The role of baroclinic waves in the zonal mean general circulation

This thesis investigates the role of large-scale, baroclinic eddies in shaping the zonal mean circulation in the troposphere. The zonal mean circulation can be partitioned into an east-west component of the wind (zonal wind, hereafter) and a meridionally overturning circulation (MMC, hereafter) that includes both the north-south and vertical components of the velocity field. This thesis is partitioned into two parts, the first on the MMC and the second on the zonal wind.; 1. MMC. Adopting a strategy that combines numerical experiments with mean meridional circulation (MMC) equation diagnostics, we identify the nonlinear processes of baroclinic waves that can either greatly amplify or dampen the Hadley cell.; For the numerical experiments, we use a multi-level primitive equation (PE) model on the sphere. Self-consistent calculations of both axisymmetric and non-axisymmetric circulations allow an unambiguous estimate of baroclinic eddy effects on the structure of the Hadley cell.; Furthermore, a diagnostic analysis allows us to partition the influence of baroclinic eddies into ‘direct’ and ‘indirect’ effects. The former refers to the meridional circulation driven by the explicit eddy fluxes while the latter refers to forcing of the meridional circulation by other processes, such as surface friction and diabatic heating changes, which are induced by the baroclinic eddies. For a realistic parameter range, it is found that the influence of these indirect effects is comparable to that of the direct effects.; 2. Zonal wind structure. When there exist both the subtropical and polar-front jets in the Southern Hemisphere (SH) winter, growing baroclinic waves are found between the two jets as well as along the polar-front jet. We refer to the baroclinic waves found between the two jets as “inter-jet disturbances”.; These inter-jet disturbances are characterized by an eddy momentum flux convergence between the two jets. It is thus argued that these inter-jet disturbances blend the two jets, obscuring the distinction between the subtropical and the polar-front jets. Also, the deceleration of the westerlies on the equatorward side of the subtropical jet is associated with the above inter-jet disturbances, rather than the eddies that originated along the polar-front or subtropical jets.; These results are supported by additional observational analysis and numerical experiments.