Abstract
In this paper, the mechanisms of northern summertime teleconnections are investigated using a barotropic model. In a series of numerical experiments we study the atmospheric response over the eastern Pacific-North America to an idealized local divergence source corresponding to the northward displacement of the ITCZ in the eastern Pacific. It is found that the response is much stronger in June than in May and is strongest when the forcing is located north of about 10°N. This can be explained in terms of the refractive properties of the climatoloigical summertime subtropical jet stream over North America. In another series of experiments we examine the global response as a function of the longitudinal location of the tropical forcing. A wave train emanating from the subtropics of the western Pacific near the Philippines, arching across the Aleutians and the Gulf of Alaska, and terminating with a high anomaly over the continental United States appears over a wide longitudinal range of local forcing, suggesting the existence of a normal mode for the northern summertime climatological flow. The normal-mode concept is supported by further experiments using extratropical forcings as well as free-mode integrations. The upstream anomalous low over the Gulf of Alaska is found to be essential for the development of the anomalous high over the continental United States. These results indicate that an above-normal high over the continent may occur when the anomalous forcing (both tropical and extratropical) acts to amplify the normal-mode structure. The caveats and implications of the present results to the possible linkage between tropical forcing and United States droughts are also discussed.
