b. The tropical Madden-Julian oscillation (MJO)

In addition to the ENSO, low frequency variability in the Tropics is heavily influenced by the Madden-Julian oscillation (MJO) (e.g., Madden and Julian 1971, 1972, 1994). These oscillations are characterized by a strong eastward propagation of atmospheric features, with a typical period of 30-60 days. The oscillation is associated with variations in deep tropical convection, with the largest anomalies typically observed over the relatively warm ocean waters (greater than 28°C) of the Indian Ocean, Indonesia and the western Pacific. The MJO-related tropical convection can be masked to some degree by extreme phases of ENSO. For example, during cold episodes enhanced convection associated with the MJO is often more evident over Indonesia while suppressed convection is more evident in the vicinity of the date line. In contrast, during warm episodes enhanced convection associated with the MJO is often more evident in the vicinity of the date line while suppressed convection is more evident over Indonesia.

There is also substantial interannual variability in MJO activity, with long periods (e.g., February 1996 -July 1997) of increased activity followed by periods (e.g., August-December 1997) in which the oscillation is weak or absent (Figs. 31, 32). During March-July 1997, the active MJO was sometimes strong enough to mask the ENSO-related pattern of anomalous tropical convection (Fig. 31), and to also affect the atmospheric wind and pressure indices used to monitor the developing warm episode. Subsequently, the diminished MJO activity during August-December 1997 was presumably linked to a continued strengthening of the warm episode and its eventual control over the distribution of convection throughout the tropical Pacific and Indonesia.

As a consequence of the large-scale changes in tropical convection accompanying the MJO, the jet streams over both the North Pacific and South Pacific can be heavily impacted during the respective hemisphere's winter season (Higgins and Mo 1997). For example, the jets often exhibit a westward retraction during periods of enhanced convection over Indonesia, and an eastward extension as the enhanced convection moves over the central equatorial Pacific. In the Northern Hemisphere, these east-west modulations of the Pacific jet stream can contribute to recurring blocking activity over the high latitudes of the North Pacific (Higgins and Mo 1997), as was observed during September-December 1996 (Halpert and Bell 1997) and continued well into April 1997. This blocking activity was an important contributing factor to below-normal surface temperatures across western Canada throughout the 1996/97 cold season, and to below-normal surface temperatures and above-normal precipitation across the northwestern United States during the period. These conditions contributed to flooding in the Pacific Northwest during late 1996 and early 1997 (Halpert and Bell 1997), and to massive flooding of the Red River Valley and its tributaries in North Dakota and western Minnesota during April [see section 4a(1)].

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