-Pacific region. For example, for the mid-1970s shift, a warmer tropical Pacific in relation to the tropical Indian Ocean weakened the trade winds, reduced coupling strength, and contributed to the Pacific SSTs and Indian monsoon becoming less biennial. After the late-1990s transition of the IPO from positive to negative [Lee and McPhaden, 2008; Dai, 2012; W. Han et al., Pacific decadal sea level change patterns associated with a warming Indo-Pacific Warm Pool, submitted toJournal of Climate, 2012], it has been noted that the equatorial eastern Pacific SSTs have returned to being more biennial [Barnston et al., 2012]. The contribution from the Indo-Pacific SST gradient to trade-wind strength has likely contributed to increasing the strength of the TBO as will be discussed below.