On the Dynamics of Equatorial Forcing of Climate Teleconnections

Abstract

The dynamics of equatorially forced climate teleconnections on the sphere is studied using quasi-geostrophic wave theory and numerical models. Using the concept of a refractive index for meridional propagation of energy, it is demonstrated that for zonal mean flow with no horizontal shear, steady-state atmospheric teleconnections are composed of radiating Rossby modes which are forced in weak westerly zonal mean flow and evanescent modes in easterly zonal mean wind. For horizontally sheared zonal mean flow, westerly shear will lead to initial transient growth of wave packets with northwest?southeast tilt. These wave packets will move initially northward from the source region new the equator and subsequently become damped after turning southward at various critical latitudes. In contrast, easterly shear will always cause monotonic decay of all northbound wave packets from the tropics. The results imply that, in the case of a barotropically stable mean flow, kinematic shearing effect will focus or defocus wave energy from tropics to midlatitudes depending on whether the ambient horizontal shear is westerly or easterly. This mechanism also explains why tropical wave energy is naturally drawn toward the exit region of climatological winter jet streams. Experiments with a nonlinear barotropic spectral model with equatorial forcing shows that wave energy can still propagate away from regions of initially weak tropical easterly mean flow by the shear-induced growth mechanism which modifies the zero-wind line downstream of the source. The interaction of the winter subtropical westerly jet and the wave disturbance generated by diabatic forcing source over the equator produces a quasi-stationary wave pattern reminiscent of the Pacific?North America pattern. The gross features of the tropics and extratropical steady-state response in the radiating mode are in qualitative agreement with that predicted from linear theory.