Summertime teleconnections associated with United States climate anomalies and their maintenance.

摘要

The primary teleconnection patterns in northern summer and their climate impacts are documented. Compared to their counterparts in northern winter, the teleconnection patterns in summer are temporally less persistent, have smaller spatial scale, and tend to spatially overlap with each other. Time evolutions of most patterns show the characteristics of Rossby wave dispersion. That contrasts with wintertime teleconnection patterns, which resemble the fast growing normal modes of the winter climatological mean flow.; The wave train pattern associated with the rainfall variability in the Great Plains is investigated through analyses of observational data and numerical experiments. The wave train exhibits a stable tri-pole pattern over the East Pacific-North America sector throughout summer, while the upstream portion of the wave train in the western North Pacific varies from June to August. The temporal variation of the teleconnection pattern suggests an eastward Rossby wave dispersion from the western North Pacific to North America. A kinetic energy budget analysis indicates that Rossby wave dispersion plays an essential role in maintaining the pattern while barotropic instability is less important.; Observations and barotropic model simulations both show that when a forcing is placed within the mean westerly flows, the extratropical response tends to shift in accord with the forcing; when the forcing is embedded in the mean easterlies, the extratropical wave train pattern is insensitive to the locations of the forcing. A hypothesis is proposed to explain this phenomenon: a southerly flow in the basic state may transfer Rossby wave sources downstream and excite a stationary wave response in the extratropical westerlies. A southerly flow determines the location of the effective Rossby wave source and the extratropical response is relatively insensitive to the location of the tropical forcing.; The impacts of another remote forcing, ENSO, on US summer climate anomalies also are investigated. It is found that the extratropical responses are quite nonlinear with respect to opposite signs of the equatorial eastern Pacific SSTA. A similar phenomenon also is found in AGCM simulation. Vorticity budget analysis and barotropic model simulation suggest that transient eddy process plays an important role in maintaining the extratropical nonlinear response.