Contrasting meridional structures of stratospheric and tropospheric planetary wave variability in the Northern Hemisphere

Abstract

The meridional structures of stratospheric and tropospheric planetary wave variability (PWV) over the Northern Hemisphere (NH) extratropics were investigated and compared using reanalysis data. By performing the spherical double Fourier series expansion of geopotential height data, the horizontal structures of PWV at each vertical level could be examined in the two-dimensional (2D) wavenumber (zonal and meridional wavenumbers) space. Comparing the amplitudes of wave components during the last three decades, the results suggested that the structures of PWV in the NH troposphere significantly differ from the stratospheric counterparts. The PWV in the troposphere shows multiple meridional wave-like structures, most pronounced for the meridional dipole; while in contrast, PWV in the stratosphere mainly shows large-scale zonal wave patterns, dominated by zonal waves 1 and 2, and have little wave-like fluctuation in the latitudinal direction. The dominant patterns of the NH PWV also show contrasting features of meridional structure between the stratosphere and the troposphere. As represented in the 2D wavenumber space, the leading two empirical orthogonal functions of PWV in the stratosphere largely exhibit the zonal wave 1 pattern, while those in the troposphere clearly show meridional wave-like structures and are dominated by the dipole. The refractive index was derived based on the zonal mean basic state to qualitatively interpret the observational findings. The results suggested that the basic state in the NH troposphere is much more favourable for latitudinally propagating stationary waves than the stratosphere. The difference in meridional structure between stratospheric and tropospheric planetary waves can be well captured in a linear baroclinic model with the observed zonal mean basic state. Furthermore, both theoretical and modelling analyses demonstrated that the fact that zonal wave patterns are preferred in the NH stratosphere may be partly attributable to the vertical curvature of the stratospheric zonal mean basic state.

• planetary waves
• stratosphere
• troposphere
• spherical double Fourier series
• two-dimensional wavenumber

Acknowledgements

The authors wish to thank the anonymous reviewers for their constructive comments that contributed to improve this paper. This work was jointly supported by the NSFC Projects 41305046 and 41290255, and the Climate Change Special Project of the State Ocean Administration.