b. Tropical intraseasonal activity

Low-frequency variability in the Tropics is dominated by two phenomena: 1) the Southern Oscillation (e.g., Rasmusson and Arkin 1985), characterized by nearly-stationary features persisting for more than one season with a typical periodicity of four to five years; and 2) the Madden-Julian oscillation (e.g., Madden and Julian 1971, 1972, 1994), characterized by a strong eastward propagation of atmospheric features with a typical periodicity of 30-60 days. There is also strong year-to-year variability in MJO activity, with long periods of strong activity followed by periods in which the oscillation is weak or absent.

Both the SO and the MJO are associated with variations in deep tropical convection, and thus influence the atmospheric circulation on a global scale. The MJO features an eastward propagation of OLR anomalies (Fig. 27), with the largest anomalies typically observed over the relatively warm water (greater than 28°C) regions of the Indian and western Pacific Oceans. Inconspicuous anomalies are often observed over the cooler eastern Pacific, and weak but detectable anomalies are evident over South America and Africa (Weickmann 1983; Lau and Chan 1985).

In the tropics, a distinct pattern of lower- and upper-level atmospheric circulation anomalies accompany this eastward propagation of anomalous convection (e.g., Madden and Julian 1972; Weickmann 1983; Hsu 1996), with time-longitude sections of anomalous upper-tropospheric velocity potential (Fig. 28) clearly revealing a continuous eastward propagation of the MJO throughout the tropics (Knutson and Weickmann 1987). These intraseasonal variations can significantly impact regions that experience relatively short rainy seasons, such as northeastern Brazil, southeastern Africa, and northeastern Australia. They have also been linked to variations in the extratropical atmospheric circulation, particularly in the region of the Pacific Ocean and the Americas.

A strong coupling to the extratropical atmospheric circulation is sometimes found in the Southern Hemisphere in the austral winter and in the Northern Hemisphere during the boreal winter in the vicinity of the East Asian jet. For example, this jet often exhibits a westward retrogression toward eastern Asia during periods of enhanced convection over Indonesia and an eastward extension to east of the date line as the enhanced convection moves over the central equatorial Pacific. Higgins and Mo (1997) indicate that these east-west modulations of the East Asian jet can act as a significant source of downstream wave activity, leading to the development of persistent circulation anomalies over the North Pacific. They note that one typical manifestation of this downstream wave pattern is the development of high-latitude blocking over the central North Pacific approximately 5-10 days after the onset of enhanced convection over Indonesia.

Beginning in early 1996 there was a marked increase in intraseasonal variability in the global tropics, as shown in time-longitude sections of outgoing longwave radiation (Fig. 27 ) and 200­hPa velocity potential (Fig. 28). However, the intraseasonal character of the tropical convection was modulated by persistent cold­episode conditions over the central equatorial Pacific. Thus, regions of enhanced convection (negative OLR anomalies) were most prominent over the Indian Ocean and Indonesia, and tended to diminish over the abnormally cold waters of the central equatorial Pacific. Nonetheless, the MJO signature in the upper-level velocity potential field remained intact over the global Tropics, indicating a strong eastward progression of these features throughout the period.

During March-June 1996, the MJO featured a period of approximately 40 days. The period of the oscillation then lengthened during July and August to about 60 days, where it remained until the end of the year. The MJO was particularly strong during September-December 1996. During this period, there was considerable variability in the mid- and high-latitude atmospheric circulation patterns over the North Pacific, with pronounced periods of both high-latitude blocking and strong modulations in the eastward extent of the East Asian jet (Fig. 29 ). This variability is indicated by the time series of the 500-hPa height anomalies for the region of Alaska (Fig. 30a) which, with a lag of 5-10 days, is similar in character to the time series of tropical OLR anomalies near Indonesia (Fig. 30b).

Thus, it is likely that some aspects of midlatitude variability during September-December 1996 were linked to the strong intraseasonal activity in the Tropics. One aspect was strong blocking in the vicinity of the Bering Sea and Alaska during early November and again in late December (Figs. 29b,d). These periods coincided with times in which tropical convective activity was weaker than normal over the central Indian Ocean and stronger than normal over Indonesia. Interspersed with the periods of blocking were periods of below-normal heights in this region (Figs. 29a,c).

Another manifestation of the intraseasonal variability was strong variations in the eastward extent of the East Asian jet, which affected precipitation along the West Coast of the United States. Satellite imagery suggested that late December 1996 heavy precipitation and flooding along the west coast of the United States was directly related to an abnormally strong southwesterly flow of warm, moist air extending from the western tropical Pacific to the west coast of North America (Fig. 29d). These conditions reflected a pronounced eastward extension of the East Asian jet to well east of the date line and also appeared to be strongly connected to the eastward progression of enhanced tropical convection associated with the MJO to the central equatorial Pacific.

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