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Tropical Intraseasonal Oscillations
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Tropical Intraseasonal Oscillation
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Discussion (Updated 26 November 2001)
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Over the past several months the
pattern of tropical convection has been strongly modulated by tropical intraseasonal
(30-60 day) fluctuations associated with the Madden Julian Oscillation (MJO) (Fig. 1). Low-level wind fluctuations across the central and
western tropical Pacific have been consistent with this activity during the period (Fig. 2). Upper-level velocity potential anomalies [isolates the
divergent component of the wind], show a continuous eastward propagation of this activity
around the global Tropics (Fig. 3).
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During the first half of November MJO-related enhanced convection shifted eastward from
the eastern Indian Ocean to the western Pacific. Low-level wind fluctuations across the
central and western tropical Pacific have been consistent with this activity. In the past,
the convectively active phase of the MJO has been instrumental in producing low-level
westerly wind bursts that are linked to subsequent oceanic warming in the central and
eastern equatorial Pacific during the onset phase of warm episodes, especially during the
transition seasons (March-May and September-November).
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During the first half of November the circulation pattern in the eastern North Pacific
/ North American region featured a trough over the Gulf of Alaska and a ridge over much of
the United States, with anomalously warm conditions over much of the country. In the past
week an anomalous subtropical ridge intensified near Hawaii, providing a strong projection
onto the positive phase of the Pacific North American pattern and a good connection to
subtropical moisture for west coast storminess. This evolution is consistent with the MJO,
whose convectively active phase shifted eastward from the west Pacific towards the central
Pacific during the period.
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Outlook
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Based on a continuation of relatively strong MJO activity with a period on the order of
45-50 days, and on results from historical studies of the effects of the MJO, we expect
above-normal variability in the Tropics and subtropics (30°S-30°N) to continue from now through the end of the NH
winter 2001-2002. During the next 10-15 days a stormy pattern is likely to continue for
the Pacific Northwest and Northern California coastal areas, with high winds...mountain
snows..and heavy rains at lower elevation windward locales. This is supported by the
latest MRF guidance.
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