Large-scale circulation variability over the tropical western North Pacific
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The basic structure of the variability of the large-scale circulations over the tropical western Pacific is investigated with respect to its influence on tropical cyclone characteristics. A vector empirical orthogonal function analysis and fuzzy cluster algorithm are applied to a 9-yr dataset to define six recurrent 700-mb circulation patterns that represent large-scale variabilities associated with the monsoon trough and subtropical ridge. Five of the cluster patterns, which contain 48% of the sample, define combinations of active (inactive) monsoon trough and strong (weak) subtropical ridge circulations. AThe sixth cluster, which contains 26% of the data sample, depicts small deviations from the long-term climatology. After the cluster centers are defined, the fuzzy cluster coefficients are used to identify a seventh cluster, which contains the remaining 26% of the circulation patterns that could not be classified within any of the original six clusters. The 700-mb circulation patterns are physically consistent with outgoing longwave radiation anomalies and the 200-mb streamfunction and velocity potential anomalies. Active and inactive monsoon trough patterns are related to large-scale velocity potential anomalies over the tropical western Pacific and Indian Ocean basins. Anomalous cyclonic circulations are found to be regions of anomalous convergence at 700 mb., divergence at 200 mb, and enhanced large-scale convection. Anticyclonic anomalies are regions of anomalous 700-mb divergence, 200-mb convergence, and reduced large-scale convection. Variability of the subtropical ridge is associated with large-scale, 200-mb streamfunction anomalies that are related to variations in the midlatitude longwave pattern. Tropical cyclone activity is found to be related significantly to the variability of the monsoon trough described within the cluster framework. 10 refs., 4 tabs.
The article of record as published may be found at http://dx.doi.org/101175/1520-0493(1995)123<1225:LSCVOT>2.0.CO;2
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