The dominant modes of month-to-month variation in the troposphere are realistically simulated in a general circulation model whose lower boundary is prescribed with realistic topography and seasonally varying, climatological sea surface temperatures. In both hemispheres, these internal modes describe a nearly zonally symmetric, North-South shifting of the zonal jets as anomalous westerlies vacillate between high (50¿--60¿) and low (30¿--40¿) latitudes. The eddy structures evolve with the jets, and the corresponding eddy momentum forcings support the shifts in jet position. Stationary (synoptic) wave variations mainly account for the total eddy forcings in the Northern (Southern) Hemisphere. Pronounced eddy variations occur over the North Atlantic sector. ¿ 1999 American Geophysical Union