Interannual Variations of Tropical Upper Tropospheric Divergence and Pacific Teleconnections during Northern Winter


A 15-year (1974-1988) data set based on the US Navy's tropical global band analysis is used to study the interannual variations of 200 mb winter flow over the Pacific. The divergence fields are free from numerical model prediction influences that typically exist in operational data sets, and agree well with the mean outgoing longwave radiation data. An out-of-phase variation between two major equatorial centers of large-scale divergence anomalies, one over the central Pacific and the other over the western extreme of the western Pacific (maritime continent), is most noticeable. A third major center of divergence anomalies is located southwest of the Mexican coast. This center also shows an out-of-phase variation with that in the equatorial central Pacific. Composite and single-point correlation studies support the notion that the teleconnection patterns are related to equatorial divergence anomalies. In particular, divergence forcing from the central Pacific appears important for the PNA pattern, and forcing from the maaritime continent area appears important for a northeastward wave train pattern in the North Pacific. These two teleconnection patterns have streamfunction anomalies of the opposite sign over the northeastern Pacific. By examining the change of correlation patterns as the divergence base point is moved around the equatorial belt, the insensitivity of the teleconnection pattern with respect to tropical forcing locations as reported by several general circulation model simulations is observed only within limited regions. In the equatorial divergence base point is within 150¢XE-120¢XW. The pattern changes rapidly when the base point is moved outside of this region.

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