A summary of the broadscale tropical circulation from 70°E to 180° for the six months May to October 1999 is presented. By the start of this summary period there were indications that La Niña conditions were giving way to a neutral ENSO phase (e.g. weaker cool sea-surface temperature anomalies in the equatorial Pacific and a lower southern oscillation index). However, this was short-lived and a return to La Niña conditions was subsequently observed. Evidence for this included below average atmospheric pressure and above average convection over the western half of the region and positive values of the southern oscillation index, as well as a prolonged active summer monsoon over Asia. Four major active phases of the 30 to 60- day intraseasonal oscillation were observed, with periodicity apparently fluctuating between about 35 and 55 days. A total of 23 tropical cyclones developed during the period, close to the mean for the area, and most of them in the northwestern
Shaik et al. (Wed,) studied this question.
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