Classical Rossby wave theory for zonally symmetric or varying zonal flow in which the effects of the basic-state meridional wind are ignored predicts that stationary Rossby waves cannot propagate across easterlies, indicating that the tropics and the extratropics were relatively independent from each other due to the existence of the critical latitudes. However, a large amount of observations has indicated that there exist significant linkages in atmospheric motions between the tropics and the extratropics, and even between the Northern Hemisphere (NH) and the Southern Hemisphere (NH). Some preliminary work suggest that stationary wave can propagate through the easterlies with inclusion of zonal mean meridional wind in the basic state.
In this dissertation, the theoretical basis for stationary wave propagation across the critical latitude was considered, taking into account longitudinal varying meridional ambient flow, i.e., horizontally non-uniform (HN) flow. A methodology was presented for performing Rossby wave ray tracing in which both wave packet trajectory and phase are calculated. The influence of meridional flow in the basic state on the Rossby wave propagation in the easterlies, equatorial westerly duct, and the extratropical westerlies were extensively examined, respectively. Characteristics and seasonal changes in the cross-equatorial propagation of stationary Rossby wave in the climatological basic flow were revealed. The Rossby wave theory was applied to the explain the mechanisms of some atmospheric teleconnections, and the corresponding physical processes were also verified by the numerical simulations. Major conclusions in this dissertation can be summarized as follows:
(1) Theory of cross-equatorial propagation of planetary wave and phase evolution of wave packet in a horizontally non-uniform basic flow was developed to study interhemispheric teleconnections. Theoretical results show that stationary Rossby wave can propagate across the critical latitude just in one direction and the direction follows the direction of the meridional basic flow.
A two-dimensional spherical Rossby wave theory in a horizontally nonuniform basic state was introduced to study interhemispheric teleconnections. The wave theory shows that stationary Rossby wave can propagate across the critical latitude just in one direction, and the meridional group velocity of Rossby waves has the same direction as the meridional basic wind. A methodology was presented for performing Rossby wave ray tracing in which both wave packet trajectory and phase are calculated. The wave ray tracings method can better represent waves in the tropic and the phase evolution of the wave packet.
In the ideal basic flow experiments, the role of the tropical zonal mean meridional wind on the cross-equatorial propagation of Rossby wave was clarified. In the upper troposphere, northerly winds prevail during the boreal summer, disturbances conducive to southward propagate to SH; southerly winds prevail during the boreal winter, disturbances in favor of northward propagate to NH. The wave ray tracings follow trajectories and phases matched by the linearized barotropic vorticity equation solutions.
(2) Theoretical results suggest that Asian-Australian monsoon region is an important tunnel for cross-equatorial propagation of stationary Rossby wave. In upper troposphere, stationary Rossby wave conducive to northward propagate across the equator in boreal winter season, while southward propagate in boreal summer season.
The wave theory was applied the climatological basic flow in the upper troposphere to investigate the cross-equatorial propagation of stationary Rossby waves and the seasonal cycles. It seems that the large-scale meridional basic flows play a critical role in the Rossby wave propagations. The equatorial westerly duct is no longer a two-way propagation channel for stationary Rossby waves, the wave is more conducive to the propagation direction of the meridional basic flow. The westward propagating of large-scale Rossby waves is allowed in the westerlies. In boreal winter and summer seasons, cross-equatorial flows steer stationary waves propagating from one hemisphere to the other across the tropical easterlies, especially over the Australian–Asian monsoon region.
(3) An interhemispheric teleconnection pattern across the critical latitude from southern Africa through South Asia to the North Pacific (AAP) was revealed in boreal winter. The corresponding dynamical mechanism was investigated using cross-equatorial propagation of stationary Rossby wave theory.
Observations exhibit a teleconnection (the AAP pattern) across the critical latitude from southern Africa through South Asia to the North Pacific in boreal winter. Further partial correlation analysis suggests that the AAP teleconnection pattern is likely to be independent of the well-known teleconnections (such as the Southern Eurasian (SEA), Eurasian (EU), Pacific-North America (PNA), and Western Pacific (WP)) and is not forced by ENSO. Theoretical results suggest that the southerly flow over East Africa, the western Indian Ocean, and South Asia creates a path for the northward propagation of stationary waves across the critical latitude. Stationary wavenumber and group velocity analysis, ray tracing, and simple model experiments applied to nearly realistic boreal winter mean flows confirm that disturbances excited in southern Africa and the tropical southern Indian Ocean can propagate across the critical latitude to South Asia through the southerly duct and then continue downstream along the North African–Asian subtropical jet.
(4) Potential linkages between the boreal summer Indo-Pacific convection oscillation (IPCO) and the SH climates (including rainfall, surface air temperature and Antarctic sea ice) on interannual time scale were revealed. Observation, theory and simulation suggest that cross-equatorial propagation of stationary barotropic Rossby wave is the key process that responsible for the interhemispheric linkages.
The Indo-Pacific convection oscillation (IPCO) is the second leading mode of interannual variability in boreal summer convection over the Indo-Western Pacific, and characterized by the out-phase in the off-equatorial convection between the north Indian Ocean and western North Pacific. Observational statistical analysis reveals that there exist significant linkages between the IPCO and the rainfall, surface air temperature and Antarctic sea ice over some regions of the SH. The Rossby theory indicate that the convection heating associated with the IPCO can generate Rossby wave trains that propagate across the equator through the northerly duct in Indian Ocean in the upper troposphere. One branch extends from southern Africa, to extratropical southwest Indian Ocean and then poleward and eastward to mid-latitudes in SH; The other branch propagates from Maritime continent, to northern Australia and then poleward and eastward downstream to mid-latitudes in SH. The wave pattern matches well with both observations and simulations, indicating that the two quasi-barotropic wave trains are the key bridges that linked the IPCO and the SH climates.
(5) Theoretical and simulation results suggest that stationary Rossby wave propagation and phase evolution of wave packet are responsible for the zonally elongated cyclonic and anticyclonic anomalies of the Pacific–Japan (PJ) pattern.
The characteristics of the propagation and phase evolution of the stationary Rossby wave packet in northwest Pacific-East Asia region were investigated using the Rossby wave ray tracing in climatological horizontally non-uniform flow in boreal summer. Results show that the perturbations tend to northward propagate in the lower troposphere and tend to southward propagate in the upper troposphere. The evolution of the wave packet along the wave trajectory is conducive to the zonally elongated cyclonic and anticyclonic anomalies, which are responsible for the meridional structure of the PJ pattern. The linear baroclinic model experiments confirm the above processes and show that the wave coupling in upper and lower is also important.
In summary, theoretical, observational and simulation results suggest that the theory of Rossby wave propagation and phase evolution of wave packet in HN flow can reasonably well diagnose the trajectory and phase of the perturbation in the real atmosphere, and can be a fundamental theoretical tool to study the wave-like trains and atmospheric responses to heating source anomalies.