Meridional wave propagation in the subtropics and the relationship to the base state flow

Qualities of meridional wave propagation in the upper troposphere of the subtropics are analyzed via wavelet analysis using a meridional-temporal partial Morlet wavelet. Results show that power is increased in regions where the corresponding equatorial winds in the upper troposphere are westerly or weakly easterly. Furthermore, equatorward power is enhanced near subtropical jet exit regions whereas poleward power is enhanced in jet entrance regions. Regressions of upper-tropospheric winds, geopotential height and OLR against the wavelet transforms demonstrate that the wavelets are identifying signals with tropical-extratropical interactions that are connected to organized convection in the tropics.The relationship of power with background state flow characteristics, including the horizontal winds and shear, are evaluated. Instead of the zonal wind and meridional shear of the zonal wind, both the meridional wind and the zonal shear of the meridional wind appear to have a clearer relationship with the power. Power is favored for waves whose propagation is aligned in the same direction as the meridional wind, and reduced in the opposite direction. Additionally, power increases with increasing zonal shear of the meridional wind in the Northern Hemisphere and with decreasing zonal shear of the meridional wind in the Southern Hemisphere. Power in the equatorward direction is stronger than in the poleward direction and more heavily influenced by background flow characteristics. Furthermore, power for wavelets with smaller meridional and temporal scales tends to have a higher sensitivity to the background horizontal flow as compared to larger meridional and temporal scales.