Intraseasonal Atmospheric Teleconnection Patterns during the Northern Hemisphere Winter

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Abstract
The time-longitude cross section of 30–60 day filtered equatorial outgoing longwave radiation indicates many occasions or irregular (or even westward) movement during the five winters of 1979–84. Such occasions are defined as “NE” phase, while periods of regular eastward movement are designated as “E” phase. Global-scale behavior of the 30–60 day filtered velocity potential and streamfunction fields differ significantly from the E to NE phase. East (west) of an intense convective cell during the E phase are well-organized twin upper-tropospheric cyclonic (anticyclonic) systems straddling the equator with anomalous westerlies (easterlies) between them. A convective cell surrounded by four tropical disturbances at 200 mb constitutes a single equatorial eastward propagating low-frequency mode, which becomes prominent only during the E phase. When equatorial convection reaches the Indonesian region, a well-defined teleconnection pattern develops over the eastern Pacific and the North American continent. This teleconnection pattern is in phase (barotropic) in the vertical. The 850 mb streamfunction fields during the E phase appear to be dominated by three types of 30–60 day northerly surges in the Northern Hemisphere. The first type is directed southward along the western periphery of the Tibetan Plateau, eventually reaching the Arabian Sea and the equatorial Indian Ocean where convection becomes more pronounced than usual. The second type is associated with a strong pressure gradient between an anomalous anticyclone over Siberia and an intensified cyclone near Japan. These northerly surges, which become strongest about 15 days after the first type of northerly surge, do not appear to contribute much to the enhancement of equatorial convection. The third type is of subtropical North Pacific origin and acts as an effective regulator for equatorial convection over the western Pacific east of New Guinea. At 850 mb, no significant meridional surges occur during the NE phase. The immediate consequences are weak convective activity and an ill-defined equatorial mode in the NE phase compared with the E phase. Poleward of about 25°N, 30–60 day perturbations during the NE phase are as pronounced as in the E phase. The predominance of a well-organized wave train over the Eurasian continent is of interest. Also prominent is the teleconnection pattern over the eastern Pacific and the North American continent. Since the equatorial convective activity is depressed well below normal during the NE phase, the contribution of convection toward the evolution of these teleconnection patterns is negligible.

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