The global atmospheric response to low-frequency tropical forcing

The tropical-extratropical interactions relevant to intraseasonal time scales (10-60 days) are investigated in a linearized non-divergent barotropic model on a sphere. The primary goal of this research is to study the role of tropical time varying forcing on the generation and maintenance of extratropical low frequency variability. Both zonally averaged and zonally varying basic flows are examined. WKB theory has been utilized to obtain analytical solutions for wave energy dispersion, wave ray paths, and wave amplitude variations along the rays. Both global and local energy budgets are carried out for the low frequency forced disturbances.; The first part of this research (Chapter 2) deals with two types of zonal mean wind profiles: (1) solid body rotation flow; (2) flows with critical latitudes. An analytical analysis based on WKB and ray tracing methods shows that unlike stationary Rossby waves, westward moving, low frequency Rossby waves can propagate through the tropical easterlies into the extratropics. The difference between orientations of the stationary and low frequency ray paths is proportional to the forcing frequency and inversely proportional to the zonal wavenumber cubed. An expression for the disturbance amplitude is also derived, and shows that the ability of the forced waves to maintain their strength well into middle latitudes depends on their meridional wave scale and northward group velocity, both of which are functions of the slowly varying background flow. The analysis of the local energy budget demonstrates that the combination of energy dispersion from the forcing region and energy extraction from the equatorward flank of the midlatitude jet produces disturbances that have the greatest impact on the extratropical circulation. Under the assumption that the forcing amplitude is independent of frequency, this impact is largest when the tropical forcing period is in the 10-20 day range.; The second part of this research (Chapter 3) studies the influence of zonally varying flows on energy propagation of tropically forced waves. A 9-year (1980-1988) averaged 300 mb January basic flow is examined, which exhibits strong zonally varying jet streams in the Northern Hemisphere, and a nearly zonally uniform flow in the Southern Hemisphere. A two-dimensional WKB analysis shows that the group velocity depends on the sum of three terms: (1) the basic state wind vector; (2) a vector that is parallel to the absolute vorticity contours; and (3) the local wavevector. As the forcing frequency decreases, the ray paths approach the direction of the local wavevector. Generally, the changes of wave amplitude along ray paths are determined by the meridional variations of horizontal wave scales. However, the zonal variation of horizontal wave scale can still play an important role at the jet exit regions. A global energetics analysis demonstrates that the energy conversion from the basic flow becomes more important as forcing frequency decreases. A local energetics analysis shows that for long period ({dollar}>{dollar}30 days) forced disturbances, the energy redistribution terms, i.e., the advection and the pressure work terms, are also significant along the extratropical jet streams in the Northern Hemisphere. Hence, long period disturbances are strongly influenced by the jet streams in the Northern Hemisphere, and are much less sensitive to the position of the tropical forcing than those of intermediate period ({dollar}<{dollar}30 days). Consistent with observations, long (intermediate) period disturbances dominate in the Northern (Southern) Hemisphere, where the basic flow is more (less) zonally varying.